Differences in dynamic response of California Current salmon species to changes in ocean conditions

Abstract

Abstract While changes in the northeast Pacific Ocean in the mid-1970s apparently caused changes in salmon population growth in the Gulf of Alaska and the California Current, the responses of California Current salmon species, coho salmon (Oncorhynchus kisutch) and chinook salmon (O. tshawytscha) differed. Coho salmon catches declined dramatically along the coasts of California, Oregon and Washington, while chinook salmon catches did not. This provides an opportunity for comparative analysis, a rarity in the study of long-term changes in the ocean. Here we test one possible explanation for that difference, that chinook salmon populations are inherently more persistent because chinook salmon populations spawn over a range of ages, while coho salmon spawn predominantly at age 3 yr. We extended a previous theoretical approach that had been used to assess the long-term response of salmon populations with various spawning age structures to different means and variances in environmental variability. New results indicate that populations with environmental variability at the age of return to freshwater have the same characteristic identified earlier for populations with variability in the age of entry: populations spawning at multiple ages are more persistent, but that increased persistence is gained in the first few percent of departure from all spawning at a single age. Thus, in both cases the results are too sensitive to values of uncertain parameters to depend on as an explanation of the differences in response. We also approached this question by subjecting model populations with coho and chinook salmon spawning age structures to an empirical estimate of actual marine survival of coho salmon over the years 1970–2002, asking the question, if chinook salmon had been subjected to the same ocean survivals would they have experienced the same decline. The differences in spawning age structure made little difference in population responses. The dominant factor influencing the response of these species to a decline in ocean survival was the behavior of the freshwater spawner/smolt relationship at low abundance, a factor that has recently been intensively studied for coho salmon, but is poorly known for chinook salmon. These results suggest that the GLOBEC NEP should focus attention on the ocean phase of salmon life, to explain the observed difference in population response to changes in physical conditions.