Investigating time and space variation in catches of lobster (Homarus gammarus L.) in a local fishery on the east coast of England
In 1989, data were collected to investigate seasonal and spatial variation in the number and size distribution of lobsters (Homarus gammarus L.) caught by traps in a commercial fishery centred at Bridlington on the east coast of England. A quayside data set comprised the size distribution of legal-sized lobsters in 289 landings from 25 vessels between May and September. A sea-based data set comprised the size distribution of sub-legal and legal-sized lobsters recorded on 482 occasions in the same period when fleets of traps were hauled at sea at known locations by a selected vessel. Male, female, and egg-bearing (berried) female lobsters were distinguished, and the size distri?butions grouped prior to performing a principal component analysis (PCA) on each data set. The first four principal components (PC) explained 71% and 86% of the variance in the quayside and sea-based data respectively, and were identified as representing catch rate (PCI), size (PC2), and the proportion of small and large berried (PC3 and 4, quayside data) or sub-legal and legal-sized berried (PC3 and 4, sea-based data) lobsters. Analysis of the PC scores showed that catch rate and size distribution, though variable, were spatially homogeneous throughout the inshore fishing grounds, but landings of large lobsters from offshore grounds were apparent as outlying points. In the sea-based analysis, the PCA separated groups of points representing hauls with no berried females, small berried females, and large berried females, but the geographical distribution of these hauls was not different. Small berried females showed a seasonal reduction in number in the early summer moulting period, but large berried females were present throughout. Large berried females correlated more strongly with large males than with non-berried females. Overall, results suggest that only inshore and offshore sampling strata need be considered separately in the future.