Sub-annual cohort representation among young-of-the-year recruits of the western stock of Atlantic bluefin tuna

Abstract

The western stock of Atlantic bluefin tuna receives recruits principally from spawning habitats in the Gulf of Mexico although the recent discovery of bluefin tuna larvae in the western North Atlantic Slope Sea calls into question whether there is another key source of recruits to the western stock. We tested whether age-0 juveniles occurring in the US Mid-Atlantic Bight during the late summer might originate from Slope Sea spawning by estimating the hatch date distribution based on otolith microstructural analysis given larval production occurs approximately two months later in the Slope Sea (late June to early August) than in the Gulf of Mexico (early April to mid-June). Following a review of the literature on ageing methodology for Thunnini species, we attempted trials on both sectioned and whole un-sectioned otoliths, and adopted the latter based on higher precision levels. Estimated ages ranged from 122 to 175 (days post hatch) for fish ranging 23.2–36.3 cm fork length. Hatch dates ranged 20 March to 12 May 2010, exhibiting a unimodal distribution. Hatch dates mainly occurred in early-April 2010, which corresponded to the spawning and larval production period for the Gulf of Mexico. Estimated growth rate (size-at-age) was 1.1 mm d−1. Thus, we failed to observe evidence for recruitment from the newfound Slope Sea spawning region, albeit our sample represents only a single year and region. More conclusive inferences on recruitment from the Slope Sea spawning will depend on a greater sample of age-0 juveniles sampled across years or molecular approaches that can identify recruits in older juveniles and adults.