Search

Galicia is a maritime area and despite the economic crisis, its fishing fleets are among the most important in Europe. The Spanish and Portuguese longliners land blue shark (Prionace glauca) and shortfin mako (Isurus oxyrinchus) in the fish market at Vigo. They are the only fleets targeting sharks in Europe. Over the past two years, landings of shortfin mako have decreased, and those of blue shark increased, while the number of active longliners remained relatively constant. The sharks landed in a fish market are mainly juvenile, the maturity age being late for both species. Feeding strategies were studied for blue shark and shortfin mako on the basis of stomach content and stable isotope analyses. The high number of identified prey species confirms the generalist and opportunistic diet of these sharks. Results showed that cephalopods were the main prey for blue shark, while teleosts, especially Atlantic saury (Scomberesox saurus), were the main prey for shortfin mako. Their top-predator status in the marine pelagic ecosystem was confirmed by their estimated trophic levels from (1) stomach content analyses (4.8 for blue shark and 5.0 for shortfin mako), and (2) stable isotope ratios (5.2 and 5.3, respectively). The stable isotope ratios showed some incoherence in fractioning between sharks and their prey. This may be explained by their opportunistic diet and the migratory behaviour of sharks, associated with their metabolism. The mercury mean level in blue shark and shortfin mako muscles was below the maximum limit allowed by the European Union (1 mg/kg fresh meat). However, large sharks showed higher mercury levels. Shark meat was considered unfit for human consumption for blue shark over 248 cm length and for shortfin mako over 178 cm. Some tests were undertaken under real longline fishing conditions to investigate ways to reduce shark catches. The results showed that electromagnetic waves were unefficient for reducing catches of blue shark. In contrast, the rates of catch per unit of effort (where the unit of effort was the number of hooks) were higher for the hooks with magnets than for the control hooks. The results of this PhD thesis represent new data on the biology, ecology and conservation of these overexploited species. Their stocks have sometimes decreased so much that they are no longer able to accomplish their top-predator role in many marine pelagic ecosystems. [ABSTRACT FROM AUTHOR]