Implications of alternative spawning for bluefin tuna remain unclear

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.…”

“…According to these criteria, an additional spawning ground, younger age at maturity, and larger mature population all reduce vulnerability. Further, the analyses presented by Walter et al (1) and the International Commission for the Conservation of Atlantic Tunas (ICCAT) (13) indicate that lowering the age at maturity within the stock assessment model reduces the maximum decline in spawning stock biomass over the observed 1970-2013 period from an >80% decline (age of 12-16 y at maturity), to an ∼74% decline (age of 9 y at maturity), to an ∼66% decline (age of 4-5 y at maturity). A lower rate of decline in the face of fishing is one definition of lower vulnerability.…”

“…Our conclusion that a majority of spawning occurs outside the Gulf of Mexico is based on the conclusion of a lower age at maturity; our analysis of tagging data; and estimates of total mortality, including values from the stock assessment model used by Walter et al (1). 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).…”

“…As Walter et al (1) state in the conclusion of their letter, further testing of this model will require additional research using multiple techniques. Longline sampling of adult fish in the Slope Sea for studies of reproduction and population structure is a top priority.…”

Distribution, growth, and transport of larval fishes and implications for population dynamics

“…The response to this discovery has been mixed, with some voices advocating against the alternate life cycle hypothesis (Salina, 2016;Walter et al, 2016) and others arguing that it calls for more innovative studies to resolve our understanding of bluefin life history (Di Natale, 2017). A major argument against the classification of the Slope Sea as a third major spawning ground focuses on the nature of the larval bluefin observations as arising from a single year of opportunistic sampling (Walter et al, 2016).…”