Length and Age Trends of Chinook Salmon in the Nushagak River, Alaska, Related to Commercial and Recreational Fishery Selection and Exploitation

Long-term trends in fish age and size at maturation provide important insights into population stability as they are affected by changes in growing conditions. Fishery exploitation and selection can also induce such trends. It is important to identify the causes of changing life history patterns because they affect population dynamics and productivity. Average body sizes or ages of many western North American populations of Chinook salmon Oncorhynchus tshawytscha, including the Nushagak River population in Bristol Bay, Alaska, have declined over the past several decades. This population is caught in commercial, recreational, and subsistence fisheries and therefore represents an opportunity to compare fishery exploitation and selection and to examine their possible effects. We compiled data and examined trends in size and age at maturation between 1981 and 2009, and we then quantified commercial and recreational fishery exploitation and selection rates. The average age and lengths at most ages of maturing fish decreased over time. The commercial fishery caught the majority (73%) of the harvested fish annually compared with the recreational (9%) and subsistence fisheries (18%). Size selection by the commercial fishery was highly variable over time, but overall, smaller-than-average fish were caught, inconsistent with the declining trends in age and size of returning fish. In contrast, the recreational fishery and presumably the subsistence fishery regularly removed larger-than-average fish, consistent with the trends. However, the opposing selection by the commercial fishery suggests that the trends in age and size at maturation are unlikely to have been caused solely by fishery selection. Rather, the trends are probably also related to environmental changes affecting fish growth. Still, regulations that decrease either the allowable size of harvested Chinook salmon or the allowable number of large fish may help to decrease the size selectivity of the Nushagak River recreational and subsistence fisheries.