Roni, Philip. 1992. Life history and spawning habitat in four stocks of large-bodied chinook salmon (Oncorhynchus tshawytscha). M.S.
Chinook salmon (Oncorhynchus tshawytscha) reach the largest size of Pacific salmon and exhibit great variation in life history. Variation is found in juvenile residence in freshwater, age at maturity, siz at age, ocean migration patterns, migration time, and spawning time both within and between stocks (Ricker 1972; Healey 1991). Chinook are generally categorized as either ocean-type or stream-type based on the residence patterns of juveniles (Gilbert 1913; Healey 1983; Taylor 1990a, b). Ocean-type chinook migrate to sea as newly emerged fry or after 2-3 months of freshwater residence; stream-type migrate to sea after spending a year or more in freshwater (Taylor 1990a). Ocean-type chinook return to their natal streams in the fall a few days or weeks before spawning, and stream-type chinook return in the spring or summer in some cases several months before spawning (Healey 1991).
North American chinook salmon have experienced dramatic declines in size and age an maturity over the past five decades. The average size of chinoook salmon taken in fisheries has declined by over half and the average age at maturity declined by more than a year since the 1930s (Ricker 1980; Hankin and McKelvey 1985a). The maturation schedule and ocean distribution of chinook makes them susceptible to exploitation by commercial and sports fisheries for longer periods of time than other species of Pactific salmon (Ricker 1980). Age at maturity and growth rate have been shown to be partly heritable in chinook salmon (hard et al. 1980), and there ahs been selection against fish that mature at an older age, fish of greater size, and fish with specific migration patterns (Ricker 1980). At least 64 stocks of chinook salmon in the lower 48 states are in danger of extinction and more stocks of chinook have been extirpated than any other species of Pacific salmon (Nehlsen et al. 1991).
In spite of these declines in size and age at maturity, some stocks still produce individuals of exceptional size. Kenai River late run chinook sampled in the sport fishery average about 18 kg and can exceed 40 kg (Hammarstrom 1990). In the Kitsumkalum River, a tributary to the lower Skeena River in northern British Columbia, 25 to 30 kg chinook are common and fish over 40 kg are recovered in tagging and escapment operations conducted by Department of Fisheries and Oceans (DFO) every year (Les Jantz, DFO, pers. comm.). While some data exists on individual stocks of large-bodied chinook, no study has compared the life histories or habitats of stocks of large chinook to understand why these rivers produce fish of such large size when stocks in neighboring watersheds appear to be of rather typical proportions.
The sports fisheries for "trophy" chinook contribute millions of dolllars to the economies in Alaska and British Columbia, and the general sports fishery for chinook contributes millions more to the economies in the U.S. and Canada. The Kenai River alone received over 415,000 angler hours of effort directed at chinook salmon (Hammarstrom 1990). Chinook from the Kitsumkalum and Wannock Rivers also support major sports fisheries and are destinations for anglers trying to catch a chinook salmon over 30 kg.
The purpose of this study was to characterize the life history and spawning habitat of selected stocks of large chinook to elucidate factors responsible for the selection for large size in chinook. This was done through comparison of age structure, size at age, fecundity, freshwater residence patterns and marine catch distribution of four stocks of large-bodied chinook with coastwide patterns and marine catch distribution of four stocks of large-bodied chinook with coastwide patterns for the species in North America. Physical features of the spawning habitat were also surveyed and assessed including water depth and velocity, substrate size, migration distance, temperature and flow regime.