DISTRIBUTION, ABUNDANCE AND SOME GROWTH ASPECTS OF SHARK SPECIES (PISCES: CHONDRICHTHYES) ON THE KENYAN COAST

Abstract

In Kenya and most of the Western Indian Ocean (WIO) region, substantial amounts of shark landings occur as by-catch in artisanal fisheries, prawn trawls and longlines. However, the species structure, distribution, catch rates and levels of fisheries-shark interactions are not well studied. This information is, however, necessary to assess exploitation levels of shark species and for setting regulatory, conservation and management frameworks. This study therefore aimed at filling this information gap. Data were collected from fisher landings at various sites along the Kenya coast and by observers on commercial and scientific trawl surveys. Landings at six fish landing sites were inspected for sharks for 2-weeks in a month for 12 months (June 2012 to May 2013). Specimens were identified to species level and sex, length and weight recorded for each shark landed or trawled as by-catch. Results indicated that the catches are mainly dominated by hammerhead sharks (Sphyrna lewini, 53.7%), blacktip sharks (Carcharhinus limbatus, 33.7%), and grey reef sharks (Carcharhinus amblyrhynchos, 5.5%). Catch rates of species show spatial and seasonal variation, with higher catch rates in the middle coast (Malindi-Ungwana bay). Size-frequency distributions show mostly juveniles in the catches indicating fishers are probably exploiting nursery grounds. Fin weight was found to be 7.4% of body weight in S. lewini and 5.7% in C. limbatus. The sex ratios were significantly different in S. lewini (χ 2 = 36.62; df= 1; p= 0.00), C. limbatus (χ 2 = 7.03; df= 1; p= 0.008) and C. melanopterus (χ 2 = 34.77; df= 1; p= 0.00). In all three cases the female sex was dominant. The ratios for the other species assessed were close to unity. The study also provided length-weight and length-length relationships of the common shark species. Growth and mortality parameters are provided, for the first time, for the five common shark species in coastal Kenya. The results are discussed in relation to overfishing threats and behavioural ecology of the species. There is need to continuously monitor the distribution and abundance of sharks in Kenya and the WIO region for purposes of conservation, and an urgent need to set up a national plan of action to manage the shark stocks is recommended.