Discovery of a spawning ground reveals diverse migration strategies in Atlantic bluefin tuna (Thunnus thynnus)

Abstract

Atlantic bluefin tuna are a symbol of both the conflict between preservationist and utilitarian views of top ocean predators, and the struggle to reach international consensus on the management of migratory species. Currently, Atlantic bluefin tuna are managed as an early-maturing eastern stock, which spawns in the Mediterranean Sea, and a late-maturing western stock, which spawns in the Gulf of Mexico. However, electronic tagging studies show that many bluefin tuna, assumed to be of a mature size, do not visit either spawning ground during the spawning season. Whether these fish are spawning in an alternate location, skip-spawning, or not spawning until an older age affects how vulnerable this species is to anthropogenic stressors including exploitation. We use larval collections to demonstrate a bluefin tuna spawning ground in the Slope Sea, between the Gulf Stream and northeast United States continental shelf. We contend that western Atlantic bluefin tuna have a differential spawning migration, with larger individuals spawning in the Gulf of Mexico, and smaller individuals spawning in the Slope Sea. The current life history model, which assumes only Gulf of Mexico spawning, overestimates age at maturity for the western stock. Furthermore, individual tuna occupy both the Slope Sea and Mediterranean Sea in separate years, contrary to the prevailing view that individuals exhibit complete spawning-site fidelity. Overall, this complexity of spawning migrations questions whether there is complete independence in the dynamics of eastern and western Atlantic bluefin tuna and leads to lower estimates of the vulnerability of this species to exploitation and other anthropogenic stressors.ichthyoplankton | Scombridae | large pelagic fish | pop-up satellite archival tag | population structure L ong-distance migrations pose a unique challenge to fisheries management, as conservation actions taken on a regional scale can be undermined if less stringent measures are implemented across other parts of the migratory pathway. Few species exemplify this problem better than Atlantic bluefin tuna (Thunnus thynnus). This species is harvested by the fisheries of over 20 nations, from the tropics to subarctic and coastal to international waters. Contentious international disputes have persisted for decades over how many bluefin tuna to harvest and how to allocate catch among nations. By the start of the 21st century, intense fishing pressure had driven this species to historically low population levels, a decline that has since reversed as fishing mortality has decreased under stricter management (1). However, despite this recent positive trend, many challenges remain in developing an ecologically sustainable fishery for bluefin tuna that also provides economic and social benefits to the fishing communities throughout its range. Among the most prominent of these challenges is the need for stock assessment models and management regulations that better account for the complex movements of this species.The movements of Atlantic bluefin t...

RETRACTED: Long-term aggregation of larval fish siblings during dispersal along an open coast

“…In recent years, efforts have focused on quantifying population connectivity of structured populations (3), often with the goal of designing networks of marine reserves (11,34,35) or developing more effective fisheries management of stocks across genetic or geopolitical boundaries (36). However, these approaches seldom consider the effect of fine-scale processes that are influenced by stochastic and patchy dispersal, such as high spatial coherence in the genetic structure of recruits.…”

Reply to Safina and Walter et al.: Multiple lines of evidence for size-structured spawning migrations in western Atlantic bluefin tuna

“…Walter et al (1) and Safina (2) raise numerous concerns regarding our study (3). Specifically, they question our conclusions that (i) a majority of spawning occurs outside the Gulf of Mexico, (ii) western North Atlantic bluefin tuna mature earlier than currently estimated, and (iii) additional spawning locations and younger age at maturity mean that the western Atlantic bluefin tuna are less vulnerable to anthropogenic impacts, including exploitation.…”

“…A central component of this analysis, the proportion of fish migrating to the Gulf of Mexico by size (or age), is remarkably consistent across four separate datasets: two independent electronic tagging datasets (3,8) and analyses of two fisheries-dependent longline catch datasets (7,9). Abundance of larvae in the Slope Sea compared with the Gulf of Mexico provides secondary support for the conclusion; the referenced collection of larvae in other areas outside the Gulf of Mexico (10, 11) provides additional support.…”

Implications of alternative spawning for bluefin tuna remain unclear

“…1). In other words, maintaining the same catch, size composition, and index trends with a greater spawning biomass implies that the stock is less productive, and therefore more vulnerable to exploitation, exactly the opposite conclusion of Richardson et al (1).…”