TROPHIC ECOLOGY AND EXPLOITATION STATUS OF SHARK SPECIES (PISCES: ELASMOBRANCHII) IN THE NORTH COAST OF KENYA

Abstract

Sharks are apex predators in marine ecosystems whose trophic ecology is likely to influence community structure and function in their distribution range. Despite this role, data on the trophic ecology and exploitation status of most shark species is lacking especially from the Western Indian Ocean (WIO) region. This study gives information on the trophic ecology and exploitation of shark species from coastal Kenya in an effort to bridge this gap. Six shark species (Sphyrna lewini, Carcharhinus amblyrhynchos, Carcharhinus falciformis, Carcharhinus melanopterus, Carcharhinus leucas and Carcharhinus brachyurus) were collected from Kipini and Ziwayu on the north coast of Kenya from October, 2014 to November, 2015. The species were sampled from the landings and sexed, measured for total length, weighed and the stomachs examined for diet composition. Data were analyzed for size-frequency distributions and exploitation status, trophic positions, diet overlaps, feeding strategy, and niche breadths. A majority of the specimens landed were immature and below the size at maximum possible yield per recruit (Lopt). The results indicated the sharks fed mostly on fishes supporting a piscivorous habit found in many studies. However, in this study two species (C. amblyrhynchos and C. falciformis) appeared to supplement their diet with crustaceans. Interspecies overlaps in diet were high for all species (CH>0.6), indicative of competition for food resources. The shark species recorded high trophic levels ranging from 3.90-4.238 supporting their position as apex predators. Trophic level–body size relationships showed a high positive correlation for two species, C. melanopterus, R2=97.7% and C. falciformis, R2=96.0% and a negative but poor relationship for one species (C. amblyrhynchos, R2=37.9%) and for all species combined (R2= 28.3 %). The biomass of prey items consumed by the shark species for the period 2014/2015 was highest for fish at 1.2 MT suggesting potential interaction with the local fishery. It is recommended amongst others, to apply the precautionary approach in conserving the shark populations at Kipini on the north coast of Kenya by restricting the harvests by fishers through gear and seasonal control of fishing effort.