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).