The crescent loss of oxygen in the oceans caused by climate emergency global compresses blue sharks’ habitat and increases their vulnerability by forcing them to swim in shallower waters and, therefore, with a higher risk of being caught by fishing boats. Gonzalo Mucientes, an expert from the Instituto de Investigaciones Marinas-CSIC, participates in an international study whose results have just been published in the journal eLife and during which a total of 55 specimens in North Atlantic waters were marked for monitoring.
The oxygen-deficient areas (ZMO) are expanding horizontally and vertically due to climate change and this team made up of experts from various institutions in Portugal and the United Kingdom, led by Nuno Queiroz Y David W. Sims, has determined for the first time how this phenomenon can interact with fisheries and affect threatened pelagic sharks.
The research was carried out in an OMZ in the eastern tropical Atlantic, south of Cape Verde and off the African continent, where deoxygenation is particularly intense. “We conclude that sharks dive shallower than those outside. This is due to entering these poorly oxygenated areas causes them physiological stress. Furthermore, in a context of climate change, it is predicted that these areas will expand, forcing sharks to swim in shallower waters and, therefore, be more susceptible to fishing by longliners ”, explains Mucientes, who During the development of the research, it was also part of the Center for Research on Biodiversity and Genetic Resources (Cibio) in Porto.
The blue shark or blue shark (Prionace glauca) it is a highly migratory species and the most caught pelagic shark in the world. The Red List of the International Union for Conservation of Nature classifies it as “near threatened” globally.
The 55 copies of the study were caught with longlines at different ocean locations and tagged with satellite transmitters to know their movements inside and outside the ZMO zones between the years 2009 and 2017.
The scientists also used official monitoring data from 322 Spanish and Portuguese longliners that operated in the Atlantic from January 2003 to December 2011 to find out the effort and intensity of fishing. And they also took into account the blue shark catch data from the notebooks of the Spanish fleet.
Today, Galicia leads the fishing of this species among European fleets and most of the 120 vessels are concentrated in the ports of Vigo and A Guarda.
The GPS tracking of the longliners showed that there was more fishing activity on the OMZs than in the adjacent areas and that the blue sharks they moved to surface waters to avoid oxygen deficiency. If the maximum depth of immersion in the adjacent areas was 500-1,400 meters, in the OMZs the marked specimens did not exceed 200-520 meters. And 91% of the time they stayed between 250 meters and the surface.
In light of this understanding of their habitat and considering that the blue shark represents 90% of the total reported catches of pelagic sharks in the Atlantic, the authors defend the taking of management and protection measures. For example, the definition of large marine protected areas in the high seas around OMZs, as well as more effective catch control measures to conserve populations of this species.
“It is possible that other pelagic sharks may show a similar behavior, although it also depends on their physiological response to the lack of oxygen. In the case of shortfin mako, which has a different metabolism than the blue shark, maintaining an internal temperature higher than that of the environment that surrounds it, we are working with new data in ZMO areas and we hope to have results very soon “, reveals Mucientes, who also comments on the Need to carry out specific satellite tagging studies with other species such as shark, hammerhead sharks and foxes, “many of them regulated or protected but vulnerable to accidental fishing”.
Mako conservation requires additional measures
Gonzalo Mucientes, David W. Sims, from the University of Southampton, and Nuno Queiroz, from Cibio, signed a joint article in the January issue of Science, in the section of Letters, on the situation of the porbeagle (Isurus oxyrinchus) as a result of the measures taken by ICAT, the international organization in charge of regulating fisheries for migratory species in international waters of the Atlantic.
Spain is the country with the most catches within the EU followed by Portugal. “The latest evaluation of stock estimated that the North Atlantic stock could recover by 2070 with a 60% probability if the allowable catch was less than 300 tonnes. However, the catches in the last year were still very high, multiplying that figure by 6. An annual mortality of 1,100 tons, already well exceeded in 2019, offers only an 8% chance of recovery by 2070 ”, summarizes Mucientes.
“An annual mortality of 1,100 tons, already well exceeded in 2019, offers only an 8% chance of recovery by 2070”
In 2019, porbeagle were included in Appendix II of CITES (Convention on International Trade in Endangered Species of Wild Fauna and Flora), regulating the trade of the species.
“But in our letter we pointed out that additional measures are necessary such as marine protected areas in the high seas that can conserve hot spots for these sharks. Since, even without retention, reducing catch and post-release mortality is important, ”says Mucientes.
“The reality is that the measures are late for the Atlantic. If regulations of the fishing activity had been carried out (space-time capture limitations, protection of immature areas and / or pregnant females, minimum sizes, etc.) as soon as the first signs of possible overfishing were observed, these measures could have been much less drastic and effective. Long-term sustainability would have been ensured and probably prevented shortfin mako from being included in CITES ”, he adds.
Muciente also recalls that the Galician fleet has launched an improvement project (FIP Blues) for the future certification of the fishery.
Eddie is an Australian news reporter with over 9 years in the industry and has published on Forbes and tech crunch.