Who is to blame? Plausible pressures on small pelagic fish population changes in the northwestern Mediterranean Sea

Important changes have been observed in recent decades in small pelagic fish (SPF) populations of the NW Mediterranean Sea: declines in biomass and landings of European anchovy and sardine, and a geographical expansion of round sardinella. These changes have been linked to environmental factors directly influencing annual recruitment and growth. The role of climate change in affecting the composition of plankton has also been suggested to explain declines in SPF, while other causes could be the recovery of predators, competition with other pelagic organisms that prey on early life phases of SPF (i.e. gelatinous zooplankton), interspecific competition for food, or impacts from fisheries harvest. To test the role of these potential pressures, we developed qualitative mathematical models of a NW Mediterranean pelagic food web. We used analyses of sign directed graphs and Bayesian belief networks to compare alternative hypotheses about how SPF species may have responded to combinations of different pressures. Data documenting changes in SPF populations were used to test predicted directions of change from signed digraph models. An increase in sea surface temperature (SST) that had either a positive impact on round sardinella or on gelatinous zooplankton abundance was the pressure that alone provided the most plausible insights into observed changes. A combination of various pressures, including an increase in SST, an increase of exploitation and changes to zooplankton also delivered results matching current observations. Predators of SPF were identified as the most informative monitoring variable to discern between likely causes of perturbations to populations of SPF.