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Coastal shark demise in South America intensifies

Updated: Sep 22

By Leonardo Feitosa


New research shows that an important shark species, the smalltail shark, faces population decline due to an increased threat of fishing


Photo by Elianne Dipp from Pexels


Despite the inherent fear most people have of sharks, the opposite is more realistic. Sharks kill an average of six people worldwide annually, while humans are estimated to fish more than 70 million sharks per year. This is driving their populations to severe declines, with now 25% of shark and ray species threatened with extinction.


This is not to say that fishing cannot occur. Sharks are a major source of protein, especially for low-income populations, since their meat is cheaper than other seafood. Further, they comprise a significant portion of fishers’ income in developing countries. Therefore, finding a balance between fisheries and shark stocks is paramount for sustainability and food security.


But in Brazil, due to ongoing human population growth and people increasingly occupying coastal areas, pressure on coastal fish species has increased dramatically. Indeed, this is likely the main reason why the proportion of endangered shark species (33%) in Brazilian waters is higher than the global average of 25%.[1]


In fact, shark meat is sold at moderate to high prices in some areas of the southeastern and northern regions of Brazil. The country is now considered as the biggest shark meat importer of the world by the Food and Agriculture Organization (FAO) of the United Nations [2], with further participation from the small-scale fishing fleets on the shark meat supply chain.


One of the most iconic cases is Carcharhinus porosus, commonly known as the smalltail shark. During the 1980s, the smalltail shark used to be the most abundant shark in the Brazilian Amazon Coast, comprising more than 40% of catches. Since the 2000s however, the number of smalltail sharks fished decreased by 85%, thus making it one of the most imperiled species in Brazil. Its fins were never a valuable by-product, but its meat is consumed on a regular basis by the local population, as are other shark species.


Despite its small size, it takes the smalltail shark six years to reach sexual maturity. It is also a low fecund species (with only an average of six pups per gestation), which makes it one of the least resilient to anthropogenic pressures among similar sized sharks around the world. Despite environmental protection required by federal rules, it continues to be fished and traded in the Brazilian Amazon coast.


According to a study recently published in the journal Plos One, its population in the area has also decreased more than previously thought. Scientists estimated that this population decreased by 99% in 30 years, which is striking for any species of shark, but specially for a small-sized one like the smalltail shark [3].


The severity of this scenario was made clearer by two other studies published this year by the same group of scientists. They analyzed the species' range and habitat use patterns on the Amazon coast and discovered that the area is an essential habitat, since it fulfills the whole life cycle (birth, growth, reproduction, and death) in the same region [4].


But fishing in that area takes a toll on the smalltail shark's population, diminishing its capacity to rebuild its stocks. In a third article published by the same research group in Aquatic Conservation [5], the authors analyzed the shark's geographic distribution along the western Atlantic, and also estimated the probability of catching it in the Gulf of Mexico, the Amazon Coast, and southeastern Brazil (using data from the1970s to the 2010s). They discovered that the most suitable--and most frequented--area for for the shark was along the Amazon coast, which is also where its catch probability is higher. However, the probability of catching one has decreased in all regions over time, with troubling findings in Mexico and Brazil in particular. In the Gulf of Mexico, the probability of catching a smalltail shark was calculated as only 10% in 2015, and in parts of Brazil, the last smalltail shark caught was in 1994.


This is especially concerning because it indicates that the species decline is probably widespread along its geographic distribution. Scientists have shown that the smalltail shark is now one of the most endangered small-sized shark species in the Atlantic. The species requires a joint effort by fisheries managers, policy makers, and fishers to create a feasible management plan to rebuild its populations. Right now, what used to be plentiful is rare, and the most vulnerable people are likely to suffer consequences without healthy populations of this important shark. We can and should pressure environmental authorities to make sustainable fisheries management happen. Sharks and humans can coexist, and we should try our best to make this happen.


#sharks #conservation #fisheries

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References:

[1] Barreto, RP., Bornatowski, H., Motta, FS., Santander-Neto, J., Vianna, GMS., Lessa, RP. “Rethinking use and trade of pelagic sharks from Brazil.” Marine Policy, vol. 85, 2017, pp. 114-122.

[2] Dent, F., Clarke, S. “State of the global market for shark products.” FAO Fisheries and Aquaculture Technical Paper, Ed. 590, 2015.

[3] Santana, FM., Feitosa, LM., Lessa, RP. “From plentiful to critically endangered: Demographic evidence of the artisanal fisheries impact on the smalltail shark (Carcharhinus porosus) from Northern Brazil.” Plos One, vol. 15, no. 8, 2020, e0236146.

[4] Feitosa, LM., Dressler, VL., Lessa, RP. “Habitat use patterns and identification of essential habitat for an endangered coastal shark with vertebrae microchemistry: the case study of Carcharhinus porosus.” Frontiers in Marine Science, vol. 7, 2020, pp. 1-12.

[5] Feitosa, LM., Martins, LP., Souza-Junior, LA., Lessa, RP. “Potential distribution and population trends of the smalltail shark Carcharhinus porosus inferred from species distribution models and historical catch data.” Aquatic Conservation: Marine and Freshwater Ecosystems, vol. 30, no. 5, 2020, pg. 882-891.

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