Length‐ and Age‐Selective Fishing on Chinook Salmon of the Columbia River Basin, 2002–2012

Fisheries can influence the size and age of exploited stocks if they selectively remove fish. In this study, we quantified length and age selectivity of the commercial and recreational fisheries on upriver spawning stocks of Chinook Salmon Oncorhynchus tshawytscha in the lower Columbia River from 2002 to 2012. Length and age data were sampled from fish (hatchery and wild individuals together) that were caught by the fisheries and from fish that escaped and then entered a trap in a dam immediately upstream of the fisheries. We found that age‐ and length‐based standardized selection differentials (SSDs) were generally small compared with other fisheries and varied over time. Across all years, fish that were caught were, on average, 0.006 years older and 0.19 cm longer than fish in the total return. These results were consistent with the low harvest rates over the 11 years (recreational fishery harvest rate = 4.9%, commercial fishery harvest rate = 3.7%). However, in some years the SSDs during certain fishing periods or overall were larger than in other years and harvested fish were significantly older and longer than the average fish in the total return. For most years and periods, these larger SSDs appeared to be driven by higher harvest rates by recreational fishing gear (hook and line). The SSDs were influenced by how many Chinook Salmon were available to be caught when and where fisheries were occurring, the size and age distribution of those fish, and the selectivity of the gear. We conclude that the SSDs for contemporary lower Columbia River Chinook Salmon fisheries have generally been small in magnitude and variable over time, though in some years and periods the fisheries caught larger‐ and older‐than‐average fish. Thus, the 2002–2012 lower Columbia River fisheries likely had a low influence on the fitness or potential changes in length and age at maturation of upriver Chinook Salmon. Received December 16, 2015; accepted March 30, 2016 Published online July 19, 2016