Potential distribution and population trends of the smalltail shark Carcharhinus porosus inferred from species distribution models and historical catch data

Abstract

Updated distribution ranges are crucial for conservation status assessments. Comprehensive analyses combining published literature and available data on historical catches and species distribution models (SDMs) are effective tools that could improve the prediction of more realistic scenarios for some species, especially those with limited information available and facing multiple threats. The present study aimed at generating an updated distribution for the smalltail shark Carcharhinus porosus, one of the most threatened and understudied shark species of the western Atlantic Ocean. Estimates of the key areas for this species conservation based on the SDMs, and trends in catch probabilities throughout its distribution range are provided. Four algorithms (BIOCLIM, Domain, Mahalanobis, and Maximum Entropy) were used to model the distribution of C. porosus and calculate its habitat suitability based on marine environmental variables. To assess historical catch probability trends, we built a generalized linear model from published and grey literature data. This analysis was used to estimate catch probability as an indication of population trends. SDMs suggest that the northern coast of South America (NCSA) harbours the most suitable habitats for C. porosus in the world, which was expected given its historically high catch rate in this region. In addition, there was a continuously declining catch probability trend starting in the 1970s. However, the decline was smaller for the NCSA as compared with the Gulf of Mexico and the eastern South America coast. Results indicate that the NCSA should be considered the currently most important area in the world for this species conservation. Furthermore, the lack of data throughout Central and South American marine regions hampers the evaluation of extinction risk throughout its updated distribution. Thus, research in these areas is urgently required for a more comprehensive conservation status assessment.

Habitat Use Patterns and Identification of Essential Habitat for an Endangered Coastal Shark With Vertebrae Microchemistry: The Case Study of Carcharhinus porosus

“…Additionally, the species likely spends its whole life cycle within the continental platform, but with a small degree of habitat partitioning between sexes and several different birthing grounds. Furthermore, recent analysis estimated that the Northern coast of South America (defined as the coastal extension between Guyana and Maranhão state) is actually the region with the smallest decline in catch probability along the species distribution (Feitosa et al, 2020). The last IUCN species assessment also considers this area as the center of abundance for this species (Lessa et al, 2006).…”

“…Despite its small size (150 cm estimated maximum length), its life history traits make it easily susceptible to overfishing since its fecundity is low (average of 6 pups per gestation), the reproductive cycle is biannual, and both sexes reach sexual maturity with 6 years old (Lessa and Santana, 1998). It is highly associated to turbid and dynamic areas with mangrove rich coasts (Feitosa et al, 2020), and juveniles were consistently caught near estuaries in Maranhão state's coast (Menni and Lessa, 1998). Nevertheless, its habitat use patterns are still poorly understood.…”

From plentiful to critically endangered: Demographic evidence of the artisanal fisheries impact on the smalltail shark (Carcharhinus porosus) from Northern Brazil

“…In Brazil, C. porosus original area of distribution ranged from the northern coast to Cananeia beach in São Paulo, Southeastern Brazil [18][19][20]. Recent studies demonstrated that this occurrence pattern has changed dramatically with a significant decrease in the species' distribution area throughout Brazil's coast [21][22][23]. These declines caused by possible local extinctions restricted C. porosus occurrence to the northern coast of Brazil between Maranhão and Amapá states [21].…”

“…This area is considered its global center of abundance for the high proportion of individuals caught when compared to other regions where it occurs [24]. Recent data estimates show that the Amazon coast is likely the most important area for its conservation along its geographical distribution [23]. Notwithstanding, it was the dominant species of shark landed during the 1980s and 1990s in the Brazilian Northern coast (BNC), but it is now only the third most caught shark species in the region [22].…”

“…Then, C. porosus was classified as critically endangered (CR) in Brazil due to the recent increase in fishing effort with the use of longer gillnets spanning over 10 km [34], its occurrence in several different large scale fisheries for shrimp and teleosts [30], and its decreased national occurrence area, and lack of population increase [21]. However, even though a few studies investigating C. porosus biological features such as age and growth [35], diet [36], reproduction [37,38], sexual dimorphism [39], distribution [23], and habitat use [40] exist, it remains DD according to the IUCN. Furthermore, genetic diversity data obtained from specimens in the Amazon coast point to a low allele diversity, which potentially indicates the effects of overfishing in the population [41].…”