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British Columbia’s Fraser River is one of the world’s last great sockeye strongholds. Every year, millions of salmon run a deadly gauntlet of hooks and nets as they migrate from the ocean to spawn in their natal streams.
But with poor rates of survival at sea and climate change pushing summer river temperatures ever higher, the future of the Fraser River sockeye is under threat. Ensuring salmon fisheries are putting as little pressure on the remaining fish as possible has never been more important. Enter Arthur Bass, a doctoral candidate at the University of British Columbia.
In 2014, Bass embarked on an unusual sockeye fishing expedition. He targeted an autumn sockeye run, setting up fishing stations at three points along the Fraser: near the mouth, close to the spawning grounds, and one in between. From these points, he caught and released sockeye using two commercial fishing methods: gill nets and beach seines.
Gill nets have mesh large enough to trap salmon by the head. These nets usually ensnare the fish just behind the hard plate that covers the gills, though sometimes by the mouth or fins. Gill nets are anchored to the river bottom or set to drift in the river. Beach seines, nets used primarily by First Nations, have much smaller mesh. Fishers will anchor one end of the net while a small boat pulls the other into a semicircle, corralling fish as the two ends are pulled together.
Bass wanted to know if the catch method affected the salmon’s likelihood of survival if they escaped or were released. Such survival is important because at a given time there can be multiple salmon species and stocks migrating simultaneously on the Fraser, necessitating the release of non-targeted fish.
Bass and his collaborators, including First Nations fishermen, found that gill nets on the lower river cause more injuries and deaths than beach seines, but the disparity closes as the salmon move upriver.
Gill net fishing is rougher on fish, Bass says, and often results in wounds to the body and gills. This is especially so earlier on in the salmons’ migration. This is explained in part by the physical and chemical changes the fish undergo as they migrate. When sockeye enter fresh water, for instance, stress hormones naturally increase. As their migration progresses, this makes them increasingly resistant to any wounds they sustain. By the end of their migration, “they are already so jacked up on stress hormones that it’s hard to really disturb them,” Bass says.
The corollary of this is that fish caught early in the migration fare much worse than those caught later on. Fish caught earlier are also stronger, and will typically fight to exhaustion if caught in a net. The fish that survive this encounter can be so worn out they must delay their migration to recover—which is dangerous if the river temperature is higher than normal.
Fisheries and Oceans Canada (DFO) salmon research biologist David Patterson, who worked on the study, says the research is important because it demonstrates the real-world consequences of different fishing methods, unlike most studies that simulate the impact of a single type of gear in a lab.
Bass’s findings have already been added to a DFO internal review of the subject, Patterson says, meaning the research will now be taken into consideration when deciding how the fishery will be managed—decisions that would affect the fates of the sockeye that will soon be running the deadly gauntlet.