Harbour seals have become quite a controversial creature along the Pacific Northwest, as many people have put the blame of our plummeting fishing stocks on these efficient predators. When Harbour Seal culling and hunting ended in the 1970s, populations exponentially increased but currently have remained unchanged since the mid 90s in the Strait of Georgia.While the seal populations stabilized, salmon populations in the Strait have continued to decline since the 1970s. The decline of salmon is detrimental to many species, like the Southern Resident Killer Whales. It also affects humans in multiple ways, with reduced yields for fisheries—indigenous, recreational and commercial—causing damage to livelihoods and the economy. In recent years, scientists have begun more in-depth studies into what exactly Harbour Seals are eating and how much, to determine the actual effect of seal predation on salmon populations.
How do scientists figure out the diet of these highly mobile seals? They analyze their poop! In the past, seal diet has been determined by identifying bones and other hard material remaining in scat samples (feces), like fish otoliths (ear bones) and squid beaks. But with advances in technology, scientists can combine this method with the use of DNA and stable isotope ratios, to more accurately study their diets. This process allows them to calculate the relative amounts of different fish species in Harbour Seal diet and determine what seals are eating throughout the year.
They’ve found that seals are opportunistic feeders and tend to shift their diet to what is in season (like various salmon runs and spawning events). For example, in the spring, Harbour Seal diets mainly consist of codfishes and forage fish, and only a small percentage of salmon—mostly Chinook and Coho. In the fall, there are still high percentages of codfishes and forage fish in their diet, but an increased amount of salmon, primarily adult Chum.
From this diet study, we can see that Harbour Seals are an important part of our marine food web in many ways. For example, they are one of the main predators of Pacific Hake (a type of codfish) that prey on forage fish, which are an integral source of food for many species in the ecosystem, including salmon. Marine ecosystems are complex and have many intertwined relationships, so the removal of Harbour Seals could have unforeseen impacts on many species, including salmon.
Not only does this study show that Harbour Seals prey on many fish species, but their diet is predominantly made up of species that are not salmon. Unlike the generalist Harbour Seals, other species that rely more heavily on salmon are experiencing declines. In the case of Southern Resident Killer Whales, this population of whales is endangered and facing extirpation. These killer whales exclusively hunt fish—primarily adult Chinook salmon.
There is a long list of other factors that are impacting salmon populations in the Strait of Georgia other than seal predation, such as:
- Habitat fragmentation and destruction
- Food availability
- Loss of spawning habitat
- Climate change
- Ocean acidification
In order to combat these pressures, many conservation groups have been working to restore critical habitat for multiple life-stages of salmon. Projects like restoration of spawning habitats, streams, and estuaries are essential for improving survival rates of juvenile salmon and spawning outputs.
Ultimately, we see that Harbour Seals are an essential part of the marine ecosystem and help keep many fish populations in balance. It is important to understand and study all aspects affecting salmon populations in the Strait of Georgia before intervening in the ecosystem. On Mayne Island, we can do our part and help protect habitats that juvenile salmon need to survive, like conserving our eelgrass beds and nearshore habitat.