Publication Abstract

Estimating trapping areas from trap catch data: the influence of behaviour in lobsters and crabs

M.C. Bell, J.T. Addison and R.C.A. Bannister

Conversion of catch per effort data to density of an exploited species is fundamental to stock assessment. A vital component of this conversion for trap fisheries is the effective area fished by a trap. We develop a conceptual model of interactions between traps set in strings that potentially allows estimation of trapping areas from trap-by-trap catch data. The model predicts that catch rates should be highest in traps at the ends of strings, where interactions between traps are lowest. It is also predicted that trap saturation should reduce the effects of trap interactions, particularly at high density. Analysis of catch data for lobsters Homarus gammarus and crabs Cancer pagurus fished on the east coast of England offers some support for these predictions, although the catch data were ‘noisy’. Catch rates of crabs were 6% higher in traps at the ends of strings than in other traps. At low overall catch rates, catches of lobsters were 53% higher in traps at the ends of strings than in other traps. A dependence of end trap effect on catch rate was expected in lobsters because agonistic behaviour at the trap should cause trap saturation to be more important in this species than in crabs. Assuming circular trapping areas and a linear decline in capture probability with distance from the trap, and adjusting for an overall capture probability of 0.369, effective sampling area of crab traps is estimated as 642 m². No such estimate was possible for lobsters. An individual-based simulation model is developed to explore the effects on lobster and crab catches of trap spacing, density and agonistic behaviour at the trap. The model confirms the importance of trap saturation in determining the level of trap interactions.

Reference:

M.C. Bell, J.T. Addison and R.C.A. Bannister, 2001. Estimating trapping areas from trap catch data: the influence of behaviour in lobsters and crabs. Marine and Freshwater Research, 52(8): 1233-1242 .