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.