and (4) our suggesting a framework for others to do similar analyses when conditions change, when new information is available, or with different values attributed to various outcomes and costs.
An intractably large number of threats to Atlantic salmon have been identified. For example, a Canadian group of experts recently identified 63 factors threatening the survival of Atlantic salmon in eastern North America (Cairns 2001). No feasible amount of time and resources could be enough to understand and mitigate such a large number of threats. Fortunately, we do know that some threats are more important than others, and furthermore, some threats must be mitigated before others can be addressed. For example, if barriers prevent salmon from ascending a river, then those barriers must be made passable before improving habitat above them could be of any use.
Many documents and presentations read and heard by the committee gave the impression that perhaps the biggest difficulty in knowing how to rehabilitate salmon is seeing the forest for the trees. What are the most important things to do and which of them should be done first?
To approach a solution to this problem, the committee developed a conceptual framework or risk-assessment model for thinking about it that involved identifying and ranking the threats and their contribution to salmon mortality. This framework considers a range of issues that apply across the watersheds in Maine where Atlantic salmon could potentially be restored. However, the committee has not considered in detail mitigation options for the significant issue of at-sea mortality because the committee recognizes the large knowledge gap in being able to ascribe causation. (The hatchery living gene-bank program at Maine’s Craig Brook Fish Hatchery is in part an ocean mitigation program. The parr are raised to adulthood in the freshwater of the hatchery, rather than having to become mature in the sea, where survivorship is very low.) The committee acknowledges the importance of at-sea mortality as a threat factor and strongly supports the need for further research to better understand mechanisms and possible remedial measures. The committee similarly has not attempted to evaluate the range of responses to potential threats that could be induced by climate change, because that issue is much larger than conservation planning efforts in Maine can reasonably address.
As noted earlier, the committee’s initial work focused on understanding the genetic status of Atlantic salmon in Maine (NRC 2002a) in response to its charge. At the same time, the committee was gathering, organizing, assimilating, and discussing a wide and diverse range of pertinent data and information. Inevitably, we retraced the path of earlier