Harbour seals came under federal protection in 1970. After decades of over-zealous hunts, fewer than 5000 seals remained around the Strait of Georgia. By 2008 seal numbers had rebounded – skyrocketing to about 40,000 or an eight-fold increase. Chinook and Coho salmon, on the other hand, declined sharply in the same waters in the same four decades despite enhancement programs aimed at increasing salmon stocks.
The relationship between the two is now the subject of investigation. In one study, radio frequency ID “beanies” were glued to the heads of twenty seals, and 40,000 juvenile coho salmon were tagged before they were released from the Big Qualicum Hatchery on Vancouver Island. Every time a seal with a beanie ate a tagged salmon, it was recorded.
Dr. Austen Thomas of the UBC Institute for Oceans led the study. In related research over the same period, UBC doctoral student Ben Nelson analyzed seal scats for evidence of salmon DNA.
Early findings after mathematical modelling of the data revealed a specific pattern – the seals are not going after just any salmon. They are targeting juvenile Coho and Chinook that are heading from freshwater out to the Salish Sea.
The research suggests as many as 40 to 60 per cent of all juvenile Coho and 30 to 50 per cent of juvenile Chinook in the months from May to October could be lost to the expanded harbour seal population.
Now the researchers are turning their heads to the question of why these juvenile salmon are such choice targets. Are they already compromised in some way due to lack of refuge habitat, or weakened by pathogens? Removing the seals would not necessarily solve the problem – the researchers point out another predator could just move in to fill the void.
These studies by Thomas and Nelson are part of a much larger joint Canada-US research project focussed on finding out more about the ocean life of salmon.