Common trends in two populations of the European lobster, Homarus gammarus, in Scottish waters

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Dynamic factor analysis (DFA), a multivariate time series analysis technique recently applied to fisheries data for the first time, is used to explore common trends, and the effects of sea surface temperature (SST) and wind speed (WS) on short nonstationary multivariate time series of catch rates of the European lobster in Scottish fisheries. Catch rates of undersized and legal lobsters at three locations in the Hebrides and two locations in the Southeast of Scotland, were generally highest in autumn (August-October) with strong seasonal trends during the period 1990-1997, and therefore, de-seasonalised time series were analysed. From the DFA analysis, significant relationships between catch rates (undersized lobsters in one location and legal lobsters in two locations) and SST were observed for the Hebrides. At all locations in the Southeast, catch rates (both undersized and legal size lobsters) were related to SST. The results suggest, by observations on the most important trends, that (i) catch rates in the Hebrides have been steadily decreasing, (ii) those in the Southeast have been increasing during the studied period. DFA identified two common trends not revealed by traditional approaches, and proved to be a robust technique in the study of multivariate time series for the European lobster fishery.