Trophic ecology of young‐of‐the‐year elasmobranchs in a critical habitat within the Río de la Plata outer estuarine waters

Abstract

Knowledge on the trophic ecology of elasmobranch species in all their size classes is important to determine complex trophic roles and relationships between members of the community, which ultimately promotes the development of more effective conservation measures. This study investigates the diet of young‐of‐the‐year from two shark and one skate species that are common within the Southwest Atlantic Ocean. Identification and analysis of stomach contents indicated that the broadnose sevengill shark Notorynchus cepedianus fed mostly on fishes (96.08%IRI), whereas the angular angel shark Squatina guggenheim, and the smallnose fanskate Sympterygia bonapartii preyed mainly on crustaceans (68.73–99.96%IRI). In particular, Artemesia longinaris, a widely distributed small shrimp, was an important prey item for the two crustacean feeders. The high proportion of stomachs with food and the trophic levels suggest that the three species are active predators at a very young age. A high frequency of neonates was also observed for all species, suggesting that parturition events are probably occurring nearby. Overall, this study suggest that the nearshore waters of northern Argentina are functioning as an important feeding ground for co‐occurring elasmobranch species within an important estuarine habitat.

The essential habitat role of a unique coastal inlet for a widely distributed apex predator

“…These sites have also been suggested as essential habitat for N. cepedianus life stages, e.g. estuarine conditions in Partido de La Costa may facilitate feeding for young of the year [ 80 ], Bahia Anegada may function as a nursery area due to the presence of neonates, and Ria Deseado may be a secondary nursery area for larger juveniles [ 65 ]. The presence of younger life stages to the north suggests N. cepedianus primary nurseries necessitate warmer conditions, while coastal aggregation sites to the south provide feeding opportunities for larger individuals across colder conditions.…”

Growth estimates of young‐of‐the‐year broadnose sevengill shark, Notorynchus cepedianus, a top predator with poorly calcified vertebrae

“…Therefore, vulnerability to the fishing gear of YOY of broadnose sevengills will have a deleterious effect on population stability, making this species more susceptible to fishing exploitation even when not targeted by the fishery. The SAC is also important as a feeding ground for several other YOY elasmobranch species, such as the angular angelshark, Squatina guggenheim , and the smallnose fanskate, Sympterygia bonapartii (Milessi et al ., 2019 ) . The declining population trend in the SWA (Barbini et al ., 2015; Irigoyen & Trobbiani, 2016) together with the importance of the SAC as feeding and YOY habitat makes it necessary to develop management and conservation measures (such as time/area closures, as well as fishing gear restrictions) to control fishing mortality within this critical area to ensure sustainability.…”

“…The current IUCN assessment indicates declines in parts of its range (South Atlantic, Northwest Pacific) with increasing trends in the Southwest Pacific, and an overall global population reduction of 30%–49% over three generations (Finucci et al ., 2020). N. cepedianus use the inshore bays as pupping and juveniles areas where the offspring sizes (TL 35–45 cm) make it susceptible to fishery mortality because of the selectivity pattern of the fishing gear used (Bahia Anegada, Lucifora et al ., 2005; California, Ebert, 1996; Port Phillip Bay, Schmidt‐Roach, 2018; San Antonio Cape, SAC; Milessi et al ., 2019). These life‐history characteristics and the high susceptibility to fishing mortality would make the species highly dependent on YOY survivorship and more susceptible to mortality from environmental perturbations or anthropogenic influences.…”

“…susceptible to fishery mortality because of the selectivity pattern of the fishing gear used (Bahia Anegada, Lucifora et al, 2005;California, Ebert, 1996;Port Phillip Bay, Schmidt-Roach, 2018;San Antonio Cape, SAC;Milessi et al, 2019). These life-history characteristics and the high susceptibility to fishing mortality would make the species highly dependent on YOY survivorship and more susceptible to mortality from environmental perturbations or anthropogenic influences.…”