Energy requirements for a dense population of rock lobsters Jasus lalandii: novel importance of unorthodox food sources

Abstract

ABSTRACT: Standing stocks of South African rock lobster Jasus lalandii vary considerably from site to site. In this paper 2 adjacent island sites are compared. One supports an extremely dense population of rock lobsters with an estimated biomass of some 3.9kg wet weight m-' and a calculated energetic requirement of 36 X lo3 kJ m-' yr-l, while the other has almost no rock lobsters. A key question is how the dense population can be sustained by local benthic macrofaunal food sources which have a calculated annual productivity of only one-tenth of the rock lobsters' requirement. Macrofaunal standing stock proved an insufficient basis for estimating potential food production because it does not take into account recruitment of organisms, particularly barnacles, consumed shortly after settlement. Productivity of barnacle recruitment was measured at 25 X 103 kJ m-' yr-l, 70 % of the calculated annual energetic requirements of the rock lobsters. For much of the year the gut contents of rock lobsters consisted predominantly of barnacle recruits. When barnacle recruitment declined m winter, the rock lobsters turned to other food, including another unexpected source, planktonic mysids. Rock lobsters can clearly use food sources that have until now been ignored by investigators. The rapid turnover of recruits, often undetected in surveys of benthic biomass, is important not only in the present case, but also in the broader context of energy flow through communities.