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Chapter 5 -- Anadromous Salmonids and Their Habitats (continued)

Sequence of Habitat Needs for Anadromous Life Cycles (continued)

Juvenile Summer Rearing

Over time, habitat preference of juveniles shifts to deeper areas. As the fish grow larger, they are less capable of avoiding predation by escaping into spaces between rocks. Their habitat preference shifts from stream margins and backwater areas to pool habitats. Stream sections with overhanging vegetation and undercut banks are favorable summer rearing habitat because of protection from predation by birds. As juveniles increase in size, they prefer pools where they can more easily avoid being eaten by larger fish. Juvenile salmonids in the Smith River watershed are subject to predation by coastal cutthroat trout (Oncorhynchus clarki clarki), river otter (Lutra canadensis), common merganser (Mergus merganser), great blue heron (Ardea herodias), kingfisher, and Pacific giant salamander (and "snakes"?) (Reedy 1995, Scriven personal communication 1997). Predation risk from birds is greater in aggraded areas because of the lack of deep places to hide in. A source of juvenile mortality in some tributaries is stranding as flows decline during the summer.

High water temperatures that are lethal to salmonids are infrequent in the Smith River network (Tables 32, 33, and 34, McCain et. al 1995, Waldvogel personal communication 1996). Furthermore, cold water refuges are likely to be available in most cases. Areas of colder water can be found in side channels where cold water comes in from tributaries and where groundwater emerges into the stream. As stream flows decrease, these areas presumably increase in importance.

Although water temperatures are usually tolerable, sub-lethal stream temperatures can have significant effects. For example, areas of high but tolerable water temperatures may change migration timing of juveniles. Reedy (1995) noted that juvenile chinook in the Smith River rear in freshwater for a longer period when water temperatures are closer to optimal. Therefore, when water temperatures are favorable, juvenile chinook reach a larger size before entering the ocean. Because they are larger, their survival rate in the ocean is higher.

Although production of fry varies widely from year to year, variability in abundance tends to decrease as the cohort ages because density-dependent population controls (e.g. food and space) take effect. As the fry grow, density-dependent factors limit the population so that the number of smolts migrating from the system may be similar regardless of the number of fry and juveniles. Therefore, stream restoration projects which increase early rearing habitats may allow a larger number of newly hatched fry to survive and yet have insignificant effects in the bigger picture of the cohort as a whole (McCain et al. 1995).

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