Time series modelling of landings in Northwest Mediterranean Sea

Univariate seasonal ARIMA and intervention models were developed to forecast monthly catches of 53 commercial species in the northwestern Mediterranean Sea, up to one year in advance. In general terms, there was good agreement between forecasts and observed catches of target demersal species. By contrast, models fitted to non-target demersal species and pelagic species were unsatisfactory in terms of explained variability and predicting power. Large commercial size classes were better explained than the small size classes. Intervention analysis was used to identify the significance, magnitude and form of structural shifts (interventions) of the time series for each species. Most of the fitted interventions appeared in small commercial size classes and indicated a decrease in the mean level of the catch. Seasonality of demersal species, caught mainly by trawlers, was mainly attributed to changing availability and recruitment. By contrast, gear type explained the seasonal variation in landings of pelagic and some coastal species. Catch declines of two species were also compared with the variations of their respective seasonal patterns. (C) 2000 International Council for the Exploration of the Sea.