Sockeye Salmon (Oncorhynchus Nerka)
- Assessment Summary
- Executive Summary
- COSEWIC Mandate, Membership and Definitions
- Lists of Figures and Tables
- Species Information
- Population Sizes and Trends
- Limiting Factors and Threats
- Supplementation and Restoration
- Special Significance of the Species
- Existing Protection or Other Status
- Summary of Status Report
- Technical Summary
- Acknowledgements and Literature Cited
- Biographical Summary of Contractor
Limiting Factors and Threats
There are a variety of limiting factors and threats to the survival of Sakinaw Lake sockeye, including those in the freshwater and marine environments from both natural and anthropogenic factors.
Beach spawning habitat in Sakinaw Lake is susceptible to landslides caused by rapid increases in stream flow and flooding, especially in winter when rain falls on snow. Mean annual precipitation ranges from 850 mm at lower elevations to 2500 mm at higher elevations. Maximum precipitation occurs in winter as rain with less than 10% of total precipitation falling as snow at sea level, although this proportion increases significantly with elevation (see Murray and Wood 2002 and Shortreed et al. 2003 for detailed descriptions of climate and limnology).
Human activities or natural events that reduce upwelling or reduce substrate permeability by adding silt or wood debris near spawning sites could cause mortality during incubation by interfering with the delivery of oxygenated water and the removal of metabolic wastes. Beach spawning habitat has been degraded by the construction of recreational boating facilities and the cumulative effects of log storage. 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, and that none are using Beach 2 where only 25% of the formerly suitable habitat now appears suitable. Old spawning areas not presently used by sockeye are covered with thick mud, organic debris and large logs.
Although Sakinaw Lake lies at an elevation of only 5 m, access to and from the lake can be difficult for sockeye salmon during periods of low water flow. The lake outlet has been partially or completely blocked since the early 1900s by dams built for log and water storage. A permanent dam and fishway were constructed by DFO on the outlet in 1952. Since then, lake levels have been regulated to store water for both the developing recreational and cottage community, and the sockeye migration. Low water flow and high water temperature can delay or disrupt migration into Sakinaw Lake, and sockeye enter the fishway only on the high tide at night. The presence of predators, most notably river otters, in or near the fishway can disrupt the spawning migration. When migration is disrupted, fish that return to the ocean cannot gain access until the following night because the fishway gate is closed during the daylight high tide. Passage to the fishway was improved in 1995 by the installation of two large rock weirs in the creek below the fence to create large pools. These pools act as steps and offer some protection for the migrating sockeye from illegal fishing and predation. In addition, effort has been made to restructure the outflow so that it remains concentrated in a narrow channel.
It is generally believed that most natural marine mortality is caused by predation, and that physical factors (temperature, salinity, currents) and intrinsic biotic factors (genetic adaptation, nutrition, parasites and disease) affect vulnerability to predators. Marine predators include a wide range of species from diving birds, piscivorous fish, to pinnipeds and killer whales. There is some evidence from scarring and trawl catches to suggest that Sakinaw sockeye may be especially vulnerable to marine predation by lampreys (Lampetra ayresii) that are relatively abundant in Georgia Strait near Sakinaw Lake (R. Beamish, DFO, pers. comm.). In addition, ocean growth and survival of all species 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; Mueter et al. 2002a, 2002b).
Sakinaw sockeye are killed both as directed catch in terminal fisheries and more importantly, as incidental catch in mixed-stock fisheries targeting larger populations of sockeye and pink salmon. Significant terminal fishing has probably occurred in most years but reliable estimates of terminal catch are available for only three years: 1947, 1952 and 1972. The terminal harvest rate in 1947 was only 1.4% based on a terminal catch of 50 sockeye and an estimated escapement of 3500 sockeye. In 1952, the terminal harvest rate was considerably higher at 14% based on a reported gillnet catch of 1000 sockeye by three or four row boats fishing in Lee Bay and an escapement of 6000 sockeye. The 1952 fishery was open six days per week, but both commercial and sport fishing were prohibited in a small sanctuary off the mouth of Sakinaw Lake that was patrolled to deter illegal fishing (A. Skipper, DFO, pers. comm.). Illegal fishing and poaching of sockeye from the Sakinaw fishway has been a concern to enforcement staff since their earliest records on Sakinaw Lake sockeye, but the magnitude of the illegal harvest has never been estimated. In 1972, the terminal harvest rate was higher still at 23 to 29% based on an estimated catch of 1350 to 1800 sockeye taken by 2 or 3 gillnetters, primarily in Lee Bay (followed by Middlepoint, Bargain Harbour and Sabine Channel, R.P. Kraft, DFO, pers. comm.). The escapement in 1972 was 4500 sockeye, about average over the previous 20 years.
Sockeye salmon return to Sakinaw Lake through Johnstone Strait (Figure 7). They share this migration corridor with other sockeye populations including those returning to lakes in the vicinity of Johnstone Strait (Nimpkish, Heydon, Phillips and Village Bay lakes) and the “northern diversion” component of sockeye returning to the Fraser River. “Diversion rate” refers to the proportion of the total catch of Fraser sockeye that is taken along the northern approach route (i.e., through Johnstone Strait rather than Juan de Fuca Strait). A higher diversion rate implies a higher fishing effort in fisheries that kill Sakinaw sockeye including:
- Statistical Area 11 – historically a minor troll and gillnet fishery off the north end of Vancouver Island.
- Johnstone Strait – the major fishery on the northern diversion component of Fraser River stocks. Although typically a seine fishery, troll and gillnet gear is also used. Smaller catches from native and test fishing are included in the commercial catch statistics. Johnstone Strait is sub-divided into upper (Area 12) and lower (Area 13) reaches.
- Georgia Strait – troll fishing has occurred throughout Georgia Strait but a small net fishery located in Statistical Area 16 (much of it concentrated in Sabine Channel) is most important with respect to Sakinaw sockeye.
The overall intensity of mixed-stock fishing in Johnstone and Georgia straits generally increased until the late 1990s in response to high abundance and high diversion rates of Fraser River sockeye. Fishing effort by seine boats appears to have declined in both areas since the late 1970s when measured in cumulative boat days (Figure 8), but this measure of effort does not take into account how technology has increased fishing efficiency allowing seine vessels to set their nets more often and more effectively. Moreover, gillnet effort in Johnstone Strait was consistently higher between 1989 and 1994 (average 7563 boat days, range 6003 to 9479) than in any earlier period (average 4358 boat days, range 1333 to 6104). Gillnet fishing effort in Georgia Strait (mostly in Sabine Channel near Sakinaw Lake) was also higher between 1991 and 1994 (average 1095 boat days, range 205 to 2438) than in previous years (average 212 boat days, range 5 to 529). This increased fishing effort by gillnet boats in Johnstone and Georgia straits coincided with the period of rapid decline in Sakinaw escapements. Reduced fishing rates after 1997 reflect fishing restrictions in response to conservation concerns, an overall reduction in the fishing fleet, and area-based licensing requirements. Additional closures (seaward of Lewis Point) since 1980 have succeeded in reducing harvest rate on Nimpkish sockeye in Area 11 and upper Area 12 (Starr et al. 1984). However, early fisheries in lower Area 12 (sub-areas 1 to 4) would still kill sockeye from Sakinaw Lake and other “non-Fraser” stocks (those from Fulmore, Phillips Heydon, and perhaps Village Bay lakes).
It should be noted that increased fishing effort in mixed-stock fisheries does not necessarily imply increased fishing mortality on small populations like Sakinaw sockeye. Detailed information on run timing and migration routes past fisheries are required to reliably estimate population-specific harvest rates in mixed-stock fisheries, and these data are seldom available for minor stocks. “In-season” regulation of fishing effort typically depends on test-fishing indices of the aggregate abundance, and thus, the abundance of large Fraser River sockeye populations. Sakinaw Lake sockeye are said to be managed “passively”, meaning that fishing effort is not regulated based on in-season estimates of abundance, although the spawning escapement is monitored to allow post-season evaluation of the fishing plan.
Sakinaw sockeye are harvested primarily in the Johnstone Strait and Sabine Channel net fisheries. From Murray and Wood 2002.
Figure 8: Annual Fishing Effort by Gillnet (GN) and Seine (SN) Vessels During July to Mid-September while Sakinaw Sockeye are Migrating through Mixed-stock Fishing Areas in Johnstone Strait (Areas 12 and 13) and Georgia Strait (Area 16)
From Murray and Wood 2002.
The Pacific Salmon Commission (PSC) used scale analysis to estimate the contribution of Sakinaw Lake sockeye to net fisheries in Johnstone and Georgia straits in the 1975 fishing season. These estimates of catch composition imply that the total catch and exploitation rate of Sakinaw Lake sockeye in Areas 12, 13 and 16 in that year were 14 300 fish and 47%, respectively (Argue 1975). Most (92%) of this fishing mortality occurred in Johnstone Strait prior to a general fishing strike that began in the last week of July and continued through most of August; the resulting escapement to Sakinaw Lake that year was the largest (16 000) and latest (until 30 September) on record. Comparable stock composition data for Sakinaw Lake sockeye are not available for other years. However, Starr et al. (1984) concluded from run reconstruction analyses that total exploitation rates on Sakinaw Lake sockeye varied from 20 to 67%, averaging 41% between 1970 and 1982. They estimated that harvest rates in the Johnstone Strait (Areas 11, 12, and 13) and Area 16 fisheries averaged 37% and 4%, respectively.
Murray and Wood (2002) inferred the minimum total exploitation rate on Sakinaw Lake sockeye for the periods 1986-89 and 1992-94 using weekly estimates of proportions of non-Fraser and Early Stuart sockeye in the Round Island test fishery catches (Area 12) that had been determined by scale racial analysis (Gable and Cox-Rogers 1993; PSC unpublished data). In their first method, Murray and Wood (2002) assumed that Sakinaw Lake sockeye were harvested at the same rate as co-migrating Fraser River sockeye populations (mainly early Stuart sockeye) in the northern approach fisheries. Stock composition in the Area 12 test fishery was assumed to represent stock composition in the total catch from Areas 11, 12 and 13. A fishery-specific harvest rate was estimated by dividing the estimated weekly catch of Fraser River sockeye by the weekly abundance of Fraser sockeye available to the fishery. The latter quantity was inferred from estimates of weekly abundance in the lower Fraser River, lagged to account for migration time, and multiplied by the estimated diversion rate to include only fish that had returned through Johnstone Strait. Using this method, estimates of total harvest rate on early Stuart sockeye in northern approach fisheries (and by extension, Sakinaw Lake sockeye) ranged from 1 to 56% (average 21%) assuming a 7-d migration, and 1 to 97% (average 57%) assuming a 14-d migration.
In their second method, Murray and Wood (2002) reconstructed the probable catch of Sakinaw Lake sockeye from PSC estimates of the aggregate non-Fraser sockeye catch each week. Of the non-Fraser populations, Nimpkish River sockeye are present only in Areas 11 and 12; Heydon, Fulmore and Phillips lake sockeye are present only in Areas 11, 12, and 13, whereas Sakinaw Lake sockeye are present in all four areas. Murray and Wood (2002) assumed that Sakinaw Lake sockeye accounted for 8%, 20%, and 43% of the aggregate non-Fraser catch in Areas 11 and 12, Area 13, and Area 16, respectively, reasoning that they must account for a higher proportion of the aggregate as other populations stop contributing. These rough estimates of Sakinaw catch, together with SEDS escapement data, suggest that exploitation rates on Sakinaw Lake sockeye in Johnstone and Georgia strait net fisheries averaged 49 to 57% (depending on assumption about migration time) between 1986 and 1989, and 89 to 99% between 1993 and 1994. Estimates of exploitation rates would be biased high if escapements were underestimated as seems likely for 1993 and 1994 given that enumeration at the fishway was discontinued in 1990 (until 2002).
Sockeye salmon fry were transplanted into Sakinaw Lake each year from 1902 to 1906. The sockeye fry were reared at the Fraser River Hatchery near New Westminster, which operated from 1884 to 1915. The donor stocks were the Harrison (Big Silver, Weaver Creek, Trout Lake, Harrison River Rapids sites), Pitt River (Upper and Lower) and Birkenhead rivers and Shuswap Lake (Scotch and Tappin creeks, Adams River). Approximately 380 000 fry were transplanted into Sakinaw Lake from the various donor stocks (Aro 1979). Genetic evidence indicates that these transplants were unsuccessful (see previous section on Nationally Significant Populations and Figure 2).
The British Columbia Fish and Wildlife Branch attempted to augment the natural population of sea-run cutthroat trout in Sakinaw Lake by stocking 297 931 juvenile cutthroat trout (most over 10 g in weight) between 1965 and 1989 (http://www.bcfisheries.gov.bc.ca/fishinv/db/default.asp ). Lacustrine predators can limit sockeye smolt production and cutthroat trout are known to be predators on young sockeye at all times of the year (Foerster 1968). Thus, stocking Sakinaw Lake with cutthroat trout may have decreased the survival of juvenile sockeye in Sakinaw Lake.
- Date Modified: