Discussion
Our study supports the hypothesis that temperature is a main driver of
large-scale latitudinal variation in 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
community 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.
We find no evidence that temperature fluctuations and recent warming
have impacted demersal community biomass. Even though we found no effect
of recent warming, our study provides an empirical basis for long-term
climate predictions and suggests a set of explanatory variables that are
most important.
The lack of a relationship between demersal fish biomass and the
detrital bottom flux and the positive but weak relationship between
zooplankton prey production and demersal fish biomass in the SEM were
unexpected, as prey production should ultimately constrain the energy
available to fish. From a trophodynamic perspective, the weak
relationship between prey production and biomass and the strong
relationship with temperature suggests that temperature-modulated
impacts on fish turnover rates and/or trophic transfer efficiencies are
more important than the baseline prey resources in determining demersal
fish biomass, at least for the range of systems and scales considered
here. This finding is supported by the trophodynamic model, which
required a strong negative relationship between the transfer efficiency
and temperature to obtain skillful demersal fish biomass predictions.
In retrospect, the weak relationship with prey production is not too
surprising as the data compilation covers a considerable thermal range
(-1 to 27 °C), while the studied shelf systems have moderate to high
productivity and productivity varies less than a factor 4 (Figure 1d).
It is thus expected that prey production could become more important for
predicting changes in demersal fish biomass in both the SEM and the
trophodynamic model along a broader productivity gradient. For example,
a gradient from the shelf to the deep ocean that covers larger
differences in benthic prey production (Wei et al. 2011).
It is important to note that the negative relationship between demersal
fish biomass and temperature does not necessarily imply that the
potential sustainable fishing catch will be lower in warmer shelf
systems. Catch is a flux (biomass removed per unit time) similar to
production and is differently affected by temperature compared to
biomass. Increased biomass turnover times at higher temperatures, for
example, decreases the biomass associated with a given production after
warming, e.g., du Pontavice et al . (2021). In contrast with the
demersal fish biomass results herein, estimates of plankton food web
production available to fish can provide moderately skillful fisheries
catch predictions at a global scale (Friedland et al. 2012; Stocket al. 2017). Similar to the demersal fish biomass results
herein, a strong negative dependence between the transfer efficiency and
temperature significantly improved fisheries catch estimates (Stocket al. 2017).