Vulnerability assessment of elasmobranch species to fisheries in coastal Kenya: Implications for conservation and management policies

Abstract

Ecological Risk Assessment (ERA) of species is useful for making informed management decisions based on relative vulnerability of species to fisheries. Understanding the vulnerability of species to fishing gears is important for targeted management measures especially for species known to have delicate life-history strategies such as the elasmobranchs. Productivity and Susceptibility Analysis (PSA), an ERA approach, was applied to 61 elasmobranch species (32 shark and 29 ray species) to assess their relative vulnerability (low, medium and high) to fishing by artisanal, prawn trawl and tuna longline fisheries in Kenya’s coastal waters. Overall, results showed 28 % of rays and 31 % of sharks were highly vulnerable to the fisheries, with the pelagic longline tuna fishery having the largest proportions of rays (50 %) and sharks (54 %) being highly vulnerable. Most of the species were in the medium vulnerability category. Among the artisanal gears, only gillnets had species that were highly vulnerable to the fishery, while relatively fewer species (n = 9) of both sharks and rays were highly vulnerable to the prawn trawlers compared to those in the medium vulnerability category (n = 25). Seventeen species classified as High Vulnerability Species Assemblage (HVSA), were assessed to be at high risk of overfishing by different gears, and are recommended for management as a complex. Of the HVSA group, two species; Pseudoginglymostoma brevicaudatum and Rhina ancylostoma are classified by the IUCN Red List of Threatened Species as Critically Endangered (CR), while another four; Rhinoptera jayakari, Mobula birostris, Stegostoma tigrinum and Carcharhinus plumbeus are Endangered (EN), with the rest being either Vulnerable (Gymnura poecilura, Pastinachus ater, Pateobatis jenkinsii, Taeniurops meyeni, Triaenodon obsesus, Carcharhinus falciformis, and Carcharhinus carcharias), Near Threatened (Maculabatis ambigua, Mustelus mosis, Prionace glauca) or classified as Data Deficient (Pliotrema warreni). Although majority of the species have been placed in the medium vulnerability category, some of them are those that are threatened with extinction globally, calling for a more precautionary management approach. A validation approach has been used to mitigate the problems of PSAs associated with data quality and uncertainty around estimated vulnerability indices. More robust application of PSA will require continuous updating of life-history attributes to improve on sensitivity. A precautionary Shark and Ray Management Plan (SRMP) that takes into account the outputs of the PSA is recommended for management and conservation of the elasmobranch stocks in Kenya’s marine waters.