A stock assessment of lobster (Homarus gamarus) on the Norwegian Skagerrak coast

The lobster stock on the Norwegian Skagerrak coast has become gradually less abundant since early 1950'ies. This is shown by decreasing catch per unit of effort and decreasing numbers according to cohort analysis on length groups. A modified mesh assessment model is used to model the stock and the fishery. Predicted changes in yield per recruit are supported by tagging experiments. A relationship between spawning stock and subsequent recruitment is found and yield curves are constructed. The results indicate that the long term yield would be increased by about 200% if the present fishing effort is reduced by 50% and the minimum landing length is increased from 22 cm to 26 cm. Since only lobsters around the minimum size have been tagged, the experiments are unsuitable for stock assessment. They are, however, useful for estimation of growth and mortality in the length range of the tagged animals. The two models that are used in the present analysis are the cohort analysis on length (JONES 1974, 1979) and a modification of the mesh assessment model (HOYDAL, RORVIK and SPARRE 1980, 1983). The cohort by length model is used to estimate the development of the stock since 1949. The mesh-assessment model with the modifications is used to model the stock and the fishery and to predict the consequences of changing the minimum legal size and the effort. Both models involve the von Bertalanffy growth equation and the natural mortality M. Before applying the models these parameters have to be estimated