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).