Expert Report

Each report is produced by a committee of experts selected by the Academy to address a particular statement of task and is subject to a rigorous, independent peer review; while the reports represent views of the committee, they also are endorsed by the Academy. Learn more on our expert consensus reports.

Pervasive and substantial decline of Atlantic salmon populations in Maine over the past 150 years have taken the species close to extinction. Consequently, comprehensive statewide action should be taken now to ensure their survival. The populations of Atlantic salmon have declined drastically, from an estimated half million adult salmon returning to U.S. rivers each year in the early 1800s to perhaps as few as 1,000 in 2001. It has been determined that blocked river passage up and down stream and habitat degradation are the main reasons for Maine's salmon population decline. This report recommends implementing a formalized decision-making approach to establish priorities, evaluate options, and coordinate plans for conserving and restoring the salmon.

Key Messages

  • A rich and complex network of governance institutions in Maine influences how humans affect salmon. As is often the case with complex environmental problems, more information is needed on how well governance institutions are working together, and whether the government authority is sufficient to develop and implement effective recovery programs.
  • At the next level of importance, salmon farming has the potential to adversely affect salmon populations in Maine genetically and ecologically and might already have done so. Over the long term, hatchery supplementation of salmon populations in Maine is also likely to have deleterious genetic and possibly ecological effects.
  • Because populations of wild salmon in Maine are so low, the mortality associated with research and monitoring could be problematic.
  • Current agricultural practices, including forestry, do not appear to be an important problem for Atlantic salmon in Maine, although their effects should be monitored, especially for erosion, reduction of vegetation cover, and water withdrawals.
  • Dams obstruct adult and juvenile salmon passage and alter riverine habitats, including water quality. As a result, they degrade or eliminate spawning and rearing habitat for Atlantic salmon in Maine. Although dams are not as important a problem on the DPS as on other Maine rivers, they have made an enormous amount of habitat unavailable to Maine salmon and have affected much of the habitat that is still available.
  • Despite some uncertainty about the causes of the excess mortality, the committee concludes that acidification of streams has the potential to be a major impediment to the increase of salmon populations in Maine by contributing to that mortality.
  • Despite the extensive additions of nonnative hatchery and aquaculture genotypes to Maine's rivers, the evidence is surprisingly strong that the wild salmon in Maine are genetically distinct from Canadian salmon.
  • Fishing is currently prohibited; therefore, it is not an important problem for Maine salmon.
  • Human activities that directly or indirectly threaten salmon include dams and hydropower projects, Atlantic salmon aquaculture, water extraction for agriculture, fishing, hatcheries, logging, road construction, development of land sites, acidification of their streams, and research.
  • North American Atlantic salmon are clearly distinct genetically from European salmon.
  • Predation and changes in oceanic conditions could be serious problems for salmon.
  • The pattern of genetic variation seen among Maine streams is similar to patterns seen elsewhere in salmon and their relatives where no stocking has occurred
  • There is considerable genetic divergence among populations in the eight Maine rivers where wild salmon are found.
  • There is no reason to believe that the harm to wild fish that has been documented elsewhere could not occur in Maine.
  • Wild salmon in Maine do not reflect only (or even mainly) the result of decades of hatchery stocking. It is not possible to say whether or to what degree the genetic differences reflect adaptation to local conditions as opposed to random processes associated with small population sizes or some influence of stocking.