Carrying Capacity and North Pacific Salmon Production: Stock-Enhancement Implications

Ocean-ranching programs in Asia and North America are supplementing, or have replaced, the natural production of some salmonid populations feeding and maturing in the North Pacific Ocean. These activities, coupled with favorable survival conditions, have raised many stocks to historical or near historical highs during the last decade and a half. Studies of ocean forage production and the response of nekton populations to changes in levels of forage demonstrate multi-year and interdecadal variability attributed to largescale atmospheric cycles. The period since the most recent regime shift favored salmon production in Alaska waters, but not off Washington and Oregon. We present modeled estimates of the forage demand placed on coastal and oceanic feeding areas by wild and ocean-ranched pink salmon originating from Prince William Sound, Alaska, as a case history. Annual food consumption for these stocks rose from less than 100,000 mt prior to 1976 to more than 300,000 mt after 1988, when hatchery production began dominating adult returns. Food demand was distributed nearly equally between survivors and nonsurvivors, and most of the food consumption occurred in the oceanic rather than coastal environments. Because, for all species, adults that return at smaller than average size are less fecund and produce smaller eggs and fry, increased competition for food resulting in smaller body size could influence the survival of progeny, with serious consequences for both wild and hatchery populations. Studies suggest that competition can result from increased salmon production, diminished forage production, and decreases in the size of ocean feeding domains.