Identifying critical habitat for anadromous fish is difficult for several reasons. First, the location of critical habitat changes from year to year. Because a different subset of the stream network is available to fish each year due to variations in stream flow, different habitats perform critical functions in any year. Another difficulty is that the importance of a particular habitat cannot be directly determined through fish surveys. Large numbers of fish a stream segment does not necessarily indicate critical habitat or even suitable habitat. Fish may become concentrated in poor habitat when high and moderate quality habitat are lacking or fully occupied. Identification of important productive ("source") habitats is also complicated by migration of fry and juveniles away from their spawning and initial rearing areas. Although many areas of the watershed may produce large numbers of fry, the fish that actually survive to complete their life cycle may come only from certain areas. Identifying these source habitats is difficult. Despite the undesirable degree of uncertainty caused by these difficulties, there is a need to identify and protect areas that are most likely to function as critical habitat during many years.

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