Spatial distribution, seasonal abundance and exploitation status of shark species in Kenyan coastal waters

Abstract

Efforts to conserve and manage shark populations are often hampered by a lack of basic data, such as species-specific landings and distribution ranges. We bridge this gap in coastal East Africa by providing data on the distributions, catch rates, morphometrics, and exploitation status of shark species in Kenyan coastal waters. Data were collected from artisanal fishers and from bycatch taken by shallow-water (10–50 m) prawn trawlers from Malindi-Ungwana Bay and demersal research trawlers (10–150-m depth) along the ~640-km coastline, over a 12-month period (June 2012 to May 2013). A total of 1 893 individual sharks (representing 20 species and 11 families) were sampled from the artisanal fishery (n = 1 610) and the trawlers (n = 283). The demersal trawl bycatches were dominated by the African angelshark Squatina africana (2.39 kg h^–1), shortnose spurdog shark Squalus megalops (1.48 kg h^–1) and African spotted catshark Holohalaelurus punctatus (0.11 kg h^–1). Catches of the scalloped hammerhead shark Sphyrna lewini (0.73 kg h^–1), smooth hammerhead shark Sphyrna zygaena (0.60 kg h^–1) and grey reef shark Carcharhinus amblyrhynchos (0.77 kg h^–1) dominated in the prawn trawlers. Only a few species (S. lewini, C. amblyrhynchos, and blacktip reef shark Carcharhinus melanopterus) showed a coast-wide distribution in the artisanal fishery. Artisanal fishers harvested mostly immature specimens of S. lewini, C. melanopterus and blacktip shark Carcharhinus limbatus, suggesting that the fishery might be unsustainable in the long-term. The Endangered S. lewini is the most vulnerable to overexploitation on the Kenyan coast, with most specimens landed (>90%) being below the size at maturity. Data are also presented on morphometric relationships and observed or estimated exploitation reference points (maximum observed length Lmax, asymptotic length L∞, mean length at first maturity Lm, and optimum length Lopt) for the commonly landed species. A more comprehensive coast-wide National Plan of Action is recommended for the management of shark populations in Kenya.

Keywords: artisanal harvest, bycatch, conservation, growth overfishing, life-history traits, Sphyrna lewini, western Indian Ocean