ASSEMBLAGE STRUCTURE OF DECAPOD CRUSTACEANS IN THE MALINDI-UNGWANA BAY, KENYA.

Abstract

Decapod crustaceans support both the artisanal and semi-industrial fisheries in Kenya and the Western Indian Ocean (WIO) region. Despite their commercial value, data on their assemblage structure is lacking in most of the WIO region but the data are important for stock management. This study aimed at bridging the data gaps by providing information on the seasonal variation in assemblage structure of decapod crustaceans in Malindi-Ungwana Bay, Kenya. Samples were collected during the northeast monsoon (NEM) and southeast mosoon (SEM) seasons in a two-week experimental bottom trawling survey under the South West Indian Ocean Fisheries Project (SWIOFP). Samples were collected during NEM and SEM seasons between 22 nd January to 4th February 2011 and 22nd May to 4th June 2011, respectively. A total of 43 transects covering an estimated area of 546.4 nm2 were trawled in four depth zones (0-10 m, 10-20 m, 20-40 m and 40-100 m) in both seasons. Twenty species of decapod crustaceans belonging to 7 families were sampled in both seasons. The species were distributed in the families; Penaeidae, Portunidae, Calappidae, Majidae, Matutidae, Palinuridae and Scyllaridae. The penaeid shrimps had a higher relative numerical abundance both in the NEM and SEM seasons of 89.3 and 85.3 %, respectively. Of the penaeid shrimps, Fenneropenaeus indicus, had the highest relative abundance of 57.6% during NEM and 41.5% during SEM season. Sex ratios of the penaeid shrimps were skewed towards females in the depth zones 2 (10-20 m) and 3 (20-40 m). Analysis of Similarity (ANOSIM) test indicated significant difference in total crustacean abundance (individuals/km2) between the depth zones, (R=0.375; P=0.001; considering all seasons) but no significant difference between the seasons (R= -0.031; P=0.602; considering all depths). The mean species richness in the bay was higher during SEM than NEM season for all depth zones. ANOVA indicated significant effect of depth (F=3.4773; df=2, 29; P=0.044) but not season (F=0.5155; df=1, 29; P=0.479) on species diversity. The crustacean assemblage structure in the bay was more influenced by depth profiles than seasonality. The shrimps, F. indicus, Penaeus monodon and the crab, Portunus sanguinolentus were mostly associated with depth zone 1(0-10 m), while the shrimps Metapenaeus monoceros, Penaeus japonius and the crab Ashtoret lunaris were closely associated with depth zone 2 (10-20 m). There was no clear species association with depth zone 3 and 4. Canonical Correspondence Analysis (CCA) indicated the influence of temperature, salinity, Secchi depth and dissolved oxygen in the bathymetric distribution of the crustaceans in the bay. It is recommended that seasonal distribution of the crustaceans be taken into consideration when developing crustacean fishery management plans for the bay. Additionally, surveys in the bay should examine annual changes on assemblage structure in addition to biomass changes for species.