Partial female migration and cool-water migration pathways in an overfished shark

Abstract

Knowledge about reproductive movements can be of important conservation value for over-exploited species that are vulnerable when moving between and within key reproductive habitats. Lack of knowledge persists around such movements in the overfished school shark Galeorhinus galeus in Australia. Management assumes all pregnant females migrate between adult aggregations in the Great Australian Bight, South Australia, and nursery areas around Bass Strait and Tasmania. We tracked 14 late-term pregnant females tagged in South Australia using satellite-linked pop-up archival tags to investigate extent, timing, and routes of migrations. We found partial migration, with some females (n = 7) remaining near aggregating areas throughout the pupping season, some migrating to known nursery areas (n = 3), and one migrating ∼3 000 km to New Zealand. We conclude female movements and pupping habitats are less spatially constrained than assumed and propose females use cool-water routes along the shelf break to reduce energy costs of migration. Migrating females using these routes faced greater fishing pressure than sharks in inshore areas and were not protected by inshore shark fishing closures designed to protect them. This study demonstrates the complexity of reproductive movements that can occur in wide-ranging species and highlights the value of explicit movement data.

Accounting for kin sampling reveals genetic connectivity in Tasmanian and New Zealand school sharks, Galeorhinus galeus

“…Both F ST and D est , as well as diversity and clustering analyses, did not detect differentiation between TAS and NZ neonates and juveniles. This is supported by the large dispersal abilities of school sharks (Coutin et al, ; Hurst et al, ; McAllister et al, ; McMillan et al, ; Olsen, ). Genetic diversity was similar between both sampling regions, but lower compared to previous studies (He = 0.5–0.75; Hernández et al, ; Bester‐van der Merwe et al, ; Domingues et al, ).…”

“…Due to the low sample size and missing alleles, a reliable estimate of allele frequencies could not be made and these results must be interpreted with caution. In addition, a recent publication from McMillan et al () described partial migratory behavior of Australian school sharks, where some females appeared to be resident. Consequently, the possibility of a small and localized population in Tasmania cannot be excluded.…”

“…This study builds on the many telemetry and genetic studies that have investigated movement and connectivity of school sharks within Oceania (Bester‐van der Merwe et al, ; Coutin et al, ; Hernández et al, ; Hurst et al, ; McAllister, Barnett, Lyle, & Semmens, ; McMillan et al, ; Olsen, ; Ward & Gardner, ). Based on current results, the null hypothesis of a single panmictic population cannot be rejected.…”

“…Potential siblingor sex-biased sampling could explain the observed nuclear signal of structure (Allendorf & Phelps, 1981;Benestan et al, 2017;Feutry et al, 2017;Waples, 1998). School sharks are known to school by size and sex (Francis, 2010;Olsen, 1984 Lyle, & Semmens, 2015;McMillan et al, 2018;Olsen, 1954;Ward & Gardner, 1997). Based on current results, the null hypothesis of a single panmictic population cannot be rejected.…”

“…Such a problem may exist for school sharks, which are managed independently in Australia and in New Zealand (Francis, 2010), despite tagging and genetics studies that have questioned the assumption of separate stocks. Individuals are reported crossing the Tasman Sea and migrating up to 4,500 km (Coutin, Bruce, & Paul, 1992;Francis, 2010;Hurst, Baglet, McGregor, & Francis, 1999;McMillan, Huveneers, Semmens, & Gillanders, 2018). Nevertheless, such tagging studies do not provide any information about successful reproduction of migrants.…”