Introduction
Climate change affects marine
ecosystems through multiple drivers, including changes in ocean
productivity and temperature (Kwiatkowski et al. 2020). These
changes are expected to alter fish distributions and abundances and
eventually impact the structure and functioning of marine ecosystems, as
well as their associated services for human wellbeing (Lotze et
al. 2019; Petrik et al. 2020; Tittensor et al. 2021). To
anticipate and adapt to the ecological consequences of climate change,
it is therefore important to better understand and predict how changes
in ocean productivity and temperature jointly affect fish production and
biomass.
Current model predictions of climate impacts on fish often rely upon
basic ecological theories of how energy flows from primary producers to
top predators, as well as metabolic scaling of individual vital rates
with temperature. Specifically, warmer temperatures are expected to
accelerate most physiological rates, e.g. maximum consumption rate and
metabolic rate, and, consequently, the turnover rate of biomass
(Gillooly et al. 2001; Brown et al. 2004). The increase in
metabolic rate with temperature is further expected to increase the
fraction of energy that is lost through respiration. Consequently, the
increasing metabolic costs constrain the amount of energy that flows
towards the upper trophic levels of food webs by lowering the efficiency
by which primary production is converted into fish biomass (Eddyet al. 2021).
The effect of temperature on the bioenergetics at least partly underlies
projected trophic amplification of productivity, whereby fractional
changes in primary production are amplified up through the trophic
levels (Lotze et al. 2019). Since marine fish dominate the upper
trophic levels of ocean food webs worldwide (Hatton et al. 2022),
it can further be expected that the effects of temperature on both
turnover rate and trophic transfer efficiency will drive, at least in
part, large-scale latitudinal variation in total fish community biomass.
More specifically, it can be hypothesized that fish community biomass
should increase from the tropics to the poles due to a lower turnover
rate and more efficient energy transfer in cold-water environments. This
hypothesis is endorsed by theoretical and laboratory studies (O’Connoret al. 2009; Guiet et al. 2020) and by some empirical
studies demonstrating negative relationship between temperature and fish
community biomass (Maureaud et al. 2019). However, empirical
support based on large-scale observational studies across a pronounced
temperature gradient is lacking.
There are several potential reasons why such macroecological patterns in
fish biomass have not yet been documented. Firstly, fish communities
worldwide have been exposed to long-term commercial fishing that changes
total community biomass, as well as the underlying size- and trophic
structure of fish communities (Rice & Gislason 1996; Myers & Worm
2003; Andersen 2019). Consequently, the exploitation history may mask
potential temperature effects. Secondly, energy flows from primary
producers to fish may be context- or scale-dependent, especially since
some regional variations in energy flows may themselves be driven by
temperature. Notably, warmer regions may have increased stratification
and remineralization of detritus in the water column (Pomeroy & Deibel
1986; Laufkötter et al. 2017), which increases pelagic
production, but lowers the detritus flux reaching the seafloor. This in
turn limits the energy available to support benthic prey production and
the biomass of bottom-feeding (demersal) fish (van Denderen et
al. 2018). Lastly, most previous studies focused on the more easily
estimated community catch rather than the more difficult to measure
community biomass (Friedland et al. 2012; Stock et al.2017). Most fish data collection of biomass primarily serves to monitor
trends and fluctuations in population-level abundances (especially of
commercially important species for fisheries management purposes), while
less attention is given towards representing overall community
composition and biomass (but see, for example, Maureaud et al.(2019) and Gislason et al. (2020)). Taken together, data
limitations and the inter-dependencies between predictor variables may
have complicated detecting overall relationships between ocean
productivity, temperature and fish community biomass.
In this study, we perform a large-scale empirical investigation of the
macroecological patterns and drivers of fish community biomass using an
extensive collection of scientific bottom-trawl surveys sampled across
pronounced temperature gradients in the North Atlantic and Northeast
Pacific. The studied continental shelf regions account for about 15% of
global fisheries catch (Watson 2017). We find that temperature is a main
driver of large-scale latitudinal variation in demersal fish community
biomass. This result is likely driven by a reduced trophic transfer
efficiency and a faster turnover rate of fish biomass in warmer waters.
As expected, demersal fish biomass is negatively related to fishing
exploitation and positively related to zooplankton prey production. All
these findings are consistently observed across the different spatial
scales studied.