Hundreds of Pacific Salmon Populations Now Extinct

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Scientists conservatively estimate that well over one-quarter of native Pacific salmon populations spawning in rivers and lakes from California to southern British Columbia have gone extinct.

From what they've gleaned out of archaeological reports and historical accounts left by explorers, surveyors and early settlers, researchers at the US National Marine Fisheries Service figure there were 1383 Pacific salmon populations in the western contiguous United States before Europeans arrived. Each population is a geographically cohesive group of fish that essentially does not spawn with any others, rendering it genetically isolated and distinctive from all other salmon. Over time, the individual populations have adapted to their particular environment.

Since the year 1770, a total of 406 or 29% of these genetically-distinct populations have disappeared. More than one-third of the remaining 977 populations are listed as threatened or endangered under the US Endangered Species Act.

Certain ecological regions, salmon species and life-history types have been more prone to extinction. Geographically, extinctions are proportionally fewer in coastal drainages, particularly in northern Washington and southern BC, compared with interior watersheds. Coastal rainforest areas, of the Olympic Peninsula and western Vancouver Island have their 180 original salmon populations largely intact, with a 3% loss.

Mainly due to dam building and other human alterations of aquatic habitat, migrating salmon are gone entirely from the Columbia River headwaters in BC, a total of 26 populations of sockeye, steelhead and chinook. The mid and upper sections of the Columbia River and its tributaries have experienced heavy losses entailing over half the salmon populations, including all coho stocks.

The upper Snake River watershed in Idaho is also now devoid of salmon, having lost its 51 populations of sockeye, steelhead and chinook. Farther south, California's Central Valley is missing 57% of its original Pacific salmon populations including all coho, chum and pink, most of the chinook and half of the steelhead.

Sockeye salmon have endured the greatest losses, amounting to 47% of the species' populations. Other salmon populations heavily dependant upon freshwater have also suffered. Populations of chinook that mature in fresh water rather than the ocean are depleted by 54%. Similarly steelhead populations with the same freshwater-oriented life history have been reduced by 31%.

In contrast, the ocean-maturing cohorts of these species have lost around 20% of their populations. Pink salmon with population extinctions of 18% and chum of 21% are the species least affected overall.

These extinct populations represent substantial reductions in genetic diversity. Scientists estimate that 27% of the genetic diversity in Pacific salmon from the western contiguous United States is now gone. For instance, pink and chum no longer spawn in the southernmost portion of their range in California. Also lost are the salmon that used to migrate farther than any others in the western contiguous US: coho and chum of the Columbia River basin.

Reference

Richard G. Gustafson, Robin S. Waples, James M. Myers, Laurie A. Weitkamp, Gregory J. Bryant, Orlay W. Johnson and Jeffrey J. Hard. 2007. Pacific Salmon Extinctions: Quantifying Lost and Remaining Diversity. Conservation Biology. 21(4): 1009-1020.

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