Atlantic salmon have fascinated people for centuries. Ever since Roman soldiers observed them jumping in British rivers and called them “The Leaper” people have been observing salmon and their behaviour. Although much has been learned there is still a lot we don’t know about this fascinating fish. However technology is beginning to unlock some of the mysteries. Now scientists have a new tool that will help provide some answers about this aspect of the salmon’s journey. That tool is the Ocean Tracking Network.
The Ocean Tracking Network is a $170 million project which involves setting up a system of acoustic receivers on the bottom of the world’s oceans. The Network is used in conjunction with special acoustic tags which are inserted in animals as diverse as salmon and codfish to whales and sea turtles. Off our shores the receivers are placed approximately 800 metres apart on the ocean floor to create an invisible curtain that will track anything with a tag that moves through it. This acoustic curtain stretches to the edge of the continental shelf, a distance of 180 kilometres. A second curtain is also deployed in the Cabot Strait. In the case of Atlantic salmon this curtain covers the two main migration routes for fish as they leave the Atlantic Provinces and head off to Greenland to feed.
While the original project called for a line off of Halifax and a line stretching from Cape Breton across the Cabot Strait, further work is being carried out to fine-tune fish movement in certain areas. One project is targeting salmon in the Bras d’Or lakes on Cape Breton. Researchers are trying to discover where salmon travel when they leave Middle and Baddeck Rivers. Do the fish spend some, or all of their life-cycle in the Lakes, or do they leave through the Great Bras d’Or Channel to head to Greenland? The research is in its early stages but will provided valuable information that will be useful in conserving this species.
The Ocean Tracking Network will also help answer one of the more perplexing questions facing scientists and that is why young salmon aren’t surviving. Is the mortality occurring when they leave the rivers in the spring or later on when they are in the ocean? If the data indicates that most tagged fish are living long enough to swim through the Cabot Strait or the continental shelf then researchers will be able to focus on problems farther out in the ocean.
One factor which limits how long tags will last is battery size. Young salmon can’t carry a large tag so battery life is limited to a few months. Larger fish, such as salmon kelts, fish which have spawned and are returning to the sea, can carry a larger battery which will allow them to be tracked longer. While this technology is very expensive it provides valuable and useful information to help answer questions regarding what is happening to fish and marine mammals in the sea. .
Don MacLean is an outdoor writer and biologist who lives in Pictou County.