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

Irina I Rypina,Michael M Dotzel,Lawrence J Pratt,Christina M Hernandez,Joel K Llopiz

doi:10.1016/j.pocean.2021.102514

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.

Highlights

Atlantic bluefin tuna (Thunnus thynnus; bluefin hereafter) is a migratory fish of great economic and conservation value
We explore the sensitivity of habitat suitability rates with respect to the retention criterion used to define successful larvae, i.e., strict retention inside the Slope Sea for the entire duration of bluefin larval development interval versus the less strict requirement that larvae only end up in the Slope Sea at the end of the larval development in terval
In order to quantify the influence of this variability on bluefin spawning habitat suitability throughout the region, we compare statistics of simulated habitat suitability rates in 2013 for the time-mean versus time-evolving Slope Sea domain, and for the relaxed versus strict retention criteria (Fig. 2)

Summary

Introduction

Atlantic bluefin tuna (Thunnus thynnus; bluefin hereafter) is a migratory fish of great economic and conservation value. Recent collections of very young bluefin larvae in the Slope Sea—a geographical area located off the northeast United States between the shelf break and the Gulf Stream—have increased the focus on this region as a potential third major bluefin spawning ground (Richardson et al, 2016). Motivated by this recent discovery, Rypina et al (2019) investigated the suitability of the Slope Sea for Atlantic bluefin tuna spawning using a realistic ocean circulation model. Their analyses suggest that in 2013—the year of larval collec tions by Richardson et al (2016)—water temperatures and circulation patterns in the Slope Sea generally provided suitable conditions for bluefin spawning and larval development but with clear spatial variability

Methods

Results

Conclusion