Who did it?: Assessing potential pressures on small pelagic fish populations of the NW Mediterranean Sea using a qualitative modelling approach

International Council for the Exploration of the Sea (ICES) - North Pacific Marine Science Organization (PICES) International symposium : Drivers of dynamics of small pelagic fish resources, 6-11 March 2017, Victoria, BC, CanadaSmall pelagic fishes (SPF) in the NW Mediterranean Sea are key elements of the marine food web and are dominant in terms of biomass and catches. However, important changes have been observed in recent decades: for example, declines in biomass and landings of European anchovy (Engraulis encrasicolus) and sardine (Sardina pilchardus), and an expansion of Round sardinella (Sardinella aurita). Fluctuations in populations of SPF have been linked to environmental changes that directly influence annual recruitment and growth. The role of climate change in affecting the composition of plankton has also been suggested to explain the recent declines, while other causes could be the recovery of predators, competition with other pelagic organisms that feed on zooplankton and can also prey on early life phases of SPF (e.g., jellyfish), interspecific competition for food, or impacts from over exploitation and fisheries mismanagement. To test the role of these potential pressures, we developed qualitative models of the NW Mediterranean pelagic food web. We used the methodology of sign directed graphs in combination with an analysis of Bayesian belief networks to compare alternative model structures and explore how SPF species, preys and predators abundance and life expectancy may have responded to different disturbances. Observations from the literature and available data were used to validate predicted directions of change. A combination of factors, including an increase of fishing impact, an increase of sea surface temperature and food-web interactions provide the most plausible insights into potential pressures on SPF in the study areaPeer Reviewe