Spiny lobsters at Tristan da Cunha, South Atlantic: inter-island variations in growth and population structure

Abstract

The spiny lobster Jasus tristani is exploited at the isolated volcanic islands of Tristan da Cunha, Nightingale, Inaccessible and Gough in the south central Atlantic Ocean. Tagging studies indicated consistent inter-island differences in lobster growth rates and size composition which were unrelated to fishing intensity. A combined diving/trapfishing survey was conducted during March/April 1989 to investigate possible reasons for observed differences in lobster growth, size composition and sizes at maturity at Nightingale and Inaccessible, two small islands just south of Tristan da Cunha. Differences in water temperature were not considered to be relevant owing to the two islands' proximity to each other. Although it was thought that food availability to the lobster stocks might be the underlying cause for the inter-island differences, an intensive diving survey failed to reveal any obvious differences in the benthic biota of the two islands. Predation by fish on small lobsters is intense at all islands, but at Inaccessible the topography of the shallow subtidal zone (where most puerulus larvae settle) affords more shelter to young lobsters than at Nightingale. Survival of juvenile lobsters is thus relatively enhanced, resulting in higher densities of recruiting lobsters at Inaccessible, a feature confirmed by diving as well as analysis of trap catch per unit effort and size composition. The observed inter-island differences in lobster population structure therefore appear to be caused by differences in density-dependent growth and survival of young lobsters. The implications of these findings are discussed in relation to yield per recruit, egg production per recruit and the effects of minimum size regulations.