Salmon Abundance Climate And Other Influences

Pacific salmon are anadromous fish that spawn in streams from California's Central Valley northward. In spring, juvenile salmon emerge from the freshwater environment and disperse into the coastal ocean. Some salmon stocks remain in coastal areas throughout their lives, but many others spend a year or more in a long-distance migration across the feeding grounds of the subarctic Pacific before returning to their natal streams to spawn and to die (Pearcy, 1992). There are five species of Pacific salmon, with a multitude of distinct breeding populations. All five species (chinook, coho, sockeye, pink, and chum) are present from Washington state northward, while in Oregon and California only chinook and coho spawn in significant numbers.

2 SALMON ABUNDANCE: CLIMATE AND OTHER INFLUENCES 853

In the mid-1970s, ocean conditions in the North Pacific changed dramatically. Shortly thereafter, Alaskan salmon harvests entered a period of dramatic increase, rising nearly 10-fold fi-om a low of 22 million salmon (of all species) in 1974 to three successive record highs in 1993, 1994 and 1995 (Fig. 1). At the 1995 peak, Alaska harvested a total of 217 million salmon. Harvests of most salmon species in northern British Columbia also fared well during this period, although British Columbia's commercial chinook harvests have declined steadily, and by the late 1990s it became apparent that many of British Columbia's southern and interior coho stocks are severely depleted. Southward, salmon harvests have been on a roller-coaster. Commercial chinook and coho catches in California, Oregon, and Washington dropped abruptly in the late 1970s, hitting El Niño-related lows in 1983 and 1984. A dramatic but brief recovery in 1986 and 1987 then gave way to a precipitous decline to record low harvests in recent years (Fig. 2). Production has declined to the point that some stocks are on the verge of extinction. Some observers attribute these changes in salmon productivity to human disruptions of the southern fisheries and good management of the northern ones (Royce, 1988). However, mounting evidence suggests that shifts in marine climate may have played a major role (Beamish and Bouillon 1993; Hare and Francis 1995; Mantua et al„ 1997).

Winter climate over the North Pacific is dominated by a low-pressure system centered near the Aleutian Islands. From 1977 through 1988, the Aleutian Low was frequently deeper than normal, leading to severe storms, increased mixing, and cooler temperatures in the central North Pacific (Trenberth and Hurrell, 1994) (Fig. 3). Along the west coast of North America, sea surface temperatures (SST) were unusually warm during the 1977 to 1995 period (Fig. 4).

Alaska Commercial Catch

Alaska Commercial Catch

15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

Year

Figure 1 Alaskan commercial salmon harvest—all species.

15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

Year

Figure 1 Alaskan commercial salmon harvest—all species.

Commercial Catch in Washington, Oregon, and California (Millions of fish)

Figure 2 Commercial coho and chinook Salmon harvest in Washington, Oregon, and California, millions offish.

Year

Figure 2 Commercial coho and chinook Salmon harvest in Washington, Oregon, and California, millions offish.

Possible causes of the change are the subject of much research, in the last century, the North Pacific climate varied on an mterdecadal scale, with shifts or trends in mean levels of sea-level pressure and SST that lasted for several decades (Zhang et al., 1996; Latif and Bamctt, 1996). The pattern is characterized by alternate warm

DJF1977-1988

TAnom Base Period 1951-1980 CC)

DJF1977-1988

TAnom Base Period 1951-1980 CC)

Figure 3 Twelve-year (i977—1988) average winter surface temperature anomalies (°C) shown as departures from the 1951-1980 mean. Gridded temperature data consists of air temperatures over land and sea surface temperatures over oceans (IPCC 1992; Trenberth et al., 1992). Figure courtesy of James Hurrell.

Figure 3 Twelve-year (i977—1988) average winter surface temperature anomalies (°C) shown as departures from the 1951-1980 mean. Gridded temperature data consists of air temperatures over land and sea surface temperatures over oceans (IPCC 1992; Trenberth et al., 1992). Figure courtesy of James Hurrell.

2 SALMON ABUNDANCE: CLIMATE AND OTHER INFLUENCES

Seasonal Temperatura Anomalies (dag C)

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(b) California Current

(c) Cenital North Pacific lüilltu íllh Ü, jl1 J i

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Figure 4 Seasonal surface temperature anomalies in three regions of the northeast Pacific: (a) Gulf of Alaska, (A) California current, and (c) central North Pacific. (Temperature data as described in Fig. 3.)

and cool periods in a large area of the western and central north Pacific, with shifts toward warmer temperatures in the mid-1940s and cooler temperatures in the mid-1970s (the eastern edge of this region is area C in Fig. 4). The cool periods in the central North Pacific are associated with an intensification of the Aleutian Low and a warming of coastal temperatures along the west coast of North America.

The North Pacific also is influenced by the El Niño Southern Oscillation (ENSO) phenomenon (Kiladis and Diaz, 1989). ENSO-related warming of the equatorial Pacific occurs intermittently, at intervals of about 3 to 6 years, and frequently leads to intensification of the Aleutian Low. The effects of an ENSO warm event often propagate northward, warming the west coast of North America and cooling SSTs in the central north Pacific. An unususal sequence of closely spaced ENSO warm events have occurred from 1977 to 1995, possibly evidence of a change in large-scale climate (Trenberth and Hurrell, 1994; Trenberth and Hoar, 1996). These ENSO warm events (El Niño events) have tended to reinforce the decadal-scale shift to warmer coastal SSTs and cooler SSTs in the central north Pacific.

Intensification of the Aleutian low and warming of the coastal ocean appear to have positive effects on salmon abundance in the Gulf of Alaska, but negative effects on stocks that spend a portion of their lives in the California current (Pearcy, 1992; Hare et al, 1999). In the subarctic zone, the mixed layer has become shallower. This may have enhanced the survival of Alaskan and northern British Columbian salmon smolts by increasing the productivity of zooplankton, which is a frequent food source for juvenile salmon (Polovina et al, 1995; Brodeur and Ware, 1992). A general pattern of winter warming and increased winter precipitation in Alaska over the past two decades (Mantua et al, 1997) also may have contributed to favorable stream conditions for egg-to-smolt survival. From southern British Columbia southward, El Niño events have been associated with poor feeding conditions for maturing salmon and changes in species composition, including increased abundance of some species that prey on juvenile salmon (Pearcy, 1992). In addition, recent droughts in California and the Pacific Northwest have resulted in poor conditions for spawning and migration in the salmon's freshwater phase. Changes in ocean temperatures and circulation, and associated changes in stream conditions, thus appear to have contributed to the opposite trends in northern and southern salmon production.

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