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COSEWIC assessment and status report on the coho salmon (Interior Fraser population) in Canada

Habitat

Since coho salmon spawn in freshwater and juveniles normally spend one full year in freshwater before migrating to the sea, their survival depends on having adequate habitat in freshwater as well as the ocean. The distribution of spawning habitat for coho salmon is usually clumped within watersheds, often at the heads of riffles in small streams, and in side channels of larger rivers. Females generally construct nests in shallow (30-cm) areas where the gravel is less than 15-cm diameter and has good circulation of well-oxygenated water (Sandercock 1991). Low or high flows, freezing temperatures, siltation, predation, and disease can reduce egg survival. Major episodes of fry dispersal include spring movements away from spawning sites (Chapman 1962; Gribanov 1948) and pre-winter movements into small tributaries and off-channel habitat. Juvenile densities are generally higher in pools than riffles, although as the fish grow they will occupy areas of faster moving water. Juvenile coho tend to cluster in areas of suitable habitat, most frequently in streams with gradients less than 3%. Structurally complex habitats (large organic debris and large substrate), and habitats with slow moving water are both necessary to ensure high overwinter survival of young coho. Coho utilize lakes for rearing less frequently than streams, and are usually restricted to the littoral regions of lakes. Productivity (food abundance), as well as habitat, plays a role in regulating densities in streams (Chapman 1966).

The Thompson River watershed supports most of the coho salmon of the interior Fraser and the habitat is far from pristine. Many valley bottoms were logged, and subsequently used for agriculture (mainly livestock, dairy, and animal feed crops) for at least 50 years (Burt and Wallis 1997). In some cases, riparian vegetation has been removed, livestock have destabilized stream banks, and off-channel habitats and wetlands have been destroyed or isolated by dikeing. In most non-agricultural areas the old-growth timber on the valley floors has been removed, and logging is now occurring in the headwaters of many watersheds. In addition, much of the southern and western part of the Thompson drainage is in a semi-arid area, and high rates of water withdrawal in summer for irrigation cause low flows and high water temperatures (Rood and Hamilton 1995). Specific freshwater habitat concerns, by watershed, have been collated in a series of Fraser River Action Plan (FRAP) reports (e.g., Harding et al. 1994, DFO 1998a, b).

Juvenile coho salmon from the interior migrate down the Fraser River, live for an unknown time in the highly developed and constrained estuary of the Fraser River at Vancouver (Fig. 3), and usually spend the majority of their oceanic residence near the coast in southern BC. Over two million people live along the lower Fraser River and, as a consequence, riverine and estuarine habitats have been severely impacted. For instance, most of the streams in the lower Fraser River valley are classified as threatened or endangered due to landscape alterations in watersheds, riparian zone degradation, and pollution (FRAP 1998). Fortunately, it appears that habitat loss may have slowed in recent years with the release by DFO of its national “no net loss” habitat policy in 1986 (Langer et al. 2000, Levings 2000). Interior Fraser coho salmon leave the estuary and share a marine environment with other coho salmon and myriad other species. Although marine areas used by Fraser coho are less developed than the Fraser estuary is, these fish still face a variety of habitat issues within the ocean. Coho generally remain closer to the coast than most other salmon and they have to deal with impacts resulting from a rapidly increasing human population. Effects from pulp mills, sewage effluent, and fish farms, however, are difficult to quantify.

Climate related changes have had a major influence on the ability of the marine environment to support coho salmon and other species of salmon (Beamish et al. 1999b). A shift to a lower productivity regime in 1989/90 coincided with substantial reductions in the marine survival of coho salmon (Noakes et al. 2000).