Modeling Chinook Salmon Run Timing in the Gulkana River, Alaska Using Environmental Indices

Abstract

Environmental data from the Gulkana River, in conjunction with in-season Chinook Salmon Oncorhynchus tshawytscha passage numbers, are among the many tools used by the Alaska Department of Fish and Game to assess inseason run strength and timing in the upper Copper River drainage. In this study, we refine the modeling of Chinook Salmon run timing by comparing methods based on linear, exponential, and polynomial regressions of marine and freshwater environmental indices for the Gulkana River stock. A range of temporal subsets and measurement techniques for coastal upwelling, air temperature, river discharge, and water temperature was analyzed in a stepwise process to update the in-season forecast model. Ultimately, Gulkana River water temperature proved to be the most important variable in predicting run timing. A model considering daily maximum water temperature and river discharge can accurately estimate the proportion of the total run that swam upriver before July (pseudo-R 2 = 0.97), allowing for efficient Chinook Salmon fishery management through the latter portion of the run. Recent coastwide reductions in Chinook SalmonOncorhynchus tshawytscha runs that support multiple fisheries across Alaska make the careful regulation of their harvest even more important. Several stocks throughout the state are classified as "stocks of management concern" due to a chronic inability to maintain escapements for the stocks despite the use of specific management measures (Lingnau and Bergstrom 2003;Kent and Bergstrom 2006; Fair 2018a, 2018b). When returning salmon runs fail to meet the lower escapement bounds, allowable catch