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Sockeye Salmon (Oncorhynchus Nerka)

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COSEWIC
Executive Summary

Sockeye Salmon
Oncorhynchus Nerka

Species Information

Sockeye salmon is one of seven species of the genus Oncorhynchus native to North America. In the ocean, adults have a slender, streamlined, silvery body and grow to an average of 3 kg. They undergo a distinctive transformation of external colour and body shape during their migration from the ocean back to the freshwater ecosystem from which they originated. The head becomes pale green in colour and the body becomes scarlet. Males also develop a hump, teeth and a sharply hooked jaw. The adults die soon after spawning but their progeny remain for several years in the freshwater environment (usually a lake) before migrating to the ocean. Dependence on nursery lake habitat, which is discontinuous by its nature, requires precise homing that divides the sockeye salmon species into isolated populations. The isolated populations typically evolve unique migratory, spawning and rearing behaviours as adaptations that improve survival in the natal freshwater environment. This differentiation into finely tuned, locally adapted populations accounts for the high productivity and commercial importance of the species; it also means that these populations would be very difficult, if not impossible to replace should they be lost.

This status report evaluates the distinct population of sockeye that inhabits Sakinaw Lake, British Columbia (henceforth called Sakinaw sockeye). Protein electrophoresis and molecular DNA analyses indicate that Sakinaw sockeye are genetically distinct and substantially reproductively isolated from other BC sockeye salmon populations. Sakinaw sockeye also have different life history characteristics including early, but protracted, timing of river-entry, extended lake residence prior to spawning, small body size, low fecundity and large smolt size, indicating that Sakinaw sockeye are evolutionarily distinct from other sockeye populations.

Distribution

Sakinaw sockeye are endemic to Canada, in the sense that they reproduce and rear for two or three years (over half their life) exclusively within Sakinaw Lake, situated on the Sechelt Peninsula in Georgia Strait, British Columbia (BC). Because they are anadromous, they also share marine migration corridors and foraging habitat in the north Pacific Ocean with many other sockeye salmon populations. A few non-anadromous individuals have been found in Sakinaw Lake, but it is not yet known whether these are male progeny of anadromous females (i.e., “residual sockeye”, and part of the anadromous gene pool) or members of a genetically distinct, self-perpetuating population of smaller bodied, exclusively freshwater sockeye (called “kokanee”). As a species, sockeye salmon occur in North America from the Columbia River (Oregon, Washington, and Idaho) north to Kotzebue Sound, Alaska, and in Asia, from the southern Kuril Islands north to the Anadyr River. Populations have declined in abundance or become extinct in the southern parts of their range on both sides of the Pacific Ocean. Migratory (anadromous) sockeye no longer occur naturally in California or Japan, although non-migratory kokanee populations exist.

Habitat

Sakinaw sockeye require suitable spawning and juvenile rearing habitat within Sakinaw Lake, and foraging habitat for smolt and immatures in the north Pacific Ocean to attain adult size. Like other anadromous populations, they also require unobstructed passage between these habitats. Sakinaw Lake has a surface area of only 6.9 km2, a mean depth of 43 m and a mean euphotic zone depth of just over 15 m. 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; this prevents seasonal mixing and results in strong thermal stratification. In summer, the epiliminion extends to 7 m depth and becomes too warm for sockeye, but below this there is cool, well-oxygenated habitat that is rich in zooplankton and very suitable for rearing juvenile sockeye. 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.

The availability of suitable spawning habitat probably constrains maximum population size more than the availability of lake rearing habitat. Unlike most sockeye salmon populations, Sakinaw sockeye spawn almost entirely within the lake itself on five beaches near creeks or other sources of ground water. The shoreline perimeter is 35 km but most is unsuitable for spawning because eggs and alevins require clean, well-oxygenated, gravel substrates during their development until they emerge as fry.

Sakinaw Lake has an elevation of only 5 m and drains directly into Georgia Strait by a short stream. A dam on the outlet controls water storage and adult sockeye gain access to the lake through a fishway. 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.

Biology

Most Sakinaw sockeye spawn in late November; all die after spawning and their carcasses are eaten or decompose in the lake. The females construct nests(called “redds”) in gravel substrate and bury their eggs immediately after fertilization. Eggs and alevins remain buried during the winter. Free-swimming fry emerge in early May and move to limnetic habitat where they feed primarily on zooplankton. The timing of fry emergence is likely synchronized with spring plankton blooms. Synchronization requires that for each lake, the spawning time and/or embryonic development rate be genetically adapted to ambient temperature regimes in the spawning environment.

As with fry emergence, each sockeye salmon population usually has its own life history adaptations that determine the age (size) and seasonal timing for smolting, a physiological adaptation to the saline marine environment and seaward migration. Most Sakinaw sockeye become smolts in April of their second year, and exit the lake via a short (<500 m) creek into Georgia Strait. Although Sakinaw sockeye smolts are large (100 –150 mm) at age 1+ relative to other sockeye populations, some remain in the lake for another winter and become even larger smolts, migrating at age 2+. It is presumed that Sakinaw sockeye smolts migrate northwest through Johnstone Strait into the Gulf of Alaska, together with smolts from other sockeye populations in the Fraser River. The timing of sea entry can greatly determine the mortality imposed by seasonally migrating (warm-water) marine predators.

Most Sakinaw sockeye mature at age 4 after spending two winters at sea. During the return migration, individuals migrate southeast through Johnstone and Georgia straits, but are thought to turn northeast at the end of Sabine Channel to reach Sakinaw Lake. They enter Sakinaw Lake in June through September even though peak spawning does not occur until late November. As a result, they are small at maturity compared with other sockeye populations in Canada, and their fecundity is at the low end of the species’ range, averaging only 2500 eggs.

Population Sizes and Trends

Sakinaw sockeye abundance has declined dramatically since 1987. From 1947 (when records began) to 1987, the estimated number of (maturing) adults entering Sakinaw Lake averaged about 5 000 individuals (range 750 to 16 000) with no declining trend. From 1987 to 2002, numbers declined, averaging just over 1000 adults per year between 1988 and 1992, less than 200 between 1993 and 1996, and less than 50 between 1997 and 2001 (between 1999-2002, less than 80).  In 2002, adult sockeye were carefully enumerated; only 78 were counted entering the lake, and only 44 were observed on the spawning grounds.

A statistically robust estimate of decline rate from regression analysis using 1‑generation smoothed estimates of mature abundance (based on annual counts of mature adults between 1988 and 2002, and smoothed to 1990-2001) reveals a decline rate of 33% per year, which implies a reduction of 99% over 3 generations. Using only endpoints, there has been an 87% or larger reduction in the number of adult Sakinaw sockeye estimated to enter the lake over the last 3 generations (1991-2002; 4 years per generation).

Limiting Factors and Threats

The persistence of the Sakinaw sockeye population is threatened by two primary factors: mortality from fisheries, and degradation of freshwater habitat. At present, fishing mortality is probably the single greatest threat. Sakinaw sockeye continue to be killed in fisheries, and given their very low abundance, even modest fishing mortality may jeopardize the viability of the population. Over-fishing can also be considered the proximate cause of decline in the sense that fishing effort was not reduced significantly until 1998 despite the observed decline in spawning escapements to Sakinaw Lake that began in 1987. Sakinaw sockeye are captured during their migration to Sakinaw Lake through Johnstone and Georgia straits together with sockeye and pink salmon from more productive salmon populations, in what are termed ‘mixed-stock fisheries’. The fact that Sakinaw sockeye are vulnerable to these fisheries, and that escapements decreased rapidly during a short period of consistently high fishing effort, strongly suggests that fishing mortality was excessive.

The vulnerability of Sakinaw sockeye to overfishing likely increased as their productivity was eroded by habitat degradation within Sakinaw Lake. The most significant degradation of habitat within Sakinaw Lake was probably due to logging activities in the first half of the 20th century. The lake was used as a log dump, millpond and booming ground. To assist in transporting logs to the ocean, the lake’s outlet was dammed sporadically to raise water level, and this likely reduced the access for migrating sockeye. These practices largely ended by 1952 when a permanent dam with a fishway was built. Residential development and recreational boating subsequently increased. Stream flows were diverted to prevent flooding and a boat ramp was constructed through the middle of one of the major spawning beaches. Again, most of this degradation occurred prior to 1987. The BC Fish and Wildlife Branch attempted to augment the natural population of sea-run cutthroat trout in Sakinaw Lake by stocking a quarter million juveniles between 1965 and 1989. The consequences for Sakinaw sockeye are unknown but cutthroat trout are predators of juvenile sockeye. Sockeye migration into the lake may also have been adversely affected by reduced summer flows resulting from increased human utilization of water throughout the drainage.

There is little possibility that neighbouring sockeye populations could rescue Sakinaw sockeye naturally, within a human lifetime or perhaps longer, given the extremely restricted gene flow and the degree of local adapation. It is also doubtful that humans could successfully transplant sockeye into Sakinaw Lake should Sakinaw sockeye go extinct. Several previous attempts to plant sockeye from other populations into Sakinaw Lake have almost certainly failed because there is no genetic signature of the donor populations. Thus, the extinction of Sakinaw sockeye should be considered irreversible.

Fisheries and Oceans Canada (DFO) is taking several conservation actions. Fishing effort has been reduced significantly since 1995 in Georgia Strait and since 1998 in Johnstone Strait following fleet reduction and fishing restrictions imposed for conservation reasons. Area 16 (including Sabine Channel adjacent to Sakinaw Lake) was closed to commercial fishing in 2002 to protect Sakinaw sockeye; this closure did not reduce food and ceremonial fishing by First Nations however. DFO is also co-ordinating habitat restoration activities including the clearing of logging debris from spawning beaches, modifications to the fishway to improve water flow for migrating adults, and increased surveillance to discourage poaching and natural predation at the fishway where adults are most vulnerable. Hatchery supplementation was also initiated in 2000. However, it is not known whether hatchery supplementation can restore an endangered salmon population, and thus, contributions by naturally breeding adults remain a prudent component of any recovery plan.

Special Significance of the Species

Sockeye salmon are economically the most important species of Pacific salmon, contributing to commercial, recreational, and aboriginal catches along the Pacific coast of North America. The number of extant populations has declined in the southern parts of the species’ range. Currently, ESUs of North American sockeye salmon are considered endangered in four locations: two in Canada (Sakinaw Lake and Cultus Lake, BC) and two in the United States (Ozette Lake, Washington and Snake River, Idaho). The Sakinaw Lake population is one of only two anadromous lake-type sockeye salmon populations situated in the 200-km length of Georgia Strait (the other is Village Bay Lake on Quadra Island, 100 km distant at the extreme northern end of the strait). The conservation of Sakinaw sockeye is a high priority for the Sechelt Indian Band because these fish return to reproduce within the band’s traditional territory. Sockeye salmon may also play a significant role in maintaining the productivity of the Sakinaw Lake ecosystem, including a variety of animal and plant life, by importing marine-derived nutrients. The juveniles contribute to the complexity of the lake’s food web, consuming invertebrates and serving as prey for native fish, birds and mammals. Returning adults are consumed by many species, including river otters, bears and lampreys, and the carcasses provide food for bald eagles and other species. Thus, Sakinaw sockeye play a significant role in the ecology of the Sakinaw Lake ecosystem.

Existing Protection or Other Status Designations

The federal Fisheries Act has long required that DFO authorize proposed alterations to habitat. In addition, provincial and municipal governments regulate many land and water use activities that can affect fish populations. DFO is mandated to manage fisheries to conserve the resource for the benefit of all Canadians. To date, none of these protections have prevented the collapse of Sakinaw sockeye. However, mixed stock fishing effort was significantly reduced in 1998 and DFO has recently assigned a recovery team to co-ordinate restoration activities. These actions are consistent with the federal Oceans Act (1997) that requires DFO to manage Canada’s marine resources to conserve biological diversity and natural habitats. In fall 2002, COSEWIC conducted an Emergency Assessment and listed Sakinaw sockeye as Endangered (25 October 2002). NatureServe lists sockeye salmon as Secure (G5) as a species, but Critically Imperiled in Idaho (S1), Imperiled in Washington State (S2), Apparently Secure (S4) in Oregon, Secure in Alaska (S5) and Under Review in California and British Columbia.