Bycatch assessment of vulnerable megafauna in coastal artisanal fisheries in the southwest Indian Ocean
The incidental catch of marine megafauna, including marine mammals, sea turtles and elasmobranchs, poses one of the main threats to these species at the global scale. These taxa are particularly vulnerable for biological reasons, such as late maturity and low reproduction rates. Artisanal fisheries account for more than 95% of fishers in the world, especially in developing countries where artisanal fisheries are socially and economically most important. Their impact on vulnerable megafauna may thus be significant, either as bycatch or as target in artisanal fisheries. The purpose of this study is to assess bycatch and use of vulnerable megafauna (marine mammals, sea turtles and elasmobranchs) in the SWIO artisanal fisheries using interview surveys. This study focuses on areas where bycatch of vulnerable megafauna has been previously identified, suspected or least known as for the east coast of Africa (Mozambique, Tanzania and Kenya) and Mauritius. This study also provides recommendations for future research, management and mitigation of vulnerable megafauna bycatch in artisanal fisheries of the SWIO region. A total of 961 interview surveys was conducted in the region, including in Kenya (n=330), Tanzania (n=276), Mozambique (n=296) and Mauritius (n=59). Throughout the region, eight artisanal fisheries types were sampled: mono- and multifilament drift gillnets, bottom-set gillnets, beach seines, purse seines, longlines, lining under FADs (Fish Aggregating Devices) and handlines. These fisheries were considered to be the most likely involved in sea turtle, marine mammal and elasmobranch catches. A particular effort has been devoted to sample gillnet fisheries, previously documented as the major threat to large marine megafauna in the region. The major finding of this study is the high extent of large marine vertebrate bycatch in artisanal fisheries, especially in drift, bottom-set gillnets and beach seines. At least 59 species were identified as bycatch and by-product species, including five species of sea turtles, eight species of marine mammals and 46 species of elasmobranchs. The Ecological Risk Assessment emphasized that at least 17 species were particularly vulnerable to artisanal fisheries bycatch in the southwest Indian Ocean, including all species of sea turtles (loggerhead, green, hawksbill, olive Ridley and leatherback turtles), 4 species of marine mammals (dugong, Indo-Pacific bottlenose, humpback and spinner dolphins) and 8 species of elasmobranchs. Among elasmobranchs, highest risk was identified for Manta, spotted eagle rays, giant guitarfish and hammerhead sharks (including scalloped and great hammerheads). The risk was particularly enhanced in species with low productivity (low fecundity). Line fisheries (longline and handline) have a low impact on the survival of sea turtles and marine mammals. However, these fisheries have a significant impact on elasmobranchs. As suggested by interview survey data and PSA plots, there is a difference in the scale and effect of bycatch of vulnerable megafauna among different gear types. Bycatch levels were higher in multifilament than in monofilament drift gillnets, both for cetaceans (small delphinids in particular), sea turtles and elasmobranchs, and involved more species. Sea turtles (especially green, hawksbill, olive Ridley and loggerhead turtles), manta rays, hammerhead sharks and Indo-Pacific bottlenose dolphins were the most common bycatch species, as well as the most impacted by drift gillnets. Bycatch levels were lower for these species in bottom-set gillnets (but they were still high for several species), but affected a greater number of species, especially benthic and demersal species (especially coastal rays and reef sharks). However, the risk associated with bottom-set gillnets was lower for all species (due to lower susceptibility). Concerning one of the most threatened vulnerable species in the region, the dugong, bycatch events were rarely reported, in comparison to previous studies. This may be attributed to the current rarity of this species along the east African coast and a rapid decline since the early 2000s. Beach seines were also noted as highly impacting on sea turtles, especially for the green turtle, as this gear is frequently used very close to shore, over seagrass meadows, known foraging habitats for this species). Other species of sea turtles were also impacted, including hawksbill, olive Ridley and loggerhead turtles and, surprisingly, coastal marine mammals. Bycatch levels were particularly high for inshore Indo-Pacific bottlenose dolphins, especially in Mozambique. The effect of beach seines on more pelagic and oceanic species was low, such as spinner dolphins (rarely observed in inshore waters), Manta rays, great hammerhead sharks and leatherback turtles. Finally, handlines had the lowest impact on vulnerable megafauna, especially due to the low post capture mortality. Results collected in this study are consistent with previous local studies, undertaken in the southwest Indian Ocean, both in term of species involved and bycatch incidence. However, this is the first in-depth regional study of megafauna bycatch in artisanal fisheries in the southwest Indian Ocean and has clearly highlighted that an important diversity of large and vulnerable marine vertebrates are exposed to artisanal fisheries’ bycatch in the SWIO region. It is also clear that net fisheries should be the focus of future management initiatives. A priority should be given to drift gillnet fisheries, posing the greatest threat to marine mammals, sea turtles and large elasmobranchs in the region. Therefore, limiting the use of these nets should be encouraged. These limitations could be either spatial or temporal, and based on scientific information on habitat use of bycatch species. Recently, it has been shown that the establishment of marine protected areas (MPAs) can be effective for a number of taxa, including marine mammals, sharks and probably sea turtles. Initiatives to identify marine protected areas, where gillnets and beach seines may not be used, are strongly encouraged in the SWIO region.