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

Sequence of Habitat Needs for Anadromous Life Cycles (continued)

Estuary Rearing

Although few investigations of the Smith River estuary have been undertaken, it is known that estuaries along the Pacific coast provide rearing habitat for juvenile anadromous salmonids including chinook, coho, steelhead, and coastal cutthroat trout (Healey 1982, Simenstad et al. 1982, Nicholas and Hankin 1989, Healey 1991). Estuary habitats include flooded salt marshes, mudflats, and tidal channels. Patterns of estuary residence vary greatly between species, fish size, tidal stage, and time of year (Healey 1982, Levings et al. 1986, Macdonald et al. 1987, Nicholas and Hankin 1989). The length of time that a juvenile salmonid resides in the estuary depends on size at entry, availability of preferred prey, river discharge, and estuarine topography (Iwamoto and Salo 1977). Despite variability in estuary use, it is generally agreed that estuarine habitats are important to juvenile salmonids for feeding, physiological adaptation from fresh to salt water, and protection from predation (Moser et al. 1991).

The size of juvenile salmonids when they enter the estuary is important and is influenced by many factors including time of adult spawning, stream temperatures during and after incubation, fry size and condition at emergence from the gravel, population density in rearing areas, food resources, stream discharge, turbidity, tidal cycles, and photoperiod (Simenstad et al. 1982). Studies in estuaries in Washington suggest that smaller juveniles prefer salt marshes and also utilize mudflats (Simenstad et al. 1982). Smaller juveniles were also found in shallow areas in the Sacramento-San Joaquin estuary (Kjelson et al. 1982). In Washington estuaries and the Sacramento-San Joaquin estuary, larger juveniles were most commonly found in deeper channels in the estuaries (Simenstad et al. 1982, Kjelson et al. 1982). In the Campbell River estuary in British Columbia, larger fish were also more abundant in deeper waters with higher salinity, farther from shore (Macdonald et al. 1987). In the Campbell River estuary, the differences in size of fish and habitat were accompanied by differences in diet. Smaller fish ate more food of freshwater origin, while larger fish ate more food of marine origin (Macdonald et al. 1987).

Favorable estuary conditions allow juvenile salmonids to increase in size before entering the ocean. Larger size when entering the ocean results in higher rate of survival in the marine environment. Rearing in the estuary is an essential component of dominant chinook life history patterns. Studies in Oregon and Washington indicate that chinook salmon reside in estuaries for longer periods of time than other salmonids (Healey 1982, Simenstad 1982). Habitat conditions in the Smith River estuary are also believed to be critical for chinook (Waldvogel 1996). Waldvogel (1985, 1988) estimates estuarine residence for juvenile chinook to be three to four months. Although there has been no documentation of residence in the Smith River estuary by coho and chum salmon, estuaries in Oregon and Washington are important for these species (Healey 1982, Simenstad 1982, Durkin 1982, Myers and Horton 1982). This suggests that coho and chum salmon may also reside for some time in the Smith River estuary.

The number of anadromous salmonids reared in the estuary depends not only on quality and quantity of estuarine habitat but also on connectivity between freshwater habitats and the estuary. Connectivity determines the survival rate of juveniles during migration to the estuary. If the mortality rate on juvenile salmonids is too high during migration from spawning or initial rearing areas to the estuary, rearing habitat in the estuary will not be fully occupied (Trush 1995). Mortality during estuary residence is high in some estuaries. For example, in the Nanaimo and Nitinat River systems on Vancouver Island, British Columbia, a maximum of 30% of the estimated downstream migrant coho could be accounted for in the estuary (Healey 1991). Rates and causes of juvenile salmonid mortality in the Smith River estuary are not documented.

Evidence from other estuaries suggests many estuary features are important for anadromous salmonids including mouths of sloughs, submerged vegetation, large rocks, and unvegetated habitat in low tide areas. Most of the food items consumed by juvenile anadromous salmonids in estuaries are highly dependent on habitat and decaying matter supplied by vegetation, including emergent vegetation such as sedges and rushes, as well as riparian shrubs and trees. In the Carnation Creek estuary in British Columbia, 80% of the observed coho smolts were within one meter of large woody debris (McMahon and Holtby 1992). This strongly suggests that large woody debris is beneficial for coho salmon in Carnation Creek estuary. Similarly, large woody debris may be important or potentially important for coho salmon in the Smith River estuary. The importance of habitat features and habitat diversity to juvenile salmonids in the Smith River estuary is an important subject for further research (Levings and Macdonald 1991).


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