Increasing Occurrence of Atlantic Bluefin Tuna on Atlantic Herring Spawning Grounds: A Signal of Escalating Pelagic Predator–Prey Interaction?

Predation can be a significant source of natural mortality for small pelagic fish species, rivaling or exceeding fishery removals. Failure to account for changes in natural mortality can introduce uncertainty in the assessment and management of these stocks. In this study, a 10‐year span of hydroacoustic data was used to detect Bluefin Tuna Thunnus thynnus on two major fall spawning grounds of Atlantic Herring Clupea harengus , an economically and ecologically valuable forage fish species in the southern Gulf of St. Lawrence (sGSL). Average Bluefin Tuna detections increased 22‐fold from 2002 to 2012 on both spawning grounds independently of Atlantic Herring density or aggregation size. This increase is directionally consistent but larger than changes in other Bluefin Tuna population indices. Preliminary estimates of annual Atlantic Herring consumption doubled across the time series, reaching values of 4,300–20,000 metric tons in recent years. This would suggest that Bluefin Tuna are among the most important consumers of Atlantic Herring in the sGSL. These findings are key for an ecosystem‐based approach to the assessment and management of both Atlantic Herring and Bluefin Tuna in the sGSL.