Influence of Seasonality and Bathymetry on Assemblage Structure of Decapod Crustaceans in the Malindi-Ungwana Bay, Kenya

Decapod crustaceans support both artisanal and semi-industrial fisheries in the Western Indian Ocean (WIO) region. Despite their commercial value, data on assemblage structure is lacking in most of the WIO region but are important for stock management. This study provides information on seasonal and bathymetric variation in assemblage structure of decapod crustaceans in Malindi-Ungwana Bay, a biodiversity rich ecosystem in Kenya. Samples were collected during the northeast monsoon (NEM) and southeast monsoon (SEM) seasons during a two-week experimental bottom trawling survey in 2011 under the South West Indian Ocean Fisheries Project (SWIOFP). A total of 43 transects covering an estimated area of 546 nm2 were trawled in four depth zones (0-10, 10-20, 20-40 and 40-100 m) for 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 higher relative numerical abundance both in the NEM (89.3%) and SEM (85.3%) seasons. Of the penaeid shrimps, Fenneropenaeus indicus, had the highest relative abundance both in the NEM (58%) and SEM (42%) season. 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). Simple Correspondence Analyses segregated the population into SEM assemblages dominated by the Penaeidae and a mixed group NEM assemblage. Canonical Correspondence Analysis showed temperature, salinity, Secchi depth and dissolved oxygen to have seasonal influence on the bathymetric distribution of the crustaceans in the bay. The structure of the assemblages seems to be influenced more by depth profiles than by seasonality. It is recommended that variations in bathymetric distribution of the crustaceans be taken into consideration when developing a crustacean fishery management plan for the bay.