Post-release behavior and movement patterns of Chinook salmon ( Oncorhynchus tshawytscha ) and coho salmon ( Oncorhynchus kisutch ) after capture using alternative commercial fish gear, lower Columbia River, Washington and Oregon, 2013

Abstract

Commercial salmon Oncorhynchus spp. fishers traditionally have used gill nets, and more recently tangle nets, to capture adult salmon in the lower Columbia River, Washington and Oregon, but these gear types are not selective and can result in unintentional injury or death to non-target species, which is a problem when wild or Endangered Species Act-listed salmon are present. Gill and tangle nets capture fish through physical retention. Gill nets have mesh sizes that are slightly larger than the diameter of the head of the target species so that a fish moving through the net becomes entangled behind its operculum. Tangle nets have mesh sizes that are smaller than the diameter of the head of the target species so that a fish becomes entangled by its teeth or jaw. The Washington Department of Fish and Wildlife (WDFW) has been evaluating Merwin traps, beach seines, and purse seines during the past decade to determine if these are viable alternative commercial fishing gear types that would reduce negative effects to non-target fish, including wild salmon. As opposed to gill and tangle nets, these alternative gear types capture fish without physical restraint. The nets encircle the area where a fish or school of fish is located and eliminate the ability of those fish to escape. Because fish are not physically restrained by the gear, it is believed that the likelihood of injury and death would be reduced, allowing the safe release of non-target fish. In 2011 and 2012, WDFW conducted post-release mortality studies of steelhead ( Oncorhynchus mykiss ), Chinook salmon ( Oncorhynchus tshawytscha ) , and coho salmon ( Oncorhynchus kisutch ) that were captured using beach or purse seines. These studies were comprised of two groups of fish tagged with passive integrated transponder tags (PIT tags): (1) treatment fish that were captured by one of the gear types 9–25 river kilometers (rkm) downstream of Bonneville Dam (rkm 234); and (2) control fish that were captured at the Adult Fish Facility near the Washington shore fish ladder at Bonneville Dam, and then transported and released 8 rkm downstream of the Bonneville Dam. Fish were confirmed to have survived if they moved upstream and were detected on PIT-tag antennas at or upstream of Bonneville Dam, were recovered at hatcheries or at the dam, or were captured by commercial or sport fishers. Post-release survival estimates were higher for steelhead (89–98 percent) than for Chinook salmon and coho salmon (50–90 percent; Washington Department of Fish and Wildlife, unpub. data, 2014). However, some Chinook salmon and coho salmon return to hatcheries, or spawn in the mainstem Columbia River and in tributaries downstream of Bonneville Dam. The proportion of Chinook salmon and coho salmon in the treatment group that were destined for areas downstream of Bonneville Dam likely was higher than in the control group because the control fish were collected as they were attempting to pass the dam. If this assertion was true, mortality would have been overestimated in these studies, so WDFW developed a study plan to determine the post-release movements and intended location of Chinook salmon and coho salmon collected with beach and purse seines in the lower Columbia River.