Impacts of temperature changes in the Northwest Atlantic on apparent distribution shifts of Atlantic bluefin tuna (Thunnus thynnus)

The Gulf of Maine has experienced one of the fastest warming rates of any marine ecosystem. This change in temperature has impacted organisms that live in the Northwest Atlantic, especially Atlantic bluefin tuna. The International Commission for the Conservation of Atlantic Tunas has reported a decline in the catch per unit effort (CPUE) of tunas in the Gulf of Maine. Simultaneously, acoustic surveys of tunas in Canadian waters have found an increase in abundance. We explored whether these regional abundance trends and the apparent shift in Atlantic bluefin tuna distribution away from the Gulf of Maine and into Canadian waters were related to changing water temperatures. Specifically, we used NOAA Optimum Interpolation Sea Surface Temperature data to calculate monthly and annual temperature anomalies from a baseline period of (Oct. 1981-Present) for each 1/4^o degree grid cell within the two regions. We then correlated sea surface temperature anomaly time series to regional annual abundance indices from (1994-2015) to assess the relationship between ocean temperature changes and Atlantic bluefin tuna abundance. In Canadian waters, especially the Gulf of St. Lawrence, there was a significant (p = 0.01923), strong relationship between rising temperatures and acoustic survey tuna abundance estimates. In the Gulf of Maine, a significant trend appeared between temperature and rod and reel survey CPUE, but not between de-trended CPUE and temperature. These relationships may help explain the apparent distribution shift of Atlantic bluefin tuna from the Gulf of Maine into the Gulf of St. Lawrence. Including ocean temperature in Atlantic bluefin tuna movement, distribution, and population dynamics models will support improved assessments for this species as temperatures continue to warm.