Biology, fisheries and distribution of Thryssa vitrirostris (Gilchrist & Thompson 1908) and other Engraulidae along the coast of the Sofala Bank, western Indian Ocean

Abstract

The Sofala Bank (16°–21° S) forms a major discontinuity in the gradients of engraulid distribution along the south-eastern African coast. Although the number of species declines southwards, the density and the catches of species associated with higher turbidity increases within the bank, particularly for Thryssa vitrirostris and a few other Thryssa and Stolephorus species. Thryssa vitrirostris forms a single demographic population on the Sofala Bank, as judged from spatial and seasonal catch-rate data from the beach-seine fishery, the largest fishery in Mozambique. The core of the exploitable population seems to migrate towards the coast at the onset of the rainy season (November–February), and then northwards. Our results indicate that reproduction is timed with the rain regime, and length frequency data show that this is also the onset of a fast (L ∞ = 19.0 cm, k = 0.66 y−1) and highly seasonal (C = 0.86) growth. The population is largely comprised of fish up to 2 years of age and the fisheries target mostly pre-spawners (L 50 = 13 cm). The decline in standing stocks offshore, and in the catches of this species by beach-seines, may be related to an excessive exploitation rate (40–60%) brought about by the directed fishery and trawl bycatch.