Residency and reproductive status of yellowfin tuna in a proposed large‐scale pelagic marine protected area

Abstract

Abstract Since the year 2000, the designation of remote, large‐scale marine protected areas (LSMPAs) has closed over 15 × 106 km2 of ocean to commercial fishing. Yet, while these mega‐reserves have collectively made a major contribution toward meeting global targets for marine conservation, their effectiveness for the protection of highly mobile, pelagic species remains largely unknown. This study reports on the spatial behavior and reproductive status of yellowfin tuna (Thunnus albacares) inhabiting a proposed LSMPA around Ascension Island (tropical Atlantic), with the aim of evaluating the conservation benefits for this economically important species. Using a combination of satellite archival tags and conventional mark–recapture it was shown that individual tuna can remain within Ascension Island waters for periods of between 100 and 200 days, with core residency areas (50% “utilization distributions”) generally extending <200 km from shore. The vast majority of activity occurred within 92.6 km (50 nmi) of the island and coincided with a “no‐take zone” established around the island in 2016. However, dispersive movements out of this zone were observed in some satellite‐tagged individuals, possibly marking the beginning of more extensive oceanic migrations. Gonad staging of 342 individuals sampled over a 32 month period found no evidence of reproductively active (ripe) individuals, strongly suggesting that tuna foraging around Ascension Island migrate elsewhere to breed. Study results provide further evidence of the importance of oceanic islands as residency areas for pelagic megafauna, helping to justify their inclusion within LSMPAs. In the absence of local recruitment, however, these individuals will remain susceptible to exploitation on spawning grounds located outside of reserves, ultimately limiting their effectiveness and thus calling for further research to identify and protect the areas where species are most vulnerable.