Abundance and biology of the langoustine Metanephrops mozambicus (Nephropidae) on deep-water trawl grounds in eastern South Africa

Abstract

Abundance trends, population structure, and biology of the langoustine Metanephrops mozambicus (Macpherson, 1990) were investigated, based on commercial logbook information (1988-2010; 49 990 trawls) and biological samples collected off eastern South Africa. A generalized linear model (GLM) framework was used to model variation in catch rates, carapace length (CL), sex ratio, and maturation size. Standardized catch rates were inversely related to fishing effort, gradually increasing between 2002 and 2010, when fishing effort was lower. Catch rates were greatest in trawls made at 300 to 599 m depth and between June and December. Langoustines ranged in size from 17.5 to 72.6 mm CL, and the mean CL decreased with increasing depth. Males predominated in November, but equal numbers of males and females were observed in other months. The smallest egg-bearing female had a CL of 33.5 mm, and L50 was estimated at 49.4 mm, based on the incidence of external eggs. Females with freshly spawned eggs were most abundant in August, and the incidence of egg-bearing remained high until March, where after it declined. Eggs about to hatch occurred mainly in May. Growth parameter estimates (L∞ and K) were 65.5 mm and 0.7 year−1 for sexes combined, based on modal progression and the standard von Bertalanffy growth function, and longevity was estimated to be 3-4 years. Combining analyses of fisheries data with biological samples yielded insights into spatio-temporal population trends, as well as the correlation of reproduction and moulting in M. mozambicus.