Density-Dependent Mortality and the Relationship between Size Composition and Fishing Effort in Lobster Populations

Abstract

Examination of time series data from lobster fisheries shows that although in some cases fishing mortality (estimated from size composition data) is directly related to fishing effort, that of many stocks remains relatively unchanged despite changes in the pattern of exploitation. This suggests that conventional stock assessment models used for finfish populations may not be adequate for lobster populations. A model is described which incorporates into a standard yield per recruit model density-dependent mortality in large lobsters due to limited availability of suitable holes in the substrate. The model predicts that size composition may change little with changes in fishing effort, and it is shown that current stock assessment methods may fail to detect those changes. The underlying assumptions of current methods are undermined further by the possible delay of moulting of lobsters that cannot find shelters.