Environment or catches? Assessment of the decline in blackspot seabream (Pagellus bogaraveo) abundance in the Strait of Gibraltar

Abstract

In this study we carried out an analysis of the relationship between the variation in environmental and oceanographic parameters and the changes in the abundance of blackspot sea bream (Pagellus bogaraveo) in the Strait of Gibraltar between 1983 and 2016. A discrete dynamic model was applied to simulate different scenarios of blackspot sea bream biomass on a monthly time scale. ARIMA models were applied to the simulated biomass time series in order to extract the effect of the fishing component and then correlate the residual series with each of the environmental and oceanic parameter sets. Regardless of the scenario analysed, the error terms provided by the ARIMA models indicated a strong auto-regression dependence on the series and that the fishing component accounted in the worst case scenario for 73% of the variation in sea bream biomass in the Strait of Gibraltar. Throughout the study period, part of the variation not explained by the fishing component was explained in certain years by environmental parameters. These correlations were particularly significant for the water temperature anomaly, salinity anomaly and the NAO index. Our analyses therefore suggest that the main factor responsible for the drastic decline in the abundance of sea bream in the Strait of Gibraltar is over-exploitation of the resource and that environmental conditions had a one-off effect which, depending on the year, favoured or harmed the recovery of the stock.