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

Abstract

Sharks are among the most endangered and data poor vertebrates in the world. The lack of information regarding their habitat use is especially concerning since these are crucial for the establishment of priority areas for species conservation. Investigating the trace elements present in shark vertebrae has become an interesting tool to analyze species habitat use over individuals' lifetimes. Therefore, we applied vertebrae microchemistry to investigate habitat use of Carcharhinus porosus in Brazil's Northern Coast (BNC). We also discuss methodological issues that must be addressed in the future to make microchemistry studies with elasmobranchs at low latitudes more robust. Vertebrae from seventeen individuals sampled in the 1980s (n = 8), and in 2017 and 2018 (n = 9) were evaluated through laser ablation inductively coupled plasma mass spectrometry. We analyzed five elements known to reflect environmental characteristics (Ba, Ca, Mg, Mn, and Sr) 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. We also used fisheriesindependent catch data from the 1980s to test if the BNC is a nursery area for C. porosus. We found significant differences in element concentrations between sexes in both multi and single-element analysis, while decades differed only in the multi-element and Sr:Ca. Furthermore, seasons differed in both multi-element and Mg:Ca and Ba:Ca ratios. We did not find differences between life stages. Neonate multi-element signatures yielded three major groups, thus suggesting that the species has at least three birthing grounds in the area. Despite the occurrence of individuals of all sizes, adults show a more distinct occurrence pattern. 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. Therefore, the BNC is a critical area for its conservation. We reinforce that future studies with strictly tropical species should investigate the effects of metabolism, the species' growth rate, and the validation of other elements capable of demonstrating fine-scale changes in habitat to reduce the inherent noise in microchemistry studies.

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

“…Recent data using vertebrae microchemistry has demonstrated that the BNC might actually be an essential habitat, especially for species that fulfill their entire life cycle in the area [ 40 ]. Indeed, habitat use data indicate that C .…”

“…porosus is much more vulnerable to the fishing activity than other migrating coastal species such as Galeocerdo cuvier [ 90 ]. Despite this, species distribution modelling and historical catch data estimated that the Amazon coast is the area where the species has the highest catch and occurrence probability throughout its distribution range [ 40 ]. Therefore, the Amazon coast is likely the most important area for this species throughout its distribution, but is also where fisheries exposure are likely the highest [ 91 ].…”

“…Recent data using vertebrae microchemistry has demonstrated that the BNC might actually be an essential habitat, especially for species that fulfill their entire life cycle in the area [40]. Indeed, habitat use data indicate that C. porosus likely uses the waters of the Amazon coast during all life stages, with some level of sexual segregation.…”

“…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].…”

“…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 ].…”