Introduction
High latitude regions are experiencing the most rapid warming on Earth, a trend projected to continue towards year 2100 (Parmesan 2006, IPCC 2013). Rapid warming strongly affects freshwater ecosystems (O’Reilly et al. 2015), leading to changes in species abundance, phenology and distribution (Hickling et al. 2006, Comte et al. 2013, Campana et al. 2020). Temperature driven changes in physiological rates of ectotherms are expected to trigger a diverse array of ecological responses (Biro et al. 2007, Huss et al. 2019, Rezende et al. 2019), with implications for ecosystem structure and function (Benateau et al. 2019). Fish are strongly influenced by ambient temperature, with species displaying distinctive thermal niches (Hayden et al. 2014). Climate warming tends to favor fish populations currently experiencing the cold end of their species thermal range, typically in proximity of the northern limits of a species distribution (Ficke et al. 2007, Campana et al. 2020). As temperature increases, these populations are likely to outperform competing species of colder temperature guilds (Hein et al. 2014, Hayden et al. 2017). Evidence in support of, or against, these expectations is presently lacking due to a paucity of long-term ecological studies of freshwater fish communities in the rapidly warming Arctic (Amundsen et al. 2019, Zubova et al. 2020).
The impact of increasing temperatures on fish populations is mediated by direct ecophysiological effects and indirect life history responses that ultimately affect survival and reproduction. In ectotherms, temperature limits biological rates, affecting for instance food intake and metabolism and their balance determining the net energy gain of an organism (Jobling 2002). Growth rate will therefore depend on ambient temperature, with maximum growth being reached at an intermediate, optimum temperature within the thermal niche of a species (Gvoždík 2018). The growth rate of individuals living at temperatures below their optimum might increase with climate warming, given sufficient food availability (Huss et al. 2019, Smalås et al. 2020). Higher juvenile growth rates lead to larger size at age and earlier maturation. Larger size at age might increase survival, especially during early life stages, because mortality in fish is largely size-dependent (Hurst 2007, Perez & Munch 2010). Thus, faster growth increases the probability of reaching maturity, which is further enhanced by earlier maturation, overall resulting in higher transition rates to the adult, reproductive stage. Recruitment rates can be further enhanced by faster somatic growth as young adult females become larger, thereby producing larger clutches, thus climate warming is likely to boost population fecundity (Heibo et al. 2005). In addition, some critical life stages, in particular the egg and larvae, often have a narrower temperature range for survival and successful development than other life stages (Dahlke et al. 2020). Populations living close to the northern end of their distribution, might in colder years suffer from temperatures that are too low for successful development, especially in critical periods such as survival over the first winter, and should therefore benefit from climate warming (Hurst 2007, Dahlke et al. 2020).
The effects of climate warming on high latitude lakes go beyond increasing water temperatures, and predicted changes in the aquatic environments such as increased productivity, decreased dissolved oxygen levels and altered seasonality, are likely to favor cool-water species more than cold-water fish (Ficke et al. 2007, Rolls et al. 2017). Increase in temperature and productivity will expectedly first favor percids, and later cyprinids, over salmonids (Hayden et al. 2017). These cool-water species have been shown to redistribute northwards and to higher altitudes over the last few decades of rapid warming (Comte et al. 2013, Hayden et al. 2014, Rolls et al. 2017). Therefore, it exists an urge to document changes, but also to understand the mechanisms behind climate driven changes in high latitude fish communities in order to develop climate adaptation strategies that mitigate the possible eradication or displacement of cold-water species in the Arctic. One cool-water species moving northwards is the European perch (Perca fluviatilis ), hereafter perch, (Hayden et al. 2014), it has its northern range edge in subarctic regions of Eurasia around 70°N, but with a wide temperature range for growth, between 5-33°C, and an optimum between 16-27 °C (Hokanson 1977, Karås 1990) (more detailed information in Supplementary information).
Here, we address the effects of climate warming on perch populations at the northern end of the species distribution, using long-term surveys of high latitude freshwater fish communities (68-70°N). As a cool-water adapted species, we expect perch to benefit from increasing temperatures and a prolonged productive season, leading to increased abundance and importance relative to cold-water adapted fish co-inhabiting the sampled lakes. Several mechanisms underlie our expectation of an increase in the relative importance of perch following the recent rapid warming. Specifically, we focus on the life history and ecological implications of temperature-induced increase in somatic growth rate, anticipating that higher growth rates i) reduce the duration of critical life stages, ii) increase size at age, and iii) anticipate maturation age; overall improving survival and increasing recruitment rates and total population fecundity (see Supplementary information, Fig. S2 for schematic representation of mechanisms).