Prince William Sound herring : an updated synthesis of population declines and lack of recovery
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Prince William Sound herring : an updated synthesis of population declines and lack of recovery

Filetype[PDF-2.94 MB]


  • Alternative Title:
    Exxon Valdez Oil Spill Restoration Project final report
  • Description:
    The PWS herring population collapsed 4 years after the Exxon Valdez oil spill, igniting debate about the cause. Fishermen who once depended on this stock for income and some investigators are convinced that the spill was causal, others are not. Our re-examination of the data demonstrates that polynuclear aromatic hydrocarbons (PAH) are highly toxic and that the oil spill significantly damaged herring embryos in 1989. These effects were no longer detectable after 1990 and strong recruitment of the 1988 year-class (in 1991) marked population recovery from the direct toxic effects of the spill. No plausible oil-related mechanisms have been developed to explain a delayed response after intervening years of no response. By 1993, recruitment to an expanding population (plus an additional 1 to 2% because the stock was not fished in 1989) helped precipitate a catastrophic disease outbreak. Epidemiological analysis identifies three significant risk factors for the 1993 population crash: 1) relatively large biomass from 1988 to 1992 (i.e., a susceptible host); 2) relatively low zooplankton production in 1991 and 1992 (i.e., environmental conditions contributing to poor overwinter condition); and 3) the presence of disease (VHSV and filamentous bacteria). Timing of the population collapse was questioned by some, who extrapolated hydroacoustic data (a time series that began in the mid-1990s) to dates earlier than sampled and suggested the collapse began in 1989, adding fuel to the controversy that the oil spill was linked to the collapse. However, this particular hindcast fails to explain the observation of lethargic survivors with external hemorrhages in 1993 and is inconsistent with a competing age-structured model. Although linkage of the 1993 collapse with the oil spill cannot be proved or disproved with certainty, reasons for poor recovery since the collapse remain perplexing. Natural factors, including climate, inter-species competition, suboptimal recruitment, condition prior to entering the winter starvation period, disease, and predation may be important. Disease measurements through 2002 continued to indicate the population was restricted by chronic disease; recruitment was negatively affected by VHSV and life spans were shortened by Ichthyophonus, although reasons why disease continues to cycle in the population are unresolved. Aside from limited measurement in a reference population (SOutheast Alaska), comparable disease measurements in other fish populations do not exist. The PWS herring population is not genetically discrete within the Gulf of Alaska. Thus, genetic diversity within the population and exchange with surrounding populations is adequate and does not explain the lack of population recovery, despite the sudden population collapse in 1993. Continued disease cycles may be the most parsimonious reason why the PWS herring population has failed to recover in the past 13 years but the root causes for this are unknown. The resulting lack of recovery is more than likely the contribution of several factors, not just one factor.
  • Content Notes:
    S.D. Rice and M.G. Carls.

    "September 2007."

    "Restoration Project 050794 final report."

    Also available in electronic format via Internet.

    Includes bibliographical references.

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    Public Domain
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