Species composition, distribution patterns and population structure of penaeid shrimps in Malindi-Ungwana Bay, Kenya, based on experimental bottom trawl surveys

Abstract

The species composition, distribution patterns and abundance of penaeid shrimps in Malindi-Ungwana Bay, Kenya, were investigated after six years of no bottom trawling in the area. Two surveys undertaken during the dry Northeast Monsoon (NEM) and wet Southeast Monsoon (SEM) seasons in 2011 identified areas with abundant shrimps near the outflows ofthe Sabaki and Tana rivers. Distinct species composition and abundance patterns were found at the two areas, attributed mainly to depth, turbidity and season. Penaeus semisulcatus was more abundant at the Sabaki area, where it was deeper with a muddy bottom and less turbid waters. Fenneropenaeus indicus was more abundant in the Tana area, a shallower, more turbid area with sandy-mud sediments. Penaeus monodon, Penaeus japonicus and Metapenaeus monoceros were found in both areas, suggesting wider tolerance to environmental conditions. Shrimp total biomass and catch rates were significantly greater during the SEM survey, and decreased as depth increased beyond 10 m. Small-sized M. monoceros and P. monodon individuals were abundant during the SEM survey, whereas large ones with ripe and spent gonads were more common during the NEM survey, suggesting that spawning took place between the two surveys. Seasonal patterns in gonad maturity were less clear for F. indicus and P. semisulcatus. The length at first maturity (L50) varied among species, suggesting that different species in the bay start spawning at different sizes, an important biological reference for sustainable resource exploitation. This study confirms the importance of the Sabaki and Tana areas as important habitats for penaeid shrimps in Malindi-Ungwana Bay.