Ecology of Munida gregaria (Decapoda, Anomura): distribution and abundance, population dynamics and fisheries

Pelagic larvae, postlarvae and benthic adults of the galatheid crab Munida gregaria (Fabricius 1793) occur along the continental shelf of the east coast of the South Island and around the subantarctic islands of New Zealand. In the south-eastern South Island, larvae appear in June or July and develop through 5 zoeal stages. As they age, the larvae accumulate inshore and north of the Otago Peninsula. Following metamorphosis in October, the pelagic postlarvae shoal through the summer prior to settlement to the bottom. The length of the shoaling period can vary considerably from year to year, ranging from a few weeks to 6 mo or longer. The pelagic postlarvae are very patchy in spatial distribution. Postlarval biomass, as determined by aerial surveys along the south-east coast, was highest along the inner to middle shelf from Blueskin Bay to Moeraki, immediately north of the Otago Peninsula. Benthic settlement was also heavier in this area relative to south of the Peninsula. This provides evidence that a meso-scale eddy interrupts the northward drift of larvae and postlarvae in the Southland Current and retains them near the upstream boundary of the benthic population. In the Otago Peninsula area substantial benthic recruitment occurred only when and where the density of older cohorts on the bottom was low. After relatively long shoaling periods the 1976–1978 cohorts settled on inner shelf sands and migrated to middle and outer shelf bryozoan-covered bottoms within a few months. In contrast, after relatively brief shoaling periods the 1979 and 1980 cohorts settled directly on the middle and outer shelf bryozoan area which was vacant due to high mortality in previously dense older cohorts. It is proposed that density dependent regulation of O+ cohort settlement by older cohorts occurs, possibly by a combination of cannibalism and non-lethal agonistic behaviour. In the absence of these effects, settlement occurs early, accounting for the short or non-existent shoaling seasons observed each 3 to 5 yr in a 30 yr record. There is presently consideration of a fishery for pelagic M. gregaria in New Zealand, primarily for use as food stock for cultured salmon. Because the density of newly settled O+ animals was nearly maximal by the end of January each year, pelagic animals remaining after that time could be fished without seriously affecting replenishment of the benthic stock. Available biomass would be highly variable and probably non-existent once each 3 to 5 yr. T