Decades of overfishing led to a collapse of the Northwest Atlantic cod fishery, and despite a moratorium in the 1990s, the Atlantic cod population has struggled to rebound to historical, sustainable levels.
Izzy Langley, a PhD candidate at the University of St. Andrews, recently completed her Visiting Fellowship with the Ocean Frontier Institute, where she teamed up with Dr. Sara Iverson at Dalhousie University, as well as Dr. Damian Lidgard and Dr. Nell den Heyer from Fisheries and Oceans Canada (DFO) to investigate the movement behavior of Atlantic cod on the Eastern Scotian Shelf and better understand their interactions with grey seals.
Challenges to cod recovery
"Cod stocks in the Gulf of St. Lawrence and Eastern Scotian Shelf have failed to recover over the past 30 years,” says Langley.
“There are likely multiple reasons as to why, however, warming ocean temperatures and increased grey seal predation may be playing a role."
Grey seal populations have been experiencing exponential growth across the North Atlantic. These adaptable and generalist top predators are known to feed extensively on cod, particularly juvenile cod. "This predation could be contributing to the cod species' failure to recover," explains Langley.
Leveraging cutting-edge animal-tracking technologies
Langley’s project leveraged an extensive dataset of acoustically tagged cod, acoustic receivers, and animal-borne acoustic transceiver tags deployed on grey seals.
Acoustic tags are underwater tracking devices that emit acoustic signals detected by underwater receivers. Contrastingly, animal-borne transceivers are attached directly to the animal and both emit and receive acoustic information that can then be transmitted and stored.
"There exists an amazing and expansive range of acoustic receivers throughout the Gulf of Saint Lawrence and Scotian Shelf. However, these are fixed in space, so we can only detect cod when they come into range of these receivers,"
Langley explains.
New information about cod movement
To gain a more comprehensive picture of how the cod and grey seal species are overlapping and interacting, the researchers equipped grey seals with specialized tags that can detect acoustically tagged cod.
"This gives us a window into parts of the ocean we couldn't access before - by seeing where the grey seals go, we can learn more about where the cod are migrating," says Langley.
While the work is ongoing, preliminary results have already revealed some interesting findings.
“We have found evidence of cod stock mixing between the Gulf of St. Lawrence and Scotian Shelf, as well as seasonal migrations of cod into deeper waters than expected. We are also getting detections of cod in areas where we wouldn't otherwise get them if it wasn't for the grey seals that have been tagged.”
This work is an important step towards understanding how commercially valuable cod are responding to factors like ocean warming and predation pressure.
"We just don't know enough about the Scotian Shelf cod stock, and that's a problem if we want to have any hope of a sustainable fishery in the future," emphasizes Langley.
About Izzy Langley
Izzy Langley is a PhD Candidate at the University of St. Andrews for the Sea Mammal Research Unit. Her research interests are in population biology and behavioural ecology of marine top predators.
Her current work employs a cross-disciplinary approach to investigate the potential role of grey seals in regional harbour seal declines in Scotland.
