Genetic Diversity And Population Abundance Of The Wild Penaeid Shrimps Of Malindi–Ungwana Bay In Kenya: Implications For Aquaculture And Conservation

Abstract

Penaeid shrimps have recently become one of the most valuable fishery resources in Malindi–Ungwana Bay in the north coast of Kenya that resulted to the adoption of a Prawn Fisheries Management Plan 2010. The genetic diversity and population abundance of penaeid shrimps were studied based on morphometric length–weight relationships during monsoon seasons in 2011 from Malindi–Ungwana Bay with southeast monsoon (SEM) being cooler than northeast monsoon (NEM). A total of 1364 penaeid shrimps were randomly collected from the four main artisanal fishing areas in the estuaries and two other areas within the shallow waters trawling grounds. In the estuaries, sampling was done using a 25.4 mm stretched mesh size seine net, while in the shallow waters' trawling grounds, a 70 mm mesh size towing net was used onboard the trawler. Fenneropenaeus indicus was the most abundant species with a relative proportion of 30.8%, followed by Metapenaeus monoceros (24.9%), Penaeus semisulcatus (15.2%), Marsupenaeus japonicus (10.3%), Penaeus monodon (9.7%) and Penaeus canaliculatus (0.9%). The results of this study showed higher shrimp abundance during SEM compared to NEM season despite its proportional decrease with increasing water depths. The overall combined sex ratio deviated from the expected ratio of 1:1 (p < 0.05) with females dominating the total catch (1 male: 1.50 female), while the length–weight relationship of most penaeid species exhibited positive allometric growth significantly different from 3.0 (p < 0.05) with strong relationship between lengths and weights of these species. The carapace length of six penaeid shrimps at first maturity (L50) suggested that their spawning starts at different sizes with P. monodon achieving L50 at the largest size and M. monoceros at the smallest size. The results of this study further showed xv several penaeid shrimps displayed intraspecific morphological variability where shape differentiation increased with size. Following good condition factors and high abundance of M. monoceros and F. indicus with matured gonads in both seasons, this study confirmed the two species as resilient and suitable for shrimp mariculture breeding programmes in Kenya. Generally, high variability among individual shrimps within each population indicated the presence of high genetic diversity which has given rise to genetically panmictic populations in Malindi–Ungwana Bay. However, the population of P. canaliculatus in the Malindi–Ungwana Bay is considered as unique evolutionary taxa for conservation. This study further recommends conservation measures to prevent growth overfishing in the estuarine area. Key words: abundance; condition factor; genetic diversity; morphometric variation; penaeid shrimp; phenotypic plasticity; population; seasonal; sex ratio; variability