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

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 major spawning ground for Atlantic bluefin tuna (Thunnus thynnus). The degree to which this habitat varies interannually is an open question. Here, we perform a decade-long (2009-2018) analysis of model output to investigate the interannual variability in the water temperature and circulation patterns that are necessary for successful spawning. We also quantify the influence of Gulf Stream meanders and overshoot events on larval retention and their effect on spawning success rates throughout the Slope Sea, and we show that success rates for simulated larvae originating outside of the Slope Sea are significantly lower than those for larvae originating inside this region. Average spawning conditions over the decade are most favorable in the western Slope Sea, specifically in the Slope Gyre and away from the immediate vicinity of the Gulf Stream. Variability in domain- and summer-time-averaged yearly success rates is up to 25% of the mean decadal-averaged success values. Yearly spawning success correlates strongly with the Gulf Stream overshoot and 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 success.