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Sockeye Salmon (Oncorhynchus Nerka)

Biology

Life History Forms

As a species, sockeye salmon make greater use of lakes for juvenile rearing than do other Pacific salmon. With the exception of river-type and sea-type populations that are widespread but not abundant, the vast majority of sockeye salmon spawn in or near lakes. Sockeye salmon are typically anadromous, but non-anadromous forms of the species also occur, maturing, spawning and dying in fresh water without entering the ocean. These forms are called kokanee when they are genetically distinct from anadromous sockeye, or “residual sockeye” when they are the (mostly male) progeny of anadromous sockeye.A few non-anadromous males have been found in Sakinaw Lake, but it is not yet known whether these are residual sockeye or kokanee. Two specimens provided to the author in April 2002 had the mtDNA haplotype#5 that is predominant among anadromous sockeye. Thus, there is not yet evidence to argue that these non-anadromous individuals can persist without the anadromous form, or could rescue the sockeye population in Sakinaw Lake. Moreover, kokanee are known to be relatively abundant in Ruby Lake that flows into Sakinaw Lake and slippage of juveniles from Ruby into Sakinaw could occur from time to time (G. McBain, DFO, pers. comm.).


Reproduction

Sockeye salmon enter Sakinaw Lake throughout the summer from June to September with peak migration ranging from 20 July to 17 August over 40 years. Spawning does not occur until late fall, peaking in late November, with mean start and end times ranging from 20 October and 11 December over the same years. This behaviour of returning early, foregoing feeding opportunity in the ocean, and holding in the natal lake for three or four months before spawning is atypical of sockeye salmon but it is not uncommon in coastal lakes, apparently as an adaptation to prevailing temperature regimes (Hodgson and Quinn 2002).

Sockeye salmon have a high fecundity (2000 - 5200) and small egg size (5.3-6.6 mm in diameter) relative to other salmon of the same size (Burgner 1991). Fecundity in the Sakinaw Lake population is at the low end of the range for sockeye salmon, averaging 2517 in 69 females collected for broodstock in 1986, 1987, 2000 and 2001; egg size averaged 5.6 mm in diameter and 300 mg in 15 females sampled in 2001 (Murray and Wood 2002).

Sakinaw Lake sockeye salmon rely on incubation habitat within the nursery lake, typically along the shoreline in areas of upwelling water near alluvial fans. Choice of incubation habitat affects the availability of dissolved oxygen and the thermal regime (hence development rate) during incubation, as well as exposure to predation and access to the nursery lake. Experiments have confirmed that both the timing of spawning and fry orientation behaviour (rheotaxis) at emergence exist as genetic adaptations to local conditions in sockeye salmon (Raleigh 1967; Brannon 1967, 1972, 1987).

Peak fry emergence would occur around 6 May (157 days after the peak spawning date of 19 November) based on empirical relationships described by Murray (1980) and the in-gravel temperature regime measured at one spawning beach in 1999-2000. Although similar at fertilization (9° C), in-gravel temperature on the spawning beach was more stable during incubation than in adjacent Mixal Creek, remaining just above 6° C from January to April. In contrast, temperatures in Mixal Creek declined to 3° C by February, then increased to 12° C by April. Lower incubation temperatures will produce larger fry for a specified egg size (Beacham and Murray 1986).


Nutrition and Growth

Throughout the species’ range, sockeye salmon fry typically emerge free-swimming at 25-32 mm. They feed initially near the lake shoreline, subsequently shifting to the deeper waters of the limnetic zone. Juvenile sockeye are visual predators, feeding primarily on copepods (Cyclops, Epischura, and Diaptomus), cladocerans (Bosmia, Daphnia, and Diaphanosoma), and insect larvae (Burgner 1991). Growth is influenced by food supply, water temperature, stratification and the length of the growing season, lake turbidity and migratory movements to avoid predation (Goodlad et al. 1974, Burgner 1991). Food availability also depends greatly on the density of juvenile sockeye (Johnson 1961) and other limnetic fish, especially threespine sticklebacks (Gasterosteus aculeatus, O’Neill et al. 1987), peamouth chub (Mylocheilus caurinus) and sympatric populations of kokanee (Wood et al. 1999). Faster growth rates can increase the survival of sockeye salmon during lake residence and subsequently through increased smolt size (Ricker 1962, Koenings and Burkett 1987, Henderson and Cass 1991)

Sakinaw Lake sockeye smolts are larger (100-150 mm) than those produced in most other nursery lakes. They are similar in size to those produced in Lake Washington, a very productive nursery lake for sockeye salmon (Doble and Eggers 1978, Burgner 1991). Comparison of scales from adult fish reveal that freshwater growth in Sakinaw Lake exceeds that for all other sockeye populations in B.C. (Y. Yole, DFO, pers comm.). Most juvenile sockeye remain in Sakinaw Lake for only one winter (as free-swimming fish) before migrating to sea. Surprisingly, some (about 3%) remain for two winters and become even larger smolts. It is widely believed that smolt age in salmon is influenced primarily by growth rate but that size thresholds for smolting are heritable (e.g., Thorpe et al. 1982) and vary as adaptations among populations, presumably reflecting different tradeoffs in size-specific survival in the freshwater and marine environments. Smolts from coastal populations are typically smaller and younger (implying a lower smolt size threshold) than interior lakes of comparable productivity. From this perspective, the Sakinaw Lake population is atypical of coastal populations.

Most anadromous SakinawLakesockeye salmon mature and return to spawn at age 4 after spending two winters at sea. This life history is denoted age 1.2 reflecting the single freshwater (winter) annulus and two marine annuli on their scales; thus, the age at maturity is the total number of annuli plus one, because no annulus is formed during the first winter of embryonic development.Age composition, by brood year, averages 3% age 3 (1.1), 87% age 4 (1.2 and 2.1), and 10% age 5 (1.3 and 2.2). Despite their large size at smolting, Sakinaw Lake sockeye are small at maturity compared with other sockeye populations in Canada and the Pacific Northwest (Gustafson et al. 1997). The mean postorbital-hypural length of 10 spawners collected in 2001 was 445 mm (468 mm for males, n=5, SD=1.8; and 428 mm for females, n=5, SD=9.6). Mean weights for sockeye salmon passing through the Sakinaw fishway from 1957 to 1972 ranged from 1.81 kg (n=29) to 2.10 kg (n=15). By comparison, sockeye salmon of the corresponding age (2 winters at sea) average 2.73 kgin theFraser River and 2.56 kg in Bristol Bay (Burgner 1991).


Survival

Juvenile survival has not been investigated in the Sakinaw Lake population but juvenile sockeye in other populations are often exposed to intense predation by a variety of fish and bird species both during lake residence and during early seaward migration (Burgner 1991). In nearshore and open ocean environments, predation by fish, birds, and marine mammals, and competition for food resources with other fish species affects growth and survival of sockeye. Ocean growth and survival of Pacific salmon can be affected by periodic, warm water events (El Niño) in local waters, and by changes in ocean conditions in the North Pacific Ocean (e.g., Francis 1993; Beamish et al. 1997, Mueter et al. 2002a, 2002b).

Potential fish predators of juvenile sockeye in or near Sakinaw Lake include cutthroat trout (O. clarki), juvenile coho salmon and chinook salmon (O. tshawytscha), prickly sculpin (Cottus asper), and lampreys (Lampetra tridentata and L. ayresi). Round scars apparently inflicted by lampreys were observed on coho salmon smolts during a snorkel survey below the outlet dam on April 26, 2002 (Bates and August 1997). Numerous lamprey scars were also observed on spawning adult sockeye in 2002. These observations lend support to earlier reports (J.D. McPhail, UBC, pers. comm.) that a parasitic, non-anadromous form of L. tridentata inhabits Sakinaw Lake. Principal bird predators include the common loon (Gavia immer), red-necked grebe (Podiceps grisegena), common merganser (Mergus merganser), belted kingfisher (Megaceryle alcyon), osprey (Pandion haliaetus), bald eagle (Haliaeetus leucocephalus) and various gulls (Larus sp.). Mammalian predators of adults likely include river otters (Londra canadensis), harbour seals (Phoca vitulina), killer whales (Orca orcensis), American mink (Mustela vison) and black bears (Ursus americanus). Seals and river otters are common near the lake outlet and likely eat both smolts and adults within the small Sakinaw estuary and nearby Agamemnon Channel. About 10-15% of adult sockeye passing through the fishway between 1957 and 1987 were scarred. Most scarring is probably due to commercial gillnets or illegal fishing; seals (T.Gjernes DFO, pers comm.) and river otters (G. McBain, DFO , pers. comm.) have been observed chasing or feeding on salmon near the lake outlet and may also contribute to scarring.

Predation on migrating salmon is typically depensatory (e.g., Wood 1987) so its role in limiting smolt-to-adult survival could have increased as the abundance of Sakinaw Lake sockeye salmon declined. However, this would depend on trends in abundance of alternative prey including other salmonids. An aquaculture site established at Daniel Point (just south of Sakinaw) during the early 1990s may also have attracted mammalian predators and increased their abundance in proximity to fish migrating to and from Sakinaw Lake.


Migratory Behaviour

Smolt migration out of Sakinaw Lake begins during early April and extends to mid-June, peaking in early May. The migration period was similar during four years of smolt enumeration (1994-1997, Bates and August 1997) with slight shifts in peak migration, which were perhaps affected by changes in lake discharge, temperature and weather.

Adult Sakinaw sockeye are known to arrive in Johnstone Strait as early as 28 June based on a 1975 study of catch composition using scale pattern analysis (Henry 1961, Argue 1975). Tagging experiments by the IPSFC indicate that Fraser River sockeye salmon migrate from the western end of Johnstone Strait to Area 16 in 7 to 14 days at a swimming speed of 40 to 56 km per day (Verhoeven and Davidoff 1962). The only tagging data available for Sakinaw sockeye is for a single fish released on 10 August 1925 in Deepwater Bay (Area 13) and recovered eight days later in Sakinaw (recorded as Sauch-en-auch) Creek (Williamson 1927). These limited data on timing of arrival in Johnstone Strait (late June) and time required for migration through Johnstone and Georgia straits (7-14 days) are consistent with more extensive observations of the timing of arrival at Sakinaw Lake. During 34 years of visual enumeration at the fishway (commencing in late June) the mean date of first arrival was 7 July, ranging from 28 June to 15 July. The mean date of the last recorded arrival was 29 August ranging from 10 August to 28 September. The mean date of peak migration was 30 July ranging from 20 July to 17 August. The mean seasonal duration of the run was 53 days (range 33 to 88 days) with the longest duration and highest abundance occurring in 1975, a year that fishing mortality was minimal because of a general fishing strike in late July and August. Low water flow and high water temperature can delay or disrupt migration into Sakinaw Lake.