ered salmon originating from rivers in Quebec, the Canadian Maritime Provinces, and New England. Cairns’s (2001) assessment was done before a workshop was held to develop research strategies. The deliberations and conclusions of that workshop were summarized by O’Neil et al. (2000). A separate report covers the potential causes of low salmon returns to Newfoundland and Labrador (Dempson and Reddin 2000).
The group of experts whose efforts were described by Cairns (2001) gave each factor a numerical score between 0 and 1 for its magnitude (proportion of habitat affected times degree to which the factor constrains survival or reproductive output) and its trend (positive numbers for increasing mortality or constraint on reproductive output and negative numbers for the reverse). Those two numbers were multiplied and the product was plotted. Five factors were ranked highest in the following order: (1) post-fishery marine mortality is higher than that assumed by fishery models (thus, the degree to which fishing reduces pre-fishery abundance is overstated); (2) smolt survival is reduced due to fish predation; (3) predation by birds and mammals is high at sea; (4) altered ocean conditions alter migration routes; and (5) bird and seal predation in rivers and estuaries affects smolts and adults. Limited spawning habitat ranked 57th and barriers to spawning migration ranked 60th out of the 63 factors. The low ranking does not mean that the factors are unimportant; it means only that their effects on salmon have not changed in a way that explains the recent declines in salmon populations. Two predictions arising from climate-change projections were listed but not scored.
The highest-ranked factor and two of the next three highest ranked were in the marine environment. The second highest-ranked factor overall was in the estuarine environment. The highest ranked factor in freshwater was ranked seventh overall. This analysis was done for all of eastern North America. Although most of the factors apply in Maine, they are not necessarily of the same rank there.
The primary causes cited by the U.S. Fish and Wildlife Service and the National Marine Fisheries Service (50 CFR 17, 224) to support listing Atlantic salmon as endangered under the Endangered Species Act (ESA) are (1) “Documented returns of adult Atlantic salmon within the DPS [distinct population segment] range are low relative to conservation escapement goals,” and (2) “densities of young-of-the-year salmon and parr remain low relative to the potential carrying capacity. These depressed juvenile abundances, where not supplemented by stocking, are a direct result of low adult returns in recent years.”
The services concluded that the threats contributing to the danger of extinction of Atlantic salmon in Maine posed by low adult return and depressed juvenile abundance are (1) predation or disease—potential for