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COSEWIC assessment and status report on the Sockeye Salmon Oncorhynchus nerka Sakinaw population in Canada

Habitat

Habitat Requirements

Sakinaw sockeye salmon have the same general habitat requirements as sockeye salmon in other populations (described by Foerster 1968, Burgner 1991). Sakinaw sockeye require suitable spawning and rearing habitat within Sakinaw Lake to reproduce, and foraging habitat in the north Pacific Ocean to attain adult size. Like other anadromous populations, they also require unobstructed passage between these habitats. Seaward migrating “smolts” must pass through the Georgia and Johnstone straits to reach the north Pacific Ocean where they spend two summers before returning to Sakinaw Lake by the same route. In the ocean, sockeye salmon typically inhabit cool (2-7ºC) surface waters (less than 15 m) and those from British Columbia generally remain north of 48ºN and east of 160ºW (French et al. 1976). Their survival is affected by conditions in all these habitats, but maximum population size is probably limited by the availability of suitable spawning and rearing habitat within Sakinaw Lake.


Limnology of Sakinaw Lake

Sakinaw Lake has a surface area of only 6.9 km2 and a perimeter of 35 km (Shortreed et al. 2003). It has two distinct basins (Figure 4). The lower basin is the largest with a maximum depth of 140 m and a mean depth of 43 m. The upper basin is small and shallow with a maximum depth of only 40 m. Both basins are clear with a mean euphotic depth of just over 15 m (Shortreed et al. 2003). The overall drainage basin is only 64 km2 but includes a number of small streams and lakes of which Ruby Lake is the largest with a maximum depth of 112 m.


Figure 4: Sakinaw Lake, its Tributaries and Spawning Beaches

Figure 4: SakinawLake, its tributaries and spawning beaches. Beach 1 (Sharon’s Creek); Beach 2 (Haskins Creek); Beach 3 (Ruby Creek Bay); Beach 4 (Kokomo Creek Bay) and Beach 5 (unnamed) (from Murray and Wood 2002).

Beach 1 (Sharon’s Creek); Beach 2 (Haskins Creek); Beach 3 (Ruby Creek Bay); Beach 4 (Kokomo Creek Bay) and Beach 5 (unnamed) (from Murray and Wood 2002).

Chemical, temperature and salinity conditions are rare and unusual because Sakinaw Lake is meromictic with a 30-m freshwater layer overlying warm, anoxic salt water (Northcote and Johnson 1964); this prevents seasonal mixing and results in strong thermal stratification (Hutchinson 1957, Walker and Likens 1975). In summer, the epiliminion extends to 7 m depth and becomes too warm for sockeye, but between 7 m and 30 m there is cool, well-oxygenated habitat that is rich in zooplankton and very suitable for rearing juvenile sockeye (Shortreed et al. 2003). Overall primary productivity in Sakinaw Lake is higher than in other coastal BC lakes but lower than in most lakes of the Fraser River system including Cultus Lake (Shortreed et al. 2003). Total dissolved solid content ranges from 113 to 140‰. Temperature, salinity and conductivity all increase markedly with depth between 30 and 60 m. Temperature increases from 5°C to a maximum of 9°C at 60m. Salinity continues to increase slightly with depth attaining a maximum value slightly over 11‰. A strong smell of hydrogen sulfide is evident in water samples from below 30 m, and samples from below 60 m may froth when brought to the surface. There is no evidence of sea water intrusion into the upper basin.


Spawning Habitat in Sakinaw Lake

Unlike most sockeye salmon populations, Sakinaw sockeye spawn almost entirely on beaches within the lake itself, presumably because the water flow in tributary streams is either inadequate (many go dry at their mouths) or especially subject to scouring during heavy rain. Upwelling ground water is probably essential for beach spawning in Sakinaw Lake. Spawning has been observed to a depth of 25 m, apparently only near alluvial fans in places where the gravel is small enough to be readily dislodged by digging (G. McBain, DFO, pers. comm).

A survey of the lakeshore carried out in 1979 revealed that only a small portion of the shoreline was suitable for beach spawning. No large spawning sites were found in the lower (main) basin and subsequent investigation there has focussed on two small spawning areas. Spawning on all beaches was restricted to depths between 0.25 and 25 m with the greatest density of nests (redds) occurring between 3 and 10 m. All major beach-spawning areas occurred near creeks or other obvious sources of ground water. There was considerable evidence of habitat degradation as all spawning beaches were littered with forest debris and supported aquatic plants to a depth of 3 m. Most spawning sockeye were observed in the upper basin of the lake; of these, almost all (95%) were observed within the area that would have been most affected by a foreshore development proposal.

The most serious habitat degradation occurred prior to the diver survey in 1979, but degradation has continued. Dive surveys in 1999 and 2000 indicate that the sockeye are now using only 15% of the area of Beach 1 (900 versus 6000 m2). Beach 2 is no longer being used, and the suitable habitat there is only 25% of that available in 1979 (1500 versus 6000 m2). Old spawning areas not presently used by sockeye are covered with thick mud, organic debris and large logs. Visual surveys of other spawning areas in 2000 and 2001 that examined the bottom looking through the water surface, suggest that similar degradation had occurred to the spawning habitat at Beaches 3, 4 and 5. (G. McBain, DFO, pers. comm.)