Abstract
The smalltail shark, Carcharhinus porosus, was the most abundant elasmobranch species in fisheries off Brazil’s northern coast (BNC) in the 1980s, but its population has been declining since the 1990s. For this reason, a demographic analysis is necessary to determine the extent of this decline and the fishing effect on the BNC’s population. Therefore, we performed a stochastic demographic analysis of the population in the BNC, and considered its global center of abundance. Smalltail shark specimens (n = 937) were collected with gillnets in Maranhão state, eastern BNC, in the 1980s with sizes ranging between 29.6 and 120.0 cm total length. Most of the individuals (90.6%) caught were juveniles (< 6 years-old), and the mortality and exploitation rates showed that the species was overexploited (92.3% above the fishing mortality corresponding to the population equilibrium threshold). The smalltail shark’s biological characteristics, such as slow growth and low fecundity, demonstrate that it is one of the least resilient species among similar sized coastal sharks in the region. All these factors yielded an annual decrease of 28% in the intrinsic population growth rate, resulting in a population decline of more than 90% in only 10 years, and much higher for the current period. This set of features comprising fishing recruitment occurring upon juveniles, overfishing, and intrinsically low resilience make the population unable to sustain fishing pressure and severely hamper biological recruitment, thus causing this drastic population decline. Furthermore, several local extinctions for this species in the northeastern and southeastern regions of Brazil highlight its concerning conservation scenario. Therefore, since similar fisheries characteristics occur throughout its distribution range, C. porosus fits the criteria E of the IUCN Red List for a critically endangered species and urgent conservation measures are needed to prevent its extinction in the near future.
Highlights
Basic studies on elasmobranch populations are crucial for conservation status assessments and the establishment of management measures
These studies are important for Data Deficient (DD) species for which little or no information is available, but populations might be under extinction risk [1,2]
This has enabled the incorporation of uncertainty and elasticity into population parameters, increasing the robustness of demographic analysis and enabling researchers to evaluate how populations would behave under different fishing scenarios
Summary
Basic studies on elasmobranch populations are crucial for conservation status assessments and the establishment of management measures. An effective way to provide this information is to perform demographic analysis based on species-specific biological data This has been done for several shark species to identify the major threats they face [3,4,5] employing stochastic demographic analysis and Monte Carlo simulations [3,6,7]. On the other hand, has an intense artisanal and semi-industrial fishery with elevated elasmobranch bycatch levels, and little to no regulation for almost ten years [9] In this context, only a handful of coastal shark species (Isogomphodon oxyrhynchus [4], Carcharhinus signatus [3]) were studied through demographic analyses and elasticities testing different exploitation scenarios. 27 shark species are under some level of threat in Brazil even with population trends mostly unknown [10]
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