What Happens Next In Brazil Is Crucial For One Shark Species

Sharks are among the most endangered and data poor vertebrates in the world. Shark populations worldwide are subjected to a myriad of threats, such as overfishing and habitat degradation, which is concerning scientists about their long-term sustainability.

Sharks in Brazil are no different. Knowledge about how sharks use their habitats is limited, and this is problematic in and of itself for their overall conservation and management. If scientists lack shark habitat use knowledge, how can they protect them as priority areas for overall species conservation? One tool that is being used to better understand how sharks use their environments is by investigating the trace elements present in their vertebrae. This allows researchers to analyze species habitat use over individuals’ lifetimes. A team of scientists decided to apply vertebrae microchemistry to investigate habitat use of one particular shark, the smalltail shark (Carcharhinus porosus) in Brazil’s Northern Coast (BNC). “We did this project because our lab has been working with vertebrae microchemistry for a while, but never with a species from the Amazon coast. The region is a great mystery to us all, especially because we know very little about the abiotic features of the habitat. Its also one of the largest fishing areas in Brazil, and Dr. Lessa felt that applying this technique to investigate habitat use was a great opportunity,” said scientist Leonardo Manir Feitosa of Universidade Federal de Pernambuco in Brazil.

This gray small (almost 5 feet long or <1.5 meters) shark occurs in the Western Atlantic and Eastern Pacific Oceans. With its long, pointed snout and large eyes, it thrives in the muddy bottoms of estuaries where they hunt small fish and invertebrates. Due to being a smaller shark, they are often preyed on by larger sharks; they are also primarily caught as incidental bycatch in the gillnets of other fisheries. Increasing directed fishing effort by artisanal fisheries is the primary reason for this species' decline.

“We chose to focus on the smalltail shark because it is a critically endangered species in Brazil from which not a lot of information is available. It was the most important shark in terms of catch composition in northern Brazil during the 1980s and, from then on, its catches plummeted around 85%. Catches never increased since, so we needed to know how it uses the habitat in the Amazon coast to determine more effective conservation actions,” explained the Feitosa and Dr. Lessa in a joint statement.

Considered rare in the south of Brazil, they are Data Deficient (DD) throughout much of its range. But given the increasing fishing pressure, population decline in their distribution, and lack of management, this species is considered to be Vulnerable in Brazil. That is why a researchers, led by Feitosa, used vertebrae microchemistry to investigate habitat use patterns of this shark and see if there is a nursery in the area. It is the first ever study using shark vertebrae microchemistry in the Neotropical region!

Smalltail shark vertebrae were sampled from specimens landed in the Raposa municipality in Maranhão state, collected according to the Brazilian environmental laws under the license (License SISBIO, 49663-1). The sample size used was somewhat small, but collecting new vertebrae samples from this elasmobranch is difficult due to their rarity. Out of all the elements analyzed in this study, only five - Barium (Ba), Calcium (Ca), Magnesium (Mg), Manganese (Mn), and Strontium (Sr) - were used for the habitat use analysis due to their reliability as proxies for environmental tracers in elasmobranchs.

“We analyzed [the] five elements known to reflect environmental characteristics by sex, seasons, decades of capture, and life stages. Since Ca is the most abundant element in the vertebrae, we calculated element:Ca ratios and employed these proportions for all statistical analysis,” the article explains. It turns out Sr is much more important in females than in males, and Ba following the inverse pattern in males. This means that the smalltail shark likely undergoes sex segregation, which isn’t exactly surprising behavior among sharks!.

The group also used fisheries-independent catch data from the 1980s to test if the BNC is a nursery area for the smalltail shark. Nurseries are considered one of the most critical areas to be protected when it comes to elasmobranch management since they provide key habitat for small sharks to grow and, eventually, reproduce in the future. The team did not find a nursery. Instead, they discovered the area is an essential habitat for these sharks because they spend their whole life cycle in the same region. “Our results point toward the hypothesis that the BNC is an essential habitat for this species since its whole life cycle occurs in this area,” the scientists state. “Neonate multi-element signatures yielded three major groups, thus suggesting that the species has at least three birthing grounds in the area.”

This data shows that the BNC is a critical area for its conservation on a global scale. Due to this, the team has a suggestion: “A short-term feasible conservation strategy [is] to mitigate its bycatch in fisheries [by releasing] live specimens, especially those under 70 cm total length (TL) and hence sexually immature.” But the scientists stress that it would not be right to ban the target fisheries, since several communities depend on them for subsistence. Sustainable management, like the suggestion above, would allow for the population to recover while still keeping fishers employed. Conservationists wouldn’t see these management strategies pay off right away, as sharks are long-living, meaning it would require a longer time to rebuild their fallen population numbers.

“Now, we know that the region is actually an essential habitat for it and it is likely to be so for other similar sized species that co-occur in the same waters. It has greatly expanded how we viewed the Amazon coast from an ecological and conservationist point of view, thus now becoming even more important than previously thought. We hope that this discovery triggers other studies with different but also endangered species and leads to better fishing practices and species rebuilding stocks,” the scientsts conclude.