Scientists and fishermen are working together to improve escapement rates of salmon incidentally caught in commercial trawl nets targeting Alaska pollock. They found that the key may lie in designing ways to prevent salmon from reaching the back end of the trawl, called the codend. That’s where fish are collected during fishing before they are hauled onboard a fishing vessel.
“We used underwater cameras to observe how salmon behaved inside the trawl,” said David Bryan, lead author of the study and a fisheries biologist at the Alaska Fisheries Science Center. “We learned that salmon escapement is relatively low if they reach the codend. If we want to improve their chances of escaping, the design of bycatch reduction devices, also called salmon excluders, needs to focus on keeping them out of the codend.”
Under the Magnuson-Stevens Act, federal fisheries must minimize bycatch to the extent practicable. In Alaska, the Bering Sea/Aleutian Islands groundfish fishery management includes a number of management measures to do that. However, because salmon and pollock intermingle, inevitably some salmon still get into nets. Salmon excluders are a helpful innovation, developed in partnership with the commercial fishing industry. This important research is trying to make them more effective.
How Salmon Excluders Work
In Alaska, resource managers and the fishing industry take a multifaceted approach to reduce salmon bycatch. One approach is for the fishing fleet to minimize salmon bycatch by avoiding certain areas or times of the year when Chinook and chum salmon are present. The use of salmon excluders in the pollock fleet is another key approach. The excluder provides an opening in the trawl that allows salmon to escape. Because salmon are stronger swimmers, they are able to reach the opening, while pollock remain in the net.
After a salmon enters the trawl mouth, they presumably are forced back towards the codend along with the rest of the catch. It is during this aft-ward travel that a salmon first encounters the excluder devices used in this fishery.
Salmon Behavior Inside the Trawl Net
Many of the excluders tested rely on the salmon’s ability to swim forward in the trawl (against the direction of the water flow) to escape.
In this study, scientists investigated salmon behavior just forward of the codend.
“We were hoping to learn more about what motivates some salmon to swim forward and exactly when this happens,” said Bryan. “This forward movement could provide an additional opportunity to access the excluders.”
Scientists attached an underwater camera inside the trawl during fishing operations. Over the course of seven tows, they collected more than 26 hours of video, observing nearly 3,000 individual salmon.
The majority of salmon (71 percent) were observed heading aft toward the codend. Just 24 percent were observed swimming forward, which would enable them to escape. Another 5 percent were observed either heading aft then forward or forward then aft.
There were a number of factors that appeared to affect whether salmon were able to swim forward in the net and escape:
Vessel speed over ground
Water flow within the trawl
Pollock abundance
More salmon (77 percent) swam forward when the vessel's speed over ground and water flow were lower. Fewer salmon swam forward as pollock abundance increased.
Only about 6.5 percent of salmon that were in the codend when fishing ended were able to swim forward and potentially escape before the trawl was on deck.
“These observations were really helpful because we learned where we should focus our energies next in improving salmon excluder design,” said Noelle Yochum, co-author, Trident Seafoods. “We need to find ways to help salmon escape during their first encounter with the excluder, considering how to make the escapement opportunity easier to detect as the salmon move past.”
