Seafood Watch

Seafood Watch® Wild Pacific Salmon Report October 8, 2010
and fishing to exterminate or endanger salmon runs. These habitat losses have been particularly
evident in the southern half of salmon’s North American range.
Second, salmon have a highly developed “homing” instinct, generally returning to the specific
lakes and rivers of their birth. This homing instinct is a fundamental component of salmon
biology, and is largely responsible for the formation of discrete populations. While the degree of
homing (and its opposite, “straying”) varies across species and locations, homing creates
reproductive isolation and helps to facilitate localized adaptation (Stewart et al. 2003). The
combination of isolation and adaptation has led to the evolution of numerous Evolutionarily
Significant Units (ESUs), which are treated under Endangered Species Act legislation as separate
species. As a consequence of the diversity of Pacific salmon ESUs, the loss of local populations
increases the chance of losing overall genetic diversity.
Third, Pacific salmon populations are subject to natural fluctuations that can increase their
vulnerability. Salmon populations are strongly influenced by changing atmospheric–oceanic
conditions on a number of different temporal scales. Changes in climate affect oceanic structure
and can generate significant and often sudden differences in salmon marine survival and returns
(Francis and Hare 1994). These include both the subdecadal variability of the El Nino Southern
Oscillation (ENSO) and the longer-scale (50-70 years) climate oscillations that have operated
over the North Pacific for at least the past three centuries.
Over the long-term, sediment cores indicate that sockeye salmon populations have undergone
significant swings during the past two millennia (Finney et al. 2002). For example, populations
were depressed from ~ 100 BC to AD 800, but consistently higher from AD 1200 to 1900.
Similarly, Bristol Bay sockeye salmon have undergone several major shifts over the past three
centuries (Finney et al. 2000). In the medium-term, regime shifts in the subarctic and California
Current ecosystems associated with the Pacific Decadal Oscillation (PDO) have strongly
influenced salmon productivity. A regime shift during the late 1970s (and again in the late
1980s) appears to have reduced oceanic survival of salmon in the Pacific Northwest, while
increasing oceanic survival in Alaska (Hare et al. 1999, Tolimieri and Levin 2004). Hilborn,
Quinn et al. (2003) note that “the productivity of Alaskan sockeye salmon populations appears to
be among the more sensitive biological systems that respond to interdecadal climate shifts and is
strongly coherent with changes in the Pacific Decadal Oscillation.”
In the near-term, smaller scale environmental conditions have significant effects on salmon
population variability. One recent study documents that early marine survival of three species of
salmon from Washington to Alaska is strongly influenced by sea surface temperature (SST)
within a few hundred kilometers of the stock’s natal stream (Mueter et al. 2002). SST is likely a
proxy for changes in ecological interactions in the marine realm. The authors found that survival
of pink, sockeye, and chum salmon was strongly affected by the oceanic processes related to
SST, and that these effects were consistent across species. Interestingly, it appears that water
conditions during the salmon’s first few months at sea have a greater influence on salmonid
survival than larger-scale variability associated with the PDO. Complicating the management
picture, stocks of even the same species may react in a non-uniform manner to changing climatic
conditions (Tolimieri and Levin 2004).
While these fluctuations demonstrate that shifts between productivity regimes occur outside of
the influence of anthropogenic factors, human pressures can add to the natural instability facing
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