The Uncertain Future of Fraser River Sockeye - Publications du ...

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Cohen Commission of Inquiry into the Decline of Sockeye Salmon in the Fraser River • Volume 2 experts were invited to attend the workshop, make presentations, and critically evaluate data and hypotheses about causes for the decline. Other observers attended, bringing total attendance to 68 participants. 26 Following the workshop, the Expert Advisory Panel grouped the possible explanations into nine categories (see Table 2.3.1). The panel rated each of the nine alternative hypotheses in terms of the relative probability or likelihood for which a given hypothesis could explain the Fraser River sockeye salmon situation both in 2009 and over the longer term. The panel concluded that the available evidence for and against each of the nine hypotheses does not point to a single cause of either the poor adult returns in 2009 or the long-term decrease in returns per spawner. The panel agreed that multiple hypothesized causal mechanisms are very likely to be operating simultaneously and that their effects may be additive or multiplicative (i.e., synergistic), or may tend to offset one another’s effects. 27 Table 2.3.1 Expert Advisory Panel’s judgment of the relative likelihood that a given hypothesis contributed to the observed spatial and temporal patterns in productivity of Fraser River sockeye populations Hypothesis Time Period Strength of Evidence 1a. Predation by marine mammals Overall Fair 2009 Fair 1b. Unreported catch in the ocean Overall Good outside of the Pacific Salmon 2009 Good Treaty area 2. Marine and freshwater Overall Fair pathogens (bacteria, parasites, 2009 Fair and/or viruses) 3a. Ocean conditions Overall Fair (physical and biological) inside 2009 Good Georgia Strait 3b. Ocean conditions Overall Fair (physical and biological) outside 2009 Fair Georgia Strait 4. Harmful algal blooms in Overall Fair the Strait of Georgia and/or 2009 Fair northern Puget Sound / Strait of Juan de Fuca 5. Contaminants in the Fraser Overall Poor River and/or Strait of Georgia 2009 Poor 6. Freshwater habitat conditions in Overall Fair the Fraser River watershed 2009 Fair 7. Delayed density dependent Overall Fair mortality 2009 Fair Relative likelihood that each hypothesis caused observed changes in productivity during the indicated time period Very Likely Likely Possible Unlikely Very Unlikely 16
Chapter 3 • Other investigations Table 2.3.1 cont’d Hypothesis 8a. En-route mortality during upstream migration (en-route mortality is already considered in estimates of total recruits, so while potentially strongly affecting spawner abundance, this hypothesis cannot explain declines in recruits per spawner) 8b. The effects of en-route mortality on fitness of the next generation 9. Competitive interactions with pink salmon Time Period Strength of evidence Overall Good 2009 Good Overall Poor 2009 Poor Overall Fair 2009 Fair Relative likelihood that each hypothesis caused observed changes in productivity during the indicated time period Very Likely Likely Possible Unlikely Very Unlikely Notes: The Pacific Salmon Commission explains that these likelihoods are based on evidence presented at its June 2010 workshop (during subgroup discussions) and on panellists’ background knowledge. The top row for each hypothesis reflects conclusions about overall productivity patterns (i.e., over the long term). Shading of multiple cells reflects a range of opinions among panel members. The second row of each hypothesis considers just the 2009 return year. The shading reflects the panel’s conclusion about the degree of importance: black = major factor; grey = contributing factor. The strength-of-evidence column reflects the quantity and quality of data available to evaluate each hypothesis / stressor. Panel members made their best judgments of the relative likelihood of each hypothesis, given the available evidence. Source: Exhibit 73, pp. 9–10. The panel concluded that physical and biological conditions inside the Strait of Georgia during the juvenile life stage are very likely the major cause of poor survival of the cohort that returned in 2009. Those conditions in the Strait of Georgia are also likely the major cause of the long-term decrease in productivity of most Fraser River sockeye stocks that has occurred since the late 1980s or early 1990s. The panel also concluded that similar physical and biological conditions affected survival outside the Strait of Georgia, but to a lesser degree. (However, it lacked certain types of information needed to identify the mechanisms more specifically.) 28 According to the panel, freshwater and marine pathogens (e.g., viruses, bacteria, and/or parasites) are an important contributor to both the poor returns in 2009 and the long-term decrease in productivity. However, there were insufficient data to allow further distinctions among those factors. 29 Only three other hypothesized mechanisms likely or possibly contributed to the declines: • Harmful algal blooms in the southern Strait of Georgia in 2007 were a possible explanation for the poor returns in 2009, and a possible to unlikely explanation of the long-term decline in productivity. • Panellists expressed conclusions ranging from likely to unlikely for the hypothesis that delayed density-dependent mortality (related to the term “over-escapement”; see the discussion below) contributed to the long-term decrease in productivity. • Competitive interactions between pink salmon and Fraser River sockeye were rated as either a likely or a possible contributor to the longterm decline. 30 The panel recommended that a coordinated, multidisciplinary two-phase research program be 17
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Abbreviations and acronyms AAA AARO
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