Exploring interannual variability in potential spawning habitat for Atlantic bluefin tuna in the Slope Sea

Abstract The Slope Sea in the Northwest Atlantic Ocean, located between the Gulf Stream and the continental shelf of the Northeast United States, is a recently-documented possible major spawning ground for Atlantic bluefin tuna (Thunnus thynnus). Larval surveys and a habitat modeling study have shown that suitable spawning habitat occurs in the Slope Sea, but the degree to which this habitat varies interannually is an open question. Here, we perform a decade-long (2009–2018) numerical modeling analysis, with simulated larvae released uniformly throughout the Slope Sea, to investigate the interannual variability in the water temperature and circulation criteria deemed necessary for successful spawning. We also quantify the influence of Gulf Stream meanders and overshoot events on larval retention and their effect on habitat suitability rates throughout the Slope Sea, defined as the percentage of simulated larvae released at a given location that satisfy criteria related to water temperature and retention near nursery habitat. Average environmental oceanographic conditions over the decade are most favorable in the western part of the Slope Sea, specifically in the Slope Gyre and away from the immediate vicinity of the Gulf Stream. Variability in domain- and summertime-averaged yearly spawning habitat suitability rates is up to 25% of the mean decadal-averaged values. Yearly habitat suitability correlates strongly with the Gulf Stream overshoot but does not correlate well with other oceanographic variables or indices, so an overshoot index can be used as a sole oceanographic proxy for predicting yearly bluefin spawning habitat suitability in the Slope Sea. Selective spawning can weaken the correlation between habitat suitability and Gulf Stream overshoot. Effort should be put towards collecting observational data against which we could validate our findings.