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\begin{document}

\title{Ontogenetic shift or not? Different foraging trade-offs within the meso-
to bathypelagic fish community}



\author[1]{Liz Loutrage}%
\author[2]{Anik Brind'Amour}%
\author[3]{Tiphaine Chouvelon}%
\author[4]{Jérôme Spitz}%
\affil[1]{La Rochelle University}%
\affil[2]{Ifremer}%
\affil[3]{Ifremer Centre Atlantique}%
\affil[4]{University of La Rochelle}%


\vspace{-1em}



  
  \date{June 15, 2024}


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\selectlanguage{english}
\begin{abstract}
During ontogeny, the increase in body size forces species to make
trade-offs between their food requirements, the conditions necessary for
growth and reproduction as well as the avoidance of predators.
Ontogenetic changes are leading species to seek out habitats and food
resources that meet their needs. These aspects are interesting to study
in the case of deep pelagic fishes, as a significant part of the
community migrates vertically at night to feed in the productive surface
layer, while other species remain at depth. To this end, ontogenetic
changes in nocturnal habitat (vertical use of the water column) and in
the type of food resources (based on stable isotopes of nitrogen) were
investigated in 12 species of deep pelagic fish from the Bay of Biscay
in the Northeast Atlantic. Our results revealed the existence of major
differences in the ontogenetic strategies employed by deep pelagic
fishes. Some species showed ontogenetic changes in both vertical habitat
use and food resources (e.g. Lampanyctus crocodilus and Melanostigma
atlanticum). In contrast, other species showed no ontogenetic change
(e.g. Searsia koefoedi and Notoscopelus kroyeri). Some species only
changed food resources (e.g. Myctophum punctatum, Arctozenus risso, and
Serrivomer beanii), while others seemed to be influenced more by depth
than by trophic features (e.g. Xenodermichthys copei and Argyropelecus
olfersii). These results suggest that to meet their increasing energy
requirements during ontogeny, some species have adopted a strategy of
shifting their food resources (larger prey or prey with a higher trophic
level), while others seemed to maintain their food resources but are
most likely increasing the quantity of prey ingested. In addition, some
species opted for a habitat change to greater depths at adult age to
limit the energy expenditure associated with migration, while others
continued to feed at the surface at night.%
\end{abstract}%



\sloppy


Ontogenetic shift or not? Different foraging trade-offs within the meso-
to bathypelagic fish community

Liz LOUTRAGE\textsuperscript{1,2,3}, Anik
BRIND'AMOUR\textsuperscript{2}, Tiphaine CHOUVELON\textsuperscript{1,4},
J\selectlanguage{ngerman}érôme SPITZ\textsuperscript{1,3}

\begin{enumerate}
\tightlist
\item
  Observatoire Pelagis, UAR 3462 La Rochelle Université/CNRS, La
  Rochelle, France
\item
  DECOD (Ecosystem Dynamics and Sustainability), IFREMER, INRAE,
  Institut Agro, Nantes, France
\item
  Centre D'Etudes Biologiques de Chizé (CEBC), UMR 7372 La Rochelle
  Université/CNRS, Villiers-en-Bois, France
\item
  IFREMER, Unité CCEM (Contamination Chimique des Écosystèmes Marins),
  Nantes, France
\end{enumerate}

\textbf{Corresponding author:}

Liz Loutrage

liz.loutrage@univ-lr.fr

Observatoire Pelagis,

UAR 3462 La Rochelle Université/CNRS,

5 allées de l'océan,

La Rochelle, France

\textbf{Keywords:} Stable Isotopes, Deep pelagic, Trophic ecology, Body
size, Variance partitioning

\textbf{Introduction}

Ontogenetic shifts in marine predators are major drivers in the
mechanisms underlying ecosystem structure and functioning (Rudolf and
Rasmussen, 2013; Nakazawa, 2015). They are also considered a determinant
of food web diversity and stability, community resilience, and responses
to disturbance (de Roos and Persson, 2013; Nakazawa, 2015; Nilsson et
al., 2018). Although the importance of these ubiquitous changes in
ecosystems is well established, community ecology has traditionally been
based on species, thereby erasing intraspecific differences. During
ontogeny, individuals must make trade-offs between their dietary needs,
conditions necessary for reproduction, and predator avoidance
(Sutherland, 1996; Kimirei et al., 2013; Sánchez-Hernández et al.,
2019). All of these needs and trade-offs change over the lifetime of
species, requiring them to find habitats that meet their needs (Werner
and Gilliam, 1984; McNamara and Houston, 1986; Werner and Hall, 1988;
Ludwig and Rowe, 1990). Thus, shifts in diet and habitat use during
ontogeny can lead to segregation in the niches occupied by individuals
of a species and thus reduce intraspecific competition within a
population (Sánchez-Hernández and Cobo, 2012; Wollrab et al., 2013). At
the interspecific level, these shifts also play an important role in
competitive interactions and niche partitioning (Woodward and Hildrew,
2002; Woodward et al., 2005; de Roos and Persson, 2013).

Fish species often show a close relationship between body size, which is
generally related to the size of the mouth opening, and the size of the
prey they consume (Dunic and Baum, 2017). Therefore, ontogenetic shifts
in resource use are very common in fish (Werner and Gilliam, 1984). In
general, early-stage fish feed on phytoplankton, zooplankton, or small
invertebrates (Nunn et al., 2012). As their vision and swimming
performance improve, fish begin to feed on macroinvertebrates and fish
(Huss et al., 2013). These shifts in food types are often associated
with or caused by a shift in habitat uses (Werner and Gilliam, 1984;
Sánchez-Hernández et al., 2019). For instance, a change in diet may be
the consequence of a change in habitat to cope with new predation risks
during ontogeny, or it may be caused by the search for more nutritious
and/or more abundant prey (Sánchez-Hernández et al., 2019). An example
of the consequences of a change in diet dictated by a change in habitat
use is that of small individuals of the Gobiidae
species\emph{Pterogobius elapoides} , feeding on abundant pelagic
copepods in the water column where predation is high. As the individuals
grow larger, they limit the risk of predation by feeding only on prey
found in the sediments of the sandy bottom (Choi and Suk, 2012).

Meso- and bathypelagic fish communities (i.e., inhabiting the
mesopelagic zone between 200-1000 m, and the bathypelagic zone below
1000 m depth) are believed to dominate the fish biomass worldwide
(Irigoien et al., 2014). The deep pelagic food web is supported solely
by phytoplankton primary production, resulting in the segregation of
deep pelagic fish trophic niches essentially along a continuum of
trophic levels (Stowasser et al., 2012; Valls et al., 2014; Chouvelon et
al., 2022; Richards et al., 2023). Three main food guilds are generally
described for midwater fishes: zooplanktivores (e.g., Myctophidae),
micronektivores (e.g., Stomiidae), and generalists (e.g.
Eurypharyngidae, in which a wide variety of prey even benthic, is found)
(Gartner Jr et al., 1997; Drazen and Sutton, 2017). In addition to these
guilds, two main foraging strategies are employed by deep pelagic
fishes. Part of the community performs diurnal vertical migrations (DVM)
at night from the mesopelagic to the epipelagic zone to feed (Clarke,
1963; Badcock and Merrett, 1976; Watanabe et al., 1999). This migratory
behaviour is very energy-consuming but is compensated by the high prey
density in the epipelagic zone and the reduction of visual predation at
night. The non-migratory part of the community remains at depth. The
non-migratory species thus live in an environment of low prey density
but have lower energy requirements and a low risk of predation
(Marshall, 1979; Herring, 2001).

Most deep pelagic species, particularly Myctophidae, spend their larval
stage in the productive epipelagic zone (Ahlstrom, 1959; Loeb, 1979;
Moser and Smith, 1993; Sassa et al., 2002, 2004; Bowlin, 2016). Within
species, individual size generally increases with depth, indicating
ontogenetic vertical migrations (Loeb, 1979; Kawaguchi and Mauchline,
1982; Badcock and Araujo, 1988; Sassa and Kawaguchi, 2006). This
ontogenetic shift along the vertical habitat is related to shifts in
morphology and pigmentation (i.e., individuals becoming darker, having
photophores, and well-developed musculature) (Moser, 1996). Similarly,
the adults of several species have a different depth distribution
according to size, with larger individuals at deeper depths (Badcock and
Merrett, 1976; Loeb, 1979; Willis and Pearcy, 1980; Auster et al., 1992;
Stefanescu and Cartes, 1992; Sassa et al., 2007; Fanelli et al., 2014).
These ontogenetic shifts in habitat use may be related to shifts in
diet, as in the case of \emph{Lampanyctus crocodilus} , where senescent
adults stop migrating and adopt benthopelagic behavior by feeding on
epibenthic prey (Stefanescu and Cartes, 1992; Fanelli et al., 2014).

Intraspecific trophic changes can be monitored from stable isotope
signatures (Hammerschlag-Peyer et al., 2011; Layman et al., 2012). For
decades, stable isotope ratios of nitrogen (\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N
values) have been widely used as an indicator of species' trophic level
(Peterson and Fry, 1987; Zanden and Rasmussen, 2001; Drazen and Sutton,
2017). This is because nitrogen isotopes undergo a significant and
relatively predictable level of fractionation during trophic transfer
between a predator and its prey, leading to a difference in
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values (\textasciitilde{}3-5%0) between two
theoretical trophic levels and allowing the relative trophic level of
species to be inferred from their \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values
(Peterson and Fry, 1987; Post, 2002; Hussey et al., 2014). Since the
pelagic ecosystem has a wide depth gradient, microbial degradation of
organic matter in suspended particles also influences
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values, with increasing values with depth (Saino
and Hattori, 1980; Casciotti et al., 2008). An enrichment
in\textsuperscript{15}N is thus found in zooplankton at greater depths
(Koppelmann et al., 2009; Hannides et al., 2013) and in deep benthic
communities (Bergmann et al., 2009; Trueman et al., 2014).

Only a few studies examined the effect of species size and depth on
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values of deep pelagic fish, such as in the
Iberian Peninsula (North-East Atlantic) and the Gulf of Mexico
(North-West, Atlantic) (Romero-Romero et al., 2019; Richards et al.,
2023), but never at the intraspecific scale. Here, we aimed to quantify
the influence of ontogeny on nocturnal habitat use and trophic ecology
for 12 deep pelagic fish belonging to nine genera, including both
migratory and non-migratory species from the Bay of Biscay, NE Atlantic.
Changes in foraging habitats were studied through shifts in the use of
the water column at night inferred from trawling data, while trophic
shifts were studied using stable isotopes of nitrogen measured in fish
muscle tissues. The first objective was to investigate whether an
ontogenetic shift in the water column utilization (i.e., relationships
between individual size and sampling depth) is observed at both the
specific and community level. The second objective was to explore if a
shift in the trophic ecology (and possibly in the trophic level) is also
observed during ontogeny (from the measurement of \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N
values). To this end, the influence of individual size and/or sampling
depth on \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values was quantified for each species.

\section*{Materials and methods}

{\label{materials-and-methods}}

\subsection*{Sampling}

{\label{sampling}}

Organisms were collected by epi- to bathypelagic trawling in canyons of
the Bay of Biscay continental slope (North-East Atlantic) during EVHOE
scientific cruises (``\emph{Evaluation Halieutique de l'Ouest de
l'Europe} ''; https://doi.org/10.18142/8) that took place between 2002
and 2021. Trawls were conducted at night between 25 m to 2000 m depth at
25 stations (Figure 1). The trawl net was 192 m long with a headline of
76 m and a foot rope of 70 m. The average vertical mean mouth opening
was about 24 m and the horizontal opening of about 58 m. The mesh size
gradually decreased from a very large 8 m (stretched mesh) at the mouth
to 20 mm (stretched mesh) in the cod-end. To allow the capture of very
small specimens, the trawl was also equipped with a 7.5 m long sock with
a 12 mm mesh size. Each haul was made at a specifically chosen immersion
depth. Once the trawl reached the selected depth it was towed
horizontally (i.e., constant immersion depth) for 1 hour at 4 kn.

\subsection*{Datasets}

{\label{datasets}}

Two different datasets were used to study ontogenetic changes in
deep-sea pelagic fish species from the Bay of Biscay. The trawling
(first) dataset included all data collected by trawls (i.e. number of
individuals per species per sampling depth and total body length of each
individual; n = 4165). To study the trophic aspects of ontogeny (see
methodology below), muscle sampling was performed on 12 species of the
trawling dataset to access the \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values of
individuals (n = 682). This constituted the isotopic (second) dataset.
The size measured for the individuals sampled for the isotopic dataset
was the standard length. The size distribution of the individuals
composing the species included in the isotopic dataset was
representative of the size distribution observed in the trawling dataset
(see appendix 1).

\subsection*{Nitrogen stable isotope
analysis}

{\label{nitrogen-stable-isotope-analysis}}

A total of 682 muscle samples belonging to 12 of the most abundant
species (seven migratory and five non-migratory (Loutrage et al., 2023)
were collected (table II). For each individual, the standard length (cm)
was measured on board and a small piece of muscle was collected and
frozen at -20\selectlanguage{ngerman}°C. To have sufficient material for stable nitrogen isotope
analysis, the muscles of the smallest individuals were pooled. Within
each of these pools, the individuals were of equivalent size and were
sampled at the same depths. At the laboratory, muscle samples were
freeze-dried (72h). To reduce the samples to a fine powder, samples
containing a single individual were manually homogenised, while samples
containing a pool of individuals were homogenised using a ball mill
(MM400 Retsch®) with zirconium oxide-coated bowls and balls. A fraction
of this powder (0.50 ± 0.05mg dry mass) was weighed in tin cups.
Analyses were then performed with an isotope ratio mass spectrometer
(Delta V Advantage with a Conflo IV interface, Thermo Scientific)
coupled to an elemental analyser (Flash EA, 2000; Thermo Scientific).
Results are presented in the usual \selectlanguage{greek}\emph{δ} \selectlanguage{english}notation relative to the
deviation from an international standard (atmospheric nitrogen,
for\selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{15}N values), in parts per thousand (%0).
Based on repeated measurements of USGS-61 and USGS-62 used as laboratory
internal standards, the experimental analytical precision
was\emph{\textless{}} 0.15%0.

The isotopic dataset included individuals sampled from different years
(i.e. between 2007 and 2021), which could have affected our data.
However, more than 90\% of the muscle samples were collected between
2019 and 2021 and nearly 75\% in 2021, which reduces the potential
inter-annual effect. Furthermore, if a significant temporal change
in\selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{15}N values had occurred, we expect that a
common pattern would have been observed for the different species
considered, which was not the case. Thus, we hypothesize that changes in
size and depth have a much greater influence on the\selectlanguage{greek}\emph{δ}
\selectlanguage{english}\textsuperscript{15}N values than does the temporal aspect. Details on
the sampling by year for each species are presented in the appendix 2.

\subsection*{Relationships between size distribution and
depth}

{\label{relationships-between-size-distribution-and-depth}}

The different depth layers were defined as follows: the epipelagic zone
between 25 and 175m, the upper mesopelagic zone between 175 and 700m,
the lower mesopelagic zone between 700 and 1000m, and the bathypelagic
zone below 1000m. This division corresponds to the one used in the
literature (Sutton, 2013) and is congruent with the depth structuration
observed in the canyons of the Bay of Biscay (Loutrage et al., 2023). To
study the changes in size distribution with depth, the trawling dataset
was used. At both community and specific levels, a linear model was
performed, the sampling depth corresponding to a continuous explanatory
variable. Results were considered significant when the \emph{p-value}was
[?]0.05.

\selectlanguage{greek}\subsection*{\texorpdfstring{Relationships between\emph{δ}
\textsuperscript{15}N values and
size}{Relationships betweenδ 15N values and size}}\selectlanguage{english}

{\label{relationships-between-ux3b415n-values-and-size}}

The relationship between \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{15}N values and
individual size was explored with the isotopic dataset at both community
and species levels. In the case of pooled samples for nitrogen isotope
analysis, the size data is the mean size of all pooled individuals. A
linear model was used in each case (species and community levels). For
non-significant relationships between fish size and\selectlanguage{greek}\emph{δ}
\selectlanguage{english}\textsuperscript{15}N values (i.e. \emph{p-value} [?]0.05) at the species
level, coefficients of variation were calculated to explore the
dispersion of values.

\subsection*{Variance partitioning}

{\label{variance-partitioning}}

Variance partitioning was used to calculate the variance explained by
the different variables included in a model (Borcard et al., 1992;
Legendre and Legendre, 2012). This is done by developing a set of
partial models (in a multivariate or univariate framework) created using
a subset of predictor variables. Here, the objective was to test to what
extent the individual size and the sampling depth influence the\selectlanguage{greek}\emph{δ}
\selectlanguage{english}\textsuperscript{15}N values at the specific level. Due to the
restricted depth range at which \emph{Aphanopus carbo} and \emph{Stomias
boa} were captured ([?] 100m), the variance partitioning was not performed
on these two species. The model results are composed of the proportion
of \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{15}N values influenced by size and depth
separately, and a third fraction representing the shared fraction of
variation explained when both variables are included in the model. An
ANOVA-type permutation test was performed for each model to test the
significance of the influence of each variable (depth and size) on
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values. Since the third fraction is deduced from
the sum of variances, it cannot be tested statistically. The R
package\emph{vegan} was used to perform the tests (Oksanen et al.,
2022). All the graphics were performed with the \emph{ggplot2} R package
and all statistical analyses were performed in the R environment version
4.3 (Wickham et al., 2016; R Core Team, 2023).

\section*{Results}

{\label{results}}

\subsection*{Relationships between size distribution and
depth}

{\label{relationships-between-size-distribution-and-depth-1}}

We observed a significant increase (\emph{p-value \textless{}} 0\emph{.}
001) in fish size (total length) with depth at the community level
(Table I). The median individual size increased consecutively between
the epipelagic, upper mesopelagic, and lower mesopelagic depth layers
(median individual size equal to 7.0, 9.0, and 10.6 cm respectively;
Figure 2). Median individual size then decreased slightly between the
lower mesopelagic and bathypelagic layers, with a median individual size
of 10 cm in the bathypelagic layer. Furthermore, while the first three
depth layers had an unimodal distribution, the bathypelagic layer
presented a bimodal distribution with a peak of around 8 cm and another
around 13 cm.

The relationship between size distribution and depth was also tested at
the species level (Table I). Only four of the 12 species showed a
significant relationship. \emph{Melanostigma atlanticum, Lampanyctus
crocodilus} , and \emph{Xenodermichthys copei} showed a significant
increase in individual size with depth. \emph{Myctophum punctatum} was
the only species showing a significant decrease in the individual size
with depth.

\emph{Lampanyctus crocodilus} showed an increase in individual size
between the upper and lower mesopelagic layers, from a median size
(total length) of 10.0 cm to 12.0 cm (Figure 3). This was followed by a
stabilisation between the lower mesopelagic and bathypelagic layers with
the same median size of 12.0 cm. \emph{Melanostigma atlanticum} showed a
continuous increase in the size of individuals with depth, with median
individual sizes of 6.0, 7.45, and 9 cm
respectively.\emph{Xenodermichthys copei} showed a maximum median size
in the lower mesopelagic layer (= 10.2cm total length). \emph{Myctophum
punctatum}showed small differences in individual size between depth
layers, with a median size of between 6.6 and 7.0 cm. Relationships for
other species are available in Appendix 3.

\selectlanguage{greek}\subsection*{\texorpdfstring{Relationships between\emph{δ}
\textsuperscript{15}N values and
size}{Relationships betweenδ 15N values and size}}\selectlanguage{english}

{\label{relationships-between-ux3b415n-values-and-size-1}}

\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values were determined for seven migratory and
five non-migratory or short migratory species, for a total of 682
individuals (= isotopic dataset). Mean \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{15}N
values ranged from 9\emph{.} 49 \selectlanguage{ngerman}± 0\emph{.} 57\selectlanguage{english}%0 for \emph{Serrivomer
beanii} to 12\emph{.} 36 +- 0\emph{.} 33\selectlanguage{english}%0 for \emph{Aphanopus carbo}
(Table II). For this dataset, mean values of standard length ranged from
6\emph{.} 3 +- 1\emph{.} 7 cm for \emph{Argyropelecus olfersii} to
77\emph{.} 3 +- 10\emph{.} 8 cm for \emph{Aphanopus carbo} ,
with\emph{Serrivomer beanii} having the widest size range (45.0 cm
between the minimum and the maximum length) and \emph{Lampanyctus
macdonaldi}the narrowest (3.3 cm).

The relationship between \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{15}N values and
individual size was first investigated at the community level (Figure
4). The linear model results showed a significant increase in\selectlanguage{greek}\emph{δ}
\selectlanguage{english}\textsuperscript{15}N values with individual size. However, the\emph{R}
\textsuperscript{2} was very low (= 0.01), indicating high variability
in the values.

The relationship between \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{15}N values and size
was then investigated at the species level (Figure 5A). The results of
the linear models showed that six species had a significant increase
of\selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{15}N values with increasing individual
size:\emph{Myctophum punctatum} , \emph{Melanostigma atlanticum}
,\emph{Lampanyctus crocodilus} , \emph{Stomias boa} , \emph{Serrivomer
beanii,} and \emph{Aphanopus carbo} . Only \emph{Arctozenus risso} had a
significant decrease of \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{15}N values with
increasing size.

The other five species had no significant relationship between the two
variables (Figure 5B). The \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{15}N values were
stable regardless of the size increase. However, the coefficient of
variations allowed to distinguish species with low interindividual
variability (e.g., \emph{N. kroyeri} ) from species with high
interindividual variability (\emph{X. copei} ).

\subsection*{Variance partitioning}

{\label{variance-partitioning-1}}

Results of the variation partitioning analyses showed that five species
(the same as above) had their \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{15}N values
significantly influenced by individual size (Figure 6, NB: \emph{S.
boa}and \emph{A. carbo} not considered in these analyses due to small
depth range). \emph{L. crocodilus} had the highest proportion of
variation in\selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{15}N values explained by size
(25.7\%), followed by \emph{M. punctatum} (24.7\%), \emph{M. atlanticum}
(10.1\%),\emph{S. beanii} (13.2\%) and \emph{A. risso} (5.8\%).
Alternatively, in\emph{A. olfersii} and \emph{X. copei} , variations
in\selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{15}N were significantly explained by depth,
at a proportion of 5.7\% and 5.5\% respectively.

\subsection*{Summary of relationships at specific and community
levels}

{\label{summary-of-relationships-at-specific-and-community-levels}}

According to the different relationships, diverse species patterns can
be described (Table III). First, two species (\emph{L. crocodilus}
and\emph{M. atlanticum} ) showed an ontogenetic change both in their
vertical distribution and their\selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{15}N values.
The largest individuals of these species were caught at greater depth
and their\selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{15}N values increased with individual
size. Four other species showed an ontogenetic change in their\selectlanguage{greek}\emph{δ}
\selectlanguage{english}\textsuperscript{15}N values but no change in their vertical
distribution (from trawling data): \emph{A.~risso} , \emph{S. beanii}
,\emph{S. boa,} and \emph{A. carbo} . Among the species that did not
show any significant change in their trophic ecology (according
to\selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{15}N values) with increasing size,
differences appeared in the dispersion of \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{15}N
values. Some species such as \emph{N. kroyeri} and \emph{L.~macdonaldi}
had a restricted range of \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{15}N values whatever
the size of individuals (i.e. variation coefficients = 2.15 and 2.68,
respectively) whereas some species such as \emph{X.~copei} showed a high
dispersion of \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{15}N values (CV = 6.57).
Finally, three species showed no relationships among the variables
tested:\emph{N.~kroyeri} , \emph{L.~macdonaldi} , and \emph{S. koefoedi}
. However, for \emph{L.~macdonaldi} , a particularly low size range was
sampled for isotopic analysis (= 3 cm between the minimum and the
maximum individual size). At the community level, an increase in
individual size with depth was observed, as well as an increase in
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values with increasing size (Table III).

\section*{Discussion}

{\label{discussion}}

The present study confirms, for the Bay of Biscay, the global trend
towards observing larger fish at greater depth for deep pelagic fish
communities, as described in other systems (Badcock and Merrett, 1976;
Willis and Pearcy, 1980; Auster et al., 1992; Stefanescu and Cartes,
1992; Gartner Jr et al., 1997). The presence or not of ontogenetic
shifts and their associated main drivers (trophic and/or habitat) were
described: i) species undergoing both ontogenetic shifts in vertical
habitat use (as inferred from trawling data) and in their trophic
ecology (as inferred by \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values):\emph{L.
crocodilus} and \emph{M. atlanticum} ; ii) species showing only an
ontogenetic shift in their trophic ecology (i.e. the significant
influence of size on \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values): \emph{M.~punctatum,
A. risso, S. beanii, S. boa,} and \emph{A. carbo} ; iii) species for
which only depth influences their \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values
(\emph{X.~copei} and\emph{A.~olfersii} ), although \emph{X. copei} also
presented an ontogenetic shift in habitat use form trawling data (with
the smallest individuals not found in the deepest stations); and iv)
species showing no ontogenetic shift: \emph{L.~macdonaldi} ,
\emph{S.~koefoedi,} and\emph{N.~kroyeri} .

When studying ontogenetic shifts in habitat use and trophic ecology
within deep pelagic fish communities, several important aspects must be
considered. The first limitation is the small depth or size range
sampled for some species. In particular, \emph{A. carbo} and \emph{S.
boa} were sampled over a depth range of less than 100m, so relationships
between \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values and/or size with depth could not
be estimated. Alternatively, \emph{L. macdonaldi} was sampled over a
small size range ([?]3 cm), which may potentially explain the lack of
significant relationships found with individual size for this species.
As the isotopic sampling took place over 14 years, a possible temporal
bias in the \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values cannot be excluded. However,
more than 90\% of the muscle samples were collected between 2019 and
2021, and almost 75\% in 2021, reducing this potential temporal
variability.

\subsection*{Community level}

{\label{community-level}}

A significant increase in the size of individuals with depth was
observed at the community level. This pattern has already been observed
for migratory species, with older stages generally found at greater
depths than younger ones, as individuals that may have reduced their
migratory range or stopped migrating with age (Badcock and Merrett,
1976; Clarke and Wagner, 1976; Nafpaktitis, 1977; Willis and Pearcy,
1980; Lancraft et al., 1988; Auster et al., 1992; Stefanescu and Cartes,
1992; Gartner Jr et al., 1997). This general trend in the deep pelagic
realm may be a consequence of the trade-off between foraging and
predation. To satisfy their energetic needs, juveniles and adults of
some species migrate to the epipelagic layer to feed at night.
Alternatively, at the senescent stage, some species undergo a reduction
in swim bladder size as they age (e.g. \emph{L. crocodilus} ), so that
the energetic cost of migration may be greater than the benefit
provided. Some of these species, therefore, adopt a benthopelagic
behaviour, which allows them to reduce the energy expended on foraging
by taking advantage of the higher concentrations of zooplankton in the
benthopelagic zone (Angel and de C. Baker, 1982; Vinogradov, 2005).

At the community level, a slight but significant increase in
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values with individual size was also found.
However, this relationship was very weak (R\selectlanguage{ngerman}² = 0.01). Indeed, it has
previously been shown that within fish communities, the increase in
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values as a function of individual size was more
strongly linked to ontogenetic changes than to the fact that the largest
species in the community fed on higher trophic-level prey (Jennings et
al., 2002; Stowasser et al., 2012). There are several possible causes:
omnivory, large predators feeding on smaller prey, large pelagic
suspension feeders feeding on small suspended particulate organic
matter, and morphological adaptations of small predators to feed on
larger prey (Jacob, 2005; Jennings, 2005; Bode et al., 2007). In
particular, in our study, \emph{S. beanii} was the second largest
species sampled (= 55 cm) but had the lowest mean \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N
values (= 9.5%0). This could be partly explained by their serpentine
morphology, with a large individual size that is not proportionally
reflected in the size of the mouth opening, limiting their ability to
capture large prey compared to other species with similar individual
sizes. Indeed, our study includes a wide variety of morphology,
including lanternfish, hatchetfish, dragonfish, and eelfish. This
diversity in shape is bound to lead to large differences in feeding
strategies. Consequently, body size may not be the best measure to infer
the trophic ecology of these species, although it is often correlated
with several other measures. For example, body mass, size, or shape of
the mouth opening may be more relevant to study this relationship at the
community level (Vill\selectlanguage{ngerman}éger et al., 2017).

\subsection*{Trophic-driven ontogenetic
shift}

{\label{trophic-driven-ontogenetic-shift}}

Intraspecific ontogenetic shift in fish trophic ecology (as inferred
from \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values variation with size) is generally a
consequence of the ability of fish to catch larger prey. This ability is
proportional to mouth size, which in turn is proportional to body size,
allowing species to feed on larger prey (Dunic and Baum, 2017). Such a
pattern has already been observed in mesopelagic fish species (Gartner
Jr et al., 1997). In the present study, half of the studied species
showed a significant change in their \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values with
the increasing size of individuals. Of these species, four had their
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values influenced solely by the size of
individuals: \emph{M.~punctatum, S.~beanii, S.~boa,} and \emph{A. carbo}
. However, in the case of \emph{A.~carbo} and \emph{S.~boa} , the depth
range sampled for these species was too small to test the other
relationships. \emph{M.~punctatum} was previously described as a
generalist feeder in the Mediterranean Sea with a mixed diet during all
stages of its development, except small individuals that seem to feed
exclusively on copepods (Scotto di Carlo et al., 1982). As they grow,
individuals become more efficient predators and begin to select larger,
more nutritious prey (Bernal et al., 2015). In another study in the
Northern Atlantic, \emph{A.~carbo} also showed an increase in
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values with individual size, confirming the
probable ontogenetic diet or trophic level shift for this species
(Farias et al., 2014). Its diet would shift from pelagic zooplankton to
bathypelagic prey, reflecting an improvement in its predatory ability
(Farias, 2014). To our knowledge, this is the first time that an
ontogenetic shift in trophic ecology is reported for \emph{S. beanii}and
\emph{S. boa} . The diet of \emph{S. beanii} is generally described as
being composed of crustaceans and small fish, whereas that of\emph{S.
boa} is composed of crustaceans and mesopelagic fish, so our results
suggest that the proportions may vary with individual size (Whitehead et
al., 1984). Finally, among the studied species, only\emph{A.~risso} has
undergone a significant but weak decrease in \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N
values with individual size. This trend has already been found for two
species of the southern Kerguelen mesopelagic community belonging to the
families Platytroctidae and Myctophidae (Woods et al., 2019). A
significant decrease in \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values was also found in
a small pelagic neritic fish species, for example, i.e. the European
sardine (\emph{Sardina pilchardus} ). This reduction was attributed to
the greater efficiency of large sardines in capturing phytoplankton,
which is less enriched in \textsuperscript{15}N than zooplankton prey
(Bode et al., 2003, 2004, 2006). In addition, several species belonging
to the Paralepididae family (which includes\emph{A. risso} ) have shown
tooth loss in adult specimens and a recent study in the western Atlantic
also found this pattern for \emph{A. risso} (Ho and Duhamel, 2019;
Devine and Van Guelpen, 2021). This tooth loss may lead to dietary
changes in this species, partially explaining the negative relationship
between \selectlanguage{greek}δ\selectlanguage{english}15N values and size found in our results.

\subsection*{Trophic and habitat-driven ontogenetic
shifts}

{\label{trophic-and-habitat-driven-ontogenetic-shifts}}

\emph{L. crocodilus} and \emph{M.~atlanticum} showed, in addition to an
increase in \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values with individual size, an
increase in individual size with depth. In the Mediterranean
Sea,\emph{L.~crocodilus} has already been shown to make a change in its
diet in relation to its changes in migratory activity (Stefanescu and
Cartes, 1992; Fanelli et al., 2014). \emph{L. crocodilus} has a diet
dominated by epipelagic crustaceans in its pelagic phase and its
migratory activity decreases or even stops when it reaches the senescent
stage. It then adopts a benthopelagic behaviour and feeds on fish at
greater depth (Stefanescu and Cartes, 1992; Valls et al., 2014; Bernal
et al., 2023). Our present results, therefore, suggest that this
behaviour may also occur in the Bay of Biscay. As for \emph{M.
atlanticum} , it has a particular mode of reproduction. This species
adopts a benthic behaviour during the spawning period and the
fertilization of its eggs takes place in burrows located under the
surface of the sea bed (Silverberg et al., 1987; Dallar\selectlanguage{ngerman}és et al., 2021).
This specific reproductive behaviour may explain our results that the
largest, and thus reproductive individuals are found at greater depth.
From a trophic perspective, it has been shown in the Mediterranean Sea
that the diet of \emph{M. atlanticum}consists almost exclusively of
pelagic prey (Dallarés et al., 2021). Larger individuals may have the
ability to capture larger prey, which can also explain the relationship
between \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values and individual size found for this
species. These two species (\emph{L. crocodilus} and \emph{M.
atlanticum} ), in addition to having these two significant relationships
(larger individuals are found deeper and have higher
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values), presented the highest percentages of the
variance in \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values explained by both size and
depth (i.e., the red portion, \textgreater{}7\%) in the partition
models. This part reflects the proportion of the model that cannot
distinguish the effect of depth and size on \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N
values. The benthopelagic behaviour of these species in the adult stage
may also partly explain this influence, as \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values
(including those of POM at the base of food webs) are higher at greater
depth and particularly in the benthic domain (Bergmann et al., 2009;
Trueman et al., 2014; Richards et al., 2020)\textbf{.}

\selectlanguage{greek}\subsection*{\texorpdfstring{Depth-driven increase in
δ\textsuperscript{15}N
values}{Depth-driven increase in δ15N values}}\selectlanguage{english}

{\label{depth-driven-increase-in-ux3b415n-values}}

Two species, \emph{X.~copei,} and \emph{A.~olfersii} had their
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values significantly influenced only by depth
(and not by individual size). In addition, in the case of \emph{X.
copei} the smallest individuals were not found at greater depth.
Like\emph{M. atlanticum, X. copei} was previously reported to spawn
demersally in the North Atlantic, with individuals in pelagic trawls
that were juveniles and larger fish that were caught in the deeper
stations near the bottom (Mauchline and Gordon, 1983). As the sampling
in our study was carried out during the spawning season of this species
(October-November), many spawning individuals were observed. The capture
of large spawning individuals of \emph{X. copei} at depth suggests that
spawning of this species also occur on the slope area at that period in
the Bay of Biscay. Although \emph{X. copei} did not show a significant
relationship with size, it stood out for its wide dispersion of values
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values (CV = 6.57). In the North Atlantic,
pelagic individuals of this species have a diet limited mainly to
copepods and ostracods, while benthic individuals show a wider variety
of food, maybe explaining in part this high variability in
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values (Mauchline and Gordon, 1983). In addition,
it has been observed that both immature and larger individuals may graze
on inactive prey or debris such as small copepods, potentially
decoupling the relationship between size and \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N
values (Roe and Badcock, 1984). In addition, the \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N
values of \emph{X. copei} were significantly influenced by depth. This
may be a consequence of the reproductive individuals residing at depth
for this period and therefore integrating the enrichment of the
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values at depth (Choy et al., 2015; Gloeckler et
al., 2018; Romero-Romero et al., 2019; Richards et al., 2020). As
for\emph{A. olfersii} , it is a short-migratory species described to
feed on crustaceans and small fish (Muus et al., 1999). It has already
been shown that non-migratory species such as \emph{A. olfersii}
integrate changes in zooplankton \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N with depth
(Koppelmann et al., 2009; Hannides et al., 2013). Indeed, individuals
located deeper are more dependent on the food web based on bacterially
degraded organic particles and thus enriched in \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N
than individuals located less deep (Choy et al., 2015; Gloeckler et al.,
2018; Romero-Romero et al., 2019; Richards et al., 2020). This result
was already observed for two non-migratory species near the island of
Hawaii: \emph{Cyclothone pallida} and \emph{Melanocetus
johnsonii}(Romero-Romero et al., 2019). Among the non-migratory species
in our study, \emph{A. olfersii} was the species with the largest range
of depth sampled ([?]1000m) which may explain the significant influence of
depth on \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values for this species. For the other
non-migratory species (i.e.~\emph{L. macdonaldi, A. risso, S. koefoedi}
), the depth range sampled was maybe too small to detect any influence
of depth on the \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values.

\selectlanguage{greek}\subsection*{\texorpdfstring{No ontogenetic change and no influence of
depth on δ\textsuperscript{15}N values
cases}{No ontogenetic change and no influence of depth on δ15N values cases}}\selectlanguage{english}

{\label{no-ontogenetic-change-and-no-influence-of-depth-on-ux3b415n-values-cases}}

Finally, five species showed no trophic-driven ontogenetic
change:\emph{X. copei, S. koefoedi, A. olfersii, L. macdonaldi,} and
\emph{N. kroyeri} . Among these species, some showed high variability in
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values (\emph{X. copei, S. koefoedi, A. olfersii}
) while others had relatively constant values across their size range
(\emph{L. macdonaldi} and\emph{N. kroyeri} ). This result could
potentially reflect differences in feeding strategies between species.
Species with a high dispersion of \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values may have
higher dietary plasticity, allowing them to feed on a wide variety of
prey. Such a pattern has already been found for several small pelagic
neritic species such as\emph{Scomber scombrus} in the Iberian Peninsula
(Bode et al., 2006). However, information on the diet of the
Platytroctidae family is very scarce in the literature. \emph{S.
koefoedi} has been reported to have a diet composed largely of copepods,
but also ostracods, chaetognaths, and polychaetes, which could partly
explain the large variability in \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values found
during its ontogeny (Hopkins et al., 1996; Novotny, 2018). In contrast,
species with low variability in \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values could have
implemented an alternative strategy to the one classically observed,
based on an increase in the size of the prey associated with an increase
in mouth size. In this case, meeting energy requirements would be based
on an increase in the quantity of resources ingested, made possible by
the increase in mouth size, while maintaining the same type (size) of
food resources rather than a higher energy content per larger prey
ingested. The two Myctophidae species of this group, \emph{N. kroyeri,}
and \emph{L. macdonaldi} showed weak variability in
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values through their ontogeny. These two species
have been reported to have a diet mainly composed of crustaceans
(Gj\selectlanguage{ngerman}øsæter, 1981; Coad and Reist, 2004). However, for the \emph{L.
macdonaldi} case, the restricted size range sampled can explain part of
this absence of relation (= 3 cm). In addition, the smaller individuals
sampled had a standard length of 11.5 cm, which is important considering
the maximum size of 16 cm reported for this species. In the case of
\emph{N. kroyeri} , individuals' diets may not be size-restricted like
in the case of filter feeders, with a strategy of increasing the
quantity of ingested food with size.

\subsection*{The Myctophidae case
study}

{\label{the-myctophidae-case-study}}

An important observation from our results is that the Myctophidae
species studied here appeared to have significant differences in their
feeding strategies during ontogeny. They were found in three of the four
groups of species formed from the different relationships.
First,\emph{L. crocodilus} differs from the other species in that it
undergoes ontogenetic changes in both its diet and depth distribution.
As large adults have a reduced swim bladder, the energy gain associated
with nocturnal migration to feed in productive surface waters may
outweigh the costs, making feeding in the benthic boundary layer more
cost-effective (Fanelli et al., 2014). While both \emph{M. punctatum}and
\emph{N. kroyeri} are known to migrate vertically at night in the
epipelagic layer, they appeared to have adopted opposite feeding
strategies, with \emph{M. punctatum} appearing to change its diet during
ontogeny (i.e. increasing \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values with size),
whereas not only was this change not seen in \emph{N. kroyeri} , but its
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values remained very stable across the species
size gradient. Thus, by not shifting its diet towards larger or more
energetic prey during ontogeny\emph{, N. kroyeri} appears to have opted
for an increase in food quantity rather than a change in quality.
Finally, \emph{L. macdonaldi} had the deepest distribution (i.e. median
depth = 2000 m), which probably explains in part the lack of
profitability for this species to move into the epipelagic layer to feed
at night. \emph{L. macdonaldi} also appeared not to have undergone any
dietary changes during ontogeny, although this remains to be confirmed
with a wider sampling across the size range of the species. All these
differences in the ontogenetic foraging strategies of these
phylogenetically related species may be partially explained by
differences in morphological traits. Indeed, differences in
morphological traits can lead to differences in swimming ability, prey
capture, detection ability visual acuity, etc (Vill\selectlanguage{ngerman}éger et al., 2017).
Differences in feeding strategies and depth distribution during ontogeny
suggest divergence within this highly diverse family to avoid
competition.

\subsection*{Conclusions}

{\label{conclusions}}

Mesopelagic species are a key element in the pelagic food web, as they
provide an intermediate link between zooplankton and top predators (e.g.
tuna, marine mammals) (Pauly et al., 1998; Pusineri et al., 2005, 2007;
Griffiths et al., 2013). Given the diversity of strategies adopted by
mesopelagic species, it seems difficult to predict the presence or not
of ontogenetic changes. However, this basic information is essential to
build functional trophic models that are relatively faithful to reality.
For these reasons, the knowledge of their ontogenetic changes must be
strengthened to better estimate the ecosystem services they provide and
to assess the environmental risks associated with their exploitation
(St. John et al., 2016; Hidalgo and Browman, 2019; Christiansen et al.,
2020; Drazen et al., 2020).

\textbf{Data Availability Statement}

All data are provided as additional ''Supporting Information for review
and publication'' and are being submitted to the PANGAEA directory
(\emph{ontogeny\_isotopy\_data.csv:} isotope
data;\emph{trawling\_dataset.csv:} trawling data; \emph{README.Rmd} :
Information on the R project; index.qmd: final report with the code to
carry out the analysis; \emph{ontogeny\_deep\_pelagic\_fish.Rproj} : R
project; \emph{README\_data.Rmd} : information on the data)

\textbf{Conflict of interest statement}

The authors declare no competing interests.

\textbf{Acknowledgments}

This work was supported by the French Ministry in charge of Environment
(Ministère de la Transition Ecologique / Direction de l'Eau et de la
Biodiversité). This study was funded by the European project H2020
SUMMER ``Sustainable Management of Mesopelagic Resources'' (grant
agreement ID: 817806). The authors are grateful to the crew of the R/V
Thalassa (https://doi. Org/10.18142/8) and to all scientists and cruise
leaders who allowed the data collection during the EVHOE scientific
surveys. We finally thank Benoît Lebreton and Gaël Guillou from the
``\emph{Plateforme de Spectrométrie Isotopique} '' of the LIENSs
laboratory (UMR 7266, CNRS -- La Rochelle University) for their
assistance during stable isotope analyses.

\textbf{ORCID}

Liz Loutrage:\emph{https://orcid.org/0009-0000-1450-403X}

Tiphaine Chouvelon:\emph{https://orcid.org/0000-0002-9497-3684}

Anik Brind'Amour:\emph{https://orcid.org/0000-0003-4072-2588}

Jérôme Spitz:\emph{https://orcid.org/0000-0003-2729-2413}

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\textbf{Table I} : Results of the linear models at both community and
species levels between size distribution and depth. Significant
relationships are shown in bold.\selectlanguage{english}
\begin{longtable}[]{@{}llllll@{}}
\toprule
Family & Species & N & Slope & R\selectlanguage{ngerman}² & \emph{p-value}\tabularnewline
\midrule
\endhead
Alepocephalidae & \emph{Xenodermichthys copei} & 1070 &
1x10\textsuperscript{-3} & 0.01 &
\textbf{\textless{}0.001}\tabularnewline
Myctophidae & \emph{Lampanyctus crocodilus} & 1200 &
2x10\textsuperscript{-3} & 0.05 &
\textbf{\textless{}0.001}\tabularnewline
& \emph{Lampanyctus macdonaldi} & 72 & 2x10\textsuperscript{-4} &
\textless{}0.001 & 0.605\tabularnewline
& \emph{Myctophum punctatum} & 397 & 3x10\textsuperscript{-4} & 0.01 &
\textbf{0.040}\tabularnewline
& \emph{Notoscopelus kroyeri} & 585 & 3x10\textsuperscript{-5} &
\textless{}0.01 & 0.912\tabularnewline
Paralepididae & \emph{Arctozenus risso} & 246 & 7x10\textsuperscript{-4}
& \textless{}0.01 & 0.226\tabularnewline
Platytroctidae & \emph{Searsia koefoedi} & 68 & 2x10\textsuperscript{-4}
& \textless{}0.01 & 0.863\tabularnewline
Serrivomeridae & \emph{Serrivomer beanii} & 63 &
6x10\textsuperscript{-4} & \textless{}0.01 & 0.866\tabularnewline
Sternophidae & \emph{Argyropelecus olfersii} & 205 &
1x10\textsuperscript{-4} & \textless{}0.01 & 0.706\tabularnewline
Stomiidae & \emph{Stomias boa} & 63 & 2x10\textsuperscript{-4} &
\textless{}0.01 & 0.939\tabularnewline
Trichiuridae & \emph{Aphanopus carbo} & 39 & 1x10\textsuperscript{-2} &
0.06 & 0.136\tabularnewline
Zoarcidae & \emph{Melanostigma atlanticum} & 157 &
5x10\textsuperscript{-3} & 0.15 &
\textbf{\textless{}0.001}\tabularnewline
Community & \emph{-} & 4165 & 3x10\textsuperscript{-3} & 0.01 &
\textbf{\textless{}0.001}\tabularnewline
\bottomrule
\end{longtable}

\textbf{Table II} . Number of samples for stable isotope analysis (N)
and the total number of individuals (n) when some pools were made for
the species, the minimum and maximum standard length of individuals
(size, cm), sampling depth range (m), and mean \selectlanguage{ngerman}± standard deviation
of\selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{15}N values for each species. Migratory
patterns are from www.fishbase.org, Lusher et al. (2016), and references
therein. DVM = Diel Vertical Migration.\selectlanguage{english}
\begin{longtable}[]{@{}llllllll@{}}
\toprule
Family & Species & N samples (n individuals) & Migratory pattern &
Sampling depth (m) & Size range (cm) & Max size FishBase (cm) &
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N (%0)\tabularnewline
\midrule
\endhead
Alepocephalidae & \emph{Xenodermichthys copei} & 114 (126) & DVM &
370-1335 & 5.6-9.5 & 31.0 & 9.83 \selectlanguage{ngerman}± 0.64\tabularnewline
Myctophidae & \emph{Lampanyctus crocodilus} & 142 (154) & DVM & 370-1600
& 6.5-14.8 & 30.0 & 10.46 \selectlanguage{ngerman}± 0.66\tabularnewline
& \emph{Lampanyctus macdonaldi} & 23 & Probably no DVM* & 1335-2000 &
11.5-14.8 & 16.0 & 11.54 \selectlanguage{ngerman}± 0.31\tabularnewline
& \emph{Myctophum punctatum} & 80 (95) & DVM & 25-1335 & 5.0-9.0 & 11.0
& 9.99 \selectlanguage{ngerman}± 0.51\tabularnewline
& \emph{Notoscopelus kroyeri} & 75 (181) & DVM & 25-780 & 3.6-11.0 &
14.3 & 11.18 \selectlanguage{ngerman}± 0.24\tabularnewline
Paralepididae & \emph{Arctozenus risso} & 78 (89) & No DVM & 370-780 &
11.0-20.5 & 30.0 & 10.53 \selectlanguage{ngerman}± 0.36\tabularnewline
Platytroctidae & \emph{Searsia koefoedi} & 20 (22) & Probably no DVM* &
750-1000 & 8.5-14.5 & 15.0 & 11.80 \selectlanguage{ngerman}± 0.57\tabularnewline
Serrivomeridae & \emph{Serrivomer beanii} & 31 (37) & DVM & 715-1335 &
26.5-75.5 & 78.0 & 9.49 \selectlanguage{ngerman}± 0.54\tabularnewline
Sternophidae & \emph{Argyropelecus olfersii} & 64 (70) & Short DVM &
370-1335 & 3.3-10.0 & 9.0 & 10.18 \selectlanguage{ngerman}± 0.47\tabularnewline
Stomiidae & \emph{Stomias boa} & 20 (30) & DVM & 680-780 & 11.8-33.6 &
32.2 & 11.61 \selectlanguage{ngerman}± 0.71\tabularnewline
Trichiuridae & \emph{Aphanopus carbo} & 9 & DVM & 750-780 & 59.0-102.0 &
151.0 & 12.36 \selectlanguage{ngerman}± 0.33\tabularnewline
Zoarcidae & \emph{Melanostigma atlanticum} & 26 (34) & No DVM & 730-1010
& 7.0-11.0 & 15.0 & 11.21 \selectlanguage{ngerman}± 0.50\tabularnewline
\bottomrule
\end{longtable}\selectlanguage{ngerman}

*No DVM, at least towards epipelagic waters at night for feeding but
possible migrations deeper into the bathypelagic zone (Moore et al.,
2003; Novotny, 2018).

\textbf{Table III} : Summary of the relationships investigated for each
species and the community considered as a whole. CV = Coefficient of
Variation.\selectlanguage{english}
\begin{longtable}[]{@{}lllll@{}}
\toprule
\begin{minipage}[b]{0.17\columnwidth}\raggedright\strut
Species\strut
\end{minipage} & \begin{minipage}[b]{0.17\columnwidth}\raggedright\strut
Family\strut
\end{minipage} & \begin{minipage}[b]{0.17\columnwidth}\raggedright\strut
Mechanism driving ontogenetic shift\strut
\end{minipage} & \begin{minipage}[b]{0.17\columnwidth}\raggedright\strut
Relationships\strut
\end{minipage} & \begin{minipage}[b]{0.17\columnwidth}\raggedright\strut
Description\strut
\end{minipage}\tabularnewline
\midrule
\endhead
\begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
\emph{L. crocodilus}\strut
\end{minipage} & \begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
Myctophidae\strut
\end{minipage} & \begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
\emph{Trophic \& and habitat-driven ontogenetic shifts}\strut
\end{minipage} & \begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
Species showing ontogenetic shifts in both their vertical distribution
(largest individuals at greater depth) and their trophic ecology
(increase of \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values with the size of the
individuals).\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\emph{M. atlanticum}\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
Zoarcidae\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\emph{M. punctatum}\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
Myctophidae\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\emph{Trophic-driven ontogenetic shift}\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
Species showing only an ontogenetic shift in their trophic ecology
(influence of individual size on \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values, but no
effect of depth whenever it could be tested).\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\emph{A. risso}\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
Paralepididae\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\emph{S. beanii}\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
Serrivomeridae\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\emph{S. boa*}\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
Stomiidae\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\emph{A. carbo*}\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
Trichiuridae\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\emph{A. olfersii}\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
Sternoptychidae\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\selectlanguage{greek}\emph{Depth-driven increase in δ\textsuperscript{15}N values}\selectlanguage{english}\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
Species showing an increase of their \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values with
depth (but no effect of size). \emph{X. copei} also showed an
ontogenetic shift in the vertical distribution of individuals (the
smallest individuals were not found in the deepest stations).\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\emph{X. copei}\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
Alepocephalidae\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\emph{N. kroyeri}\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
Myctophidae\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\emph{No ontogenetic shift}\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
Species with no observed ontogenetic shifts (no effect of depth on
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values). Moreover, \emph{N.~kroyeri} and
\emph{L.~macdonaldi} had a low CV of their \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values
(CV~\textless{}3) while \emph{S. koefoedi} had a wider dispersion of
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values (CV = 4.81).\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\emph{L. macdonaldi}\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
Myctophidae\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\emph{S. koefoedi}\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
Platytroctidae\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
Community\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
-\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\emph{Trophic \& and habitat-driven ontogenetic shifts}\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
The whole community showed an increase in individual sizes with depth. A
weak but significant relationship was also found between
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values and individual size.\strut
\end{minipage}\tabularnewline
\bottomrule
\end{longtable}

* Only the \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{15}N-size relationship was tested
for \emph{A. carbo} and \emph{S. boa} due to too low depth ranges
sampled for these two species (test with linear models only, not through
variance partitioning analyses).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure-2/Figure-2}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure-3/Figure-3}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure-4/Figure-4}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure-5/Figure-5}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure-6/Figure-6}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Appendix-3/Appendix-3}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Appendix-1/Appendix-1}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Appendix-2/Appendix-2}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure-1/Figure-1}

\end{center}
\end{figure}

\selectlanguage{english}
\FloatBarrier
\end{document}

