Marine mammal predation has also increased sharply in recent years. For decades, populations of marine mammals were depressed in the wake of significant human harvest. But populations of marine mammals have increased dramatically on the West Coast since passage of the Marine Mammal Protection Act in 1972.47 By some accounts, the populations are increasing at 10% a year, and doubling every eight years.
Seals and sea lions are major predators which consume vast numbers of salmon.48 These animals prey primarily on returning adult salmon but also consume large numbers of juveniles and subadults.49 Seals and seal lions follow migrating adults well into freshwater environments. A population of 10,000 harbor seals (the near-Columbia River population estimate) would consume 1.8 million pounds of salmon in a year. This is over 60 percent of the 2.8 million pounds landed in the 1990 Oregon troll fishery.
With respect to sea lions, a 10 percent diet of salmon is a conservative estimate. At 25 pounds of fish per day for northern sea lions (2.5 pounds of salmon) and 15 pounds of fish per day for California sea lions (1.5 pounds of salmon), the seasonally migrating California sea lions and the resident northern sea lions could consume 851,000 pounds of salmon per year. This is about 30 percent of the 2.8 million pounds of salmon landed in 1990.50
In 1994, forty percent of the adult salmon showing up at Bonneville Dam had marine mammal bites or scars on them. Only twenty percent had the marks at Lower Granite Dam.51 This suggests, not surprisingly, that the fish bitten by seals are not surviving as well as they migrate upstream—deaths that are attributed by fishery agencies to the effects of dams.
Some scientists believe that marine mammals are not a major factor in the decline of salmon, principally because salmon and marine mammals coexisted for thousands of years before the current declines in salmon. But thousands of years ago, conditions were cooler and much more favorable for salmon. And there were large marine predators on marine mammals. Today, the most significant predators, human beings, cannot lawfully kill marine mammals.
48 P2, Vol. 30, 91-25/0997 at 96-97
49 P3, Vol. 11, AF3-0168 at 18.
51 BPA, “Interim Research, Monitoring, and Evaluation Program to Support the FCPRS Biological Opinion and Recovery Plan”, at 37 (Nov. 15, 1995 Draft).
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