logical and economic). Replicated across several sites, the scientific method supplants well-intentioned trial and error as an efficient and systematic way of improving conservation and restoration efforts.

The following sections deal in more detail with specific threats.

As described in Chapter 3, dams block passage and later riverine environments both below and above them. Mitigating the threat they pose is usually most completely achieved by removing them, but enhancing passage alone can be at least somewhat effective if they affect only short stretches of river. Mitigating their effects has been discussed in more detail in NRC (1996a) and Heinz Center (2002). The decision analysis example on enhancing habitat in Chapter 4 and the discussion of the costs of dam removal at the end of this chapter provide additional information on addressing the threats to dams, as does the summary of the 1997 Conservation Plan (Maine Atlantic Salmon Task Force 1997) toward the end of this chapter.

At this stage in the decline of wild populations of Atlantic salmon in the state of Maine, the goals of hatcheries need to be explicit. The recent steep declines in salmon numbers, in spite of increases in hatchery production and the very recent change to river-specific stocking, mean that efforts need to be concentrated on rebuilding wild populations in Maine’s rivers. It is helpful to specify immediate goals aimed at dealing with the current extinction crisis as well as ongoing goals that would continue to apply even as signs of rebuilding are seen. It would also be helpful to adapt earlier assumptions and goals to current conditions and scientific knowledge.

The goal of hatcheries in response to the extinction crises in Maine should be to conserve genetic quality—a broad term that includes the concepts of genes adapted to local conditions, complementary and coadapted genes, and appropriate genetic diversity—in the remaining wild populations of Atlantic salmon, allowing these survivors to persist. In this respect, the hatcheries might serve as living gene banks. The operation of the Craig Brook National Fish Hatchery is compatible in part with this