Discussion
We find that the numerical importance of perch in fish communities at the northern edge of its distribution increased substantially during the last three decades of rapid warming. The positive trend was registered in ten out of the eleven lakes investigated. For our main two study lakes, Lake Vaggatem and Lake Skrukkebukta, the trend was driven by an increment in perch density associated with the increase in water temperature. These two perch populations showed higher recruitment with warming, with the relative proportion of 1 year old fish in catches increasing significantly with an increment in mean summer water temperature. The demographic responses to warming were concomitant with individual level effects on somatic growth, which increased with temperature across most young age classes, resulting in earlier maturation. The latter is an indirect effect of warming mediated by increased temperature-dependent growth rate of juveniles, an eco-physiological response, and phenotypic plasticity in maturation schedules, a life history adaptation. The resulting earlier maturation and larger size at age of juveniles help explain the increased recruitment rates promoting perch population density at higher temperatures. Somatic growth displayed negative density-dependence, which may mask individual and demographic responses to warming in field studies.
During the recent period of warming, the increased numerical importance of perch was accompanied by positive temperature effects on juvenile growth in our two reference lakes. The yearly mean growth significantly increased with temperature across all investigated juvenile age classes, with the exception of the 1 year old age group. The cohort mean cumulative length increment from age 1 to 4 years increased by 8.5 mm (~12%) per degree centigrade increment in summer water temperature. Individual perch growth displayed substantial negative density-dependence, decreasing by 6.8 mm per 10 CPUE units increase in relative density, in line with earlier findings for this and other species (Byström & García-Berthou 1999, Olin et al. 2017). The observed positive effect of temperature on perch somatic growth was expected considering that in our lakes mean summer water temperature varied between 10-14 °C, which, although within the species temperature tolerance range (Karås & Thoresson 1992), is well below the optimum temperature for perch growth, estimated to be within 16-27°C (Hokanson 1977). Positive effects of higher summer water temperatures on perch growth rates have been described in regions where temperature variability is within the thermal tolerance range of the species (Jeppesen et al. 2012, Huss et al. 2019), where the increased size at age was maintained also in adult age classes owing to faster growth in young stages (Huss et al. 2019, Gårdmark et al. 2020). The projected future increase in ambient temperatures will increase metabolic demands (Huey & Kingsolver 2019), but at high latitudes, lake productivity mediated by catchment greening is expected to increase with warming (O’Beirne et al. 2017), and should ensure sufficient food availability to support growth (Kao et al. 2015). The documented and projected positive effects of warming on growth of perch living at its northern range edge affect its life history, demography, and ecological interactions.
The faster growth induced by warming resulted in earlier maturation of perch. The indirect effect of warming, estimated and summarized via a structural equation model, is substantial, with maturation age (A50) decreasing by 0.8 years per cm increase in juvenile length increment (from age 1 to 4 years). The adaptive plastic response in maturation age is dependent on the eco-physiological process of somatic growth, which in turn is affected by the ambient temperature and food availability (Ward et al. 2017). A reduction in maturation age as a consequence of increased growth has been documented repeatedly in fish (Reznick 1993, Haugen 2000). Furthermore, an increase in juvenile growth rate has been associated with a subsequent increase in reproductive output in fish (Ward et al. 2017), and an increase in reproductive output is seen as a direct effect of an increase in water temperature also for perch (Heibo et al. 2005). The climate driven increase in temperature-dependent growth thus results in larger size at age and earlier maturation, increasing the perch populations reproductive output and recruitment, thus promoting population growth, as seen in other stocks (Ward et al. 2017).
Survival of individuals during different life stages is a process which could be directly affected by ambient temperature either through temperature-specific developmental rates, temperature-dependent mortality rates or time spent in different life stages mediated by individual somatic growth (Sponaugle et al. 2006, Mirth et al. 2021). We show that the proportion of 1 year olds in the populations increased with summer water temperature during their 0+ summer. Increasing summer water temperatures has been shown to increase recruitment in cool-water fish (Kokkonen et al. 2019). The effect of increased summer temperature is usually related to larger body size and condition in autumn that subsequently lower the winter mortality (Hurst 2007, Estlander et al. 2017). Developmental rates increase with water temperature in ectotherms. For instance, mortality and the duration of the perch egg-stage are decreasing with temperature, with normal development of eggs occurring in the temperature range of 7-18 °C (Küttel et al. 2002). Therefore, an increase in summer water temperature might increase the number of surviving hatchlings as more eggs might develop normally and the shorter duration of the egg-stage might decrease the predation risk at this life stage. Embryos and hatchlings are defined as critical life stages with a narrow thermal range (Dahlke et al. 2020), and at the northern edge of perch distribution an increase in spring and summer water temperature might have been pivotal for an increase in recruitment and subsequent increase in density and relative importance of the perch populations.
Considering the rapid warming experienced in the study area during the last three decades, an increased numerical importance of a cool-water species could be expected (Ficke et al. 2007, Rolls et al. 2017), but a total shift in fish community dominance was surprising. The population process outlined above helps explain the sudden response to climate warming, and suggests that similar responses should be expected in other populations of cool-water species at their northern range edge, unless kept in check by negative ecological interactions. However, many of the lakes near the northern range edge of perch are salmonid dominated systems, with little resistance capacity against percid fish at higher temperatures (Hayden et al. 2014). Shifts in dominance from cold-water fish to more cool- or warm-water fish have been documented in other freshwater systems (Jeppesen et al. 2012, Hansen et al. 2017). Climate change impact on high latitude freshwater ecosystems is predicted to further increase as warming favours resident and invasive cool-water species, potentially displacing native cold-water salmonids from these ecosystems (Hayden et al. 2017, Hansen et al. 2017, Campana et al. 2020). Perch is a generalist fish that has high capacity as a resource competitor in littoral habitats, and subsequently also as a predator, for native cold-water species such as whitefish (Hayden et al. 2014). Such ecological interactions with resident cold water species, mediating higher order effects of climate change, may change in character and outcome under warming (Urban et al. 2011). Climate induced changes in size-structured interactions may have cascading effects within the food web, and the outcome is dependent on thermal niche, population size structure and the existing ecological interactions within the ecosystem (Gårdmark et al. 2020). Cold-water sympatric species will be more vulnerable as cool-water fish increase in competitive and predatory capacity with warming, possibly causing major alterations within fish communities in high latitude lakes. In a wider perspective, cool-water perch dominance may eventually shift towards warm-water cyprinid (roach, bleak) fish dominance along increasing temperature and productivity in lakes where cyprinids are present or able to immigrate (Hayden et al. 2017).