Site fidelity and shallow genetic structure in the common smooth‐hound shark Mustelus mustelus confirmed by tag‐recapture and genetic data

Abstract

The common smooth-hound shark, Mustelus mustelus, is a widely distributed demersal shark under heavy exploitation from various fisheries throughout its distribution range. To assist in the development of appropriate management strategies, the authors evaluate stock structure, site fidelity and movement patterns along the species' distribution in southern Africa based on a combination of molecular and long-term tag-recapture data. Eight species-specific microsatellite markers (N = 73) and two mitochondrial genes, nicotinamide adenine dehydrogenase subunit 4 and control region (N = 45), did not reveal any significant genetic structure among neighbouring sites. Nonetheless, tagging data demonstrate a remarkable degree of site fidelity with 76% of sharks recaptured within 50 km of the original tagging location. On a larger geographic scale, dispersal is governed by oceanographic features as demonstrated by the lack of movements across the Benguela-Agulhas transition zone separating the South-East Atlantic Ocean (SEAO) and South-West Indian Ocean (SWIO) populations. Microsatellite data supported very shallow ocean-based structure (SEAO and SWIO) and historical southward gene flow following the Agulhas Current, corroborating the influence of this dynamic oceanographic system on gene flow. Moreover, no movements between Namibia and South Africa were observed, indicating that the Lüderitz upwelling formation off the Namibian coast acts as another barrier to dispersal and gene flow. Overall, these results show that dispersal and stock structure of M. mustelus are governed by a combination of behavioural traits and oceanographic features such as steep temperature gradients, currents and upwelling systems.

Distribution and population structure of the smooth‐hound shark, Mustelus mustelus (Linnaeus, 1758), across an oceanic archipelago: Combining several data sources to promote conservation

“…In this type of habitats, the presence of potential predators is low, but feeding resources are abundant, as it has been reported for other nearshore sharks, also in the Canary Islands, such as the angelshark, Squatina squatina (Jiménez-Alvarado et al, 2021) and several rays (Tuya et al, 2020). This species would stand to benefit from protection of these sites, which seems to play a key role in a critical life stage of M. mustelus in the Canary Islands, particularly since this species has a high degree of site fidelity, at least in other regions (da Silva et al, 2013;Klein et al, 2021) and observations of juvenile and subadults occur in successive years (da Silva, 2018).…”

“…In this type of habitats, the presence of potential predators is low, but feeding resources are abundant, as it has been reported for other nearshore sharks, also in the Canary Islands, such as the angelshark, Squatina squatina (Jiménez‐Alvarado et al, 2021) and several rays (Tuya et al, 2020). This species would stand to benefit from protection of these sites, which seems to play a key role in a critical life stage of M. mustelus in the Canary Islands, particularly since this species has a high degree of site fidelity, at least in other regions (da Silva et al, 2013; Klein et al, 2021) and observations of juvenile and subadults occur in successive years (da Silva, 2018). Most specifically, these nearshore “nursery” sites should be initially identified and then, if meeting the criteria to be considered nurseries (Heupel et al, 2007), protected of several common human perturbations, including infrastructures construction, sewage outlets, and excessive maritime traffic linked to certain tourist activities (e.g., jet skis).…”