Sockeye salmon (Oncorhynchus nerka)

Sockeye Salmon (Oncorhynchus nerka)

Original article posted in BCLSS newsletter Volume 13, Issue 3, October 2010. Updated April 2022.

With their bright red spawning colours and distinctive green head, sockeye salmon are easily the most recognizable of the 5 Pacific salmon found in BC and carry a rich cultural, economic, and ecological significance. In BC, there are about 900 sockeye stocks and over 500 kokanee (land-locked sockeye) populations. Fraser River Sockeye Salmon has a return rate of an average 9.6 million between 1980 and 2014 (DFO, 2017). However, research has shown that these salmon are 70% less diverse than they were in 1913 (Price et al., 2021). This could influence the fish’s ability to thrive as climate change and other pressures on the fish progress. The 2020 Fraser River sockeye return was less than 300,000, the worst return in history (Taylor, 2020).

The main spawning area of the sockeye extends from the Fraser River to Alaska’s Bristol Bay.  Sockeye and kokanee salmon are widely distributed in BC, with the anadromous (sea-run) form spawning in over 300 BC lakes and streams. The Fraser River is the world’s largest single river producer of sockeye; it is surpassed only by the combined sockeye production from several river systems flowing into Bristol Bay (DFO, 2017). Major spawning runs are also found in the Skeena, Nass, Stikine, Taku, and Alsek watersheds as well as those of Smith and Rivers inlets (DFO, 2019).

Sockeye display two basic life history forms: anadromous (sockeye) and a non-anadromous (kokanee), with considerable variation in each. The typical sockeye life history involves a juvenile lake-rearing phase; however, there are also populations which rear in rivers and streams, and populations which migrate directly to estuaries and then to the sea in their first year. In BC, lake-type sockeye typically spend 1 or 2 (occasionally 3) years in freshwater before migrating to sea; river-type sockeye spend at least 1 year in freshwater while sea-type sockeye migrate to the sea in their first year (Hume et al., 1995). Most Fraser River sockeye populations become smolt after 1 year of lake residence and return to spawn in their fourth year after 2 years at sea (Grant et al., 2017).

From August to December, mature kokanee move into the inlet streams of lakes, and along lake shores to spawn. After the eggs hatch, the fry will spend about a month in the gravel, then emerge and make their way to the lake. In the lake, the fry move to open water and form schools close to the surface. Kokanee are intolerant of warm water conditions and live in cooler waters 5 m – 30 m below the surface, undergoing vertical migrations at dawn and dusk to feed on surface zooplankton and insects. Kokanee mature between 3 and 5 years of age and, like other salmon, die once they have spawned (BC Ministry of Fisheries, 1999).

A distinguishing characteristic of sockeye salmon is their extensive use of lake rearing habitat during the juvenile stage of their life history. After emerging from the gravel, sockeye fry typically migrate to lakes associated with their natal spawning habitats, where they exhibit a pelagic life style, feeding mainly on zooplankton before seaward migration (Hume, 1996). The biological productivity in a sockeye nursery lake is therefore important for the survival and growth of juvenile sockeye salmon. Although sockeye salmon acquire 99% of their body weight in the ocean, survival in the freshwater stage is much lower than ocean survival. In addition, most marine mortalities of juvenile sockeye occur soon after entering the ocean and are related to the size attained in freshwater (Uchiyama et al., 2008).

Most sockeye lakes are oligotrophic, where nutrients (nitrogen and phosphorus) limit primary productivity.  In such systems, the nutrients supplied by the decomposing bodies of returning salmon can be very important in maintaining the productivity of the lakes and the carrying capacity for juvenile salmon (Naiman et al., 2002). Declines in sockeye stocks may be, in part, due to the reduced loading of these marine-derived nutrients because of low escapement (returns) of adult sockeye.  Sediment cores taken from sockeye lakes have shown substantial decreases in marine-derived nutrients in lakes since the rise of the commercial salmon fishery at the beginning of the 20th century; associated with this are apparent changes in zooplankton and phytoplankton communities (Naiman et al., 2002).

One of the most remarkable features of sockeye is a phenomenon called “cyclic dominance”. In many of the lakes of the Fraser River system in particular, sockeye are abundant in 1 of every 4 years. Sockeye can mature at ages between 2 and 6 years old but in most systems, one age group (usually 4-year-old fish) dominates, meaning most of the offspring produced in any one “brood-year” return to spawn 4 years later (DFO, 2019).  This year of increased population significance creates a cyclic dominance which leads to spectacular returns to the Adams River every 4 years (2022 is a dominant year of the migration cycle). To witness this spectacular event firsthand, be sure to take in the Salute to the Sockeye celebration at Tsútswecw Provincial Park (46 km west of Salmon Arm and 66 km east of Kamloops) from late September to late October. Visit the Adams River Salmon Society website for a full list of activities, attractions, and events planned for this year.

References

BC Ministry of Fisheries.  BC Fish Facts: Kokanee.

DFO. 2017. Summary of Fraser River Sockeye Salmon (Oncorhynchus nerka) ecology to inform pathogen transfer risk assessments in the Discovery Islands, BC. Fisheries and Oceans Canada. https://waves-vagues.dfo-mpo.gc.ca/Library/40681920.pdf

DFO. 2019. Information about Pacific Salmon. Fisheries and Oceans Canada. https://www.pac.dfo-mpo.gc.ca/fm-gp/salmon-saumon/facts-infos-eng.html

Grant, S. C. H., Holt, C., Wade, J., Mimeault, C., Burgetz, I. J., Johnson, S., & Trudel, M. 2017. Summary of Fraser River Sockeye Salmon (Oncorhynchus nerka) ecology to inform pathogen transfer risk assessments in the Discovery Islands, British Columbia. DFO Canadian Science Advisory Secretariat. Research Document74. https://waves-vagues.dfo-mpo.gc.ca/Library/40681920.pdf

Hume, J.M.B., Shortreed, K.S., Morton, K.F. 1996.  Juvenile sockeye rearing capacity of three lakes in the Fraser River system.  Can. J. Fish. Aquat. Sci. 53: 719-733.

Naiman, R.J., Bilby, R.E., Schindler, D.E., Helfield, J.M.  2002. Pacific salmon, nutrients, and the dynamics of freshwater and riparian ecosystems.  Ecosystems 5: 399-417. https://link.springer.com/content/pdf/10.1007/s10021-001-0083-3.pdf

Price, M. H., Moore, J.W., Connors, B.M., Wilson, K.L., & Reynolds, J.D. 2021. Portfolio simplification arising from a century of change in salmon population diversity and artificial production. Journal of Applied Ecology. 58(7), 1477-1486. http://www.michaelprice.org/uploads/1/0/8/9/108971119/price_etal_2021_portfolio_simplification.pdf

Taylor, G. 2020. An overview of 2020 salmon returns. Watershed Watch Salmon Society. https://watershedwatch.ca/greg-taylor-an-overview-of-2020-salmon-returns/

Uchiyama, T., Finney, B.P., Adkison, M.D.  2008. Effects of marine-derived nutrients on population dynamics of sockeye salmon (Oncorhynchus nerka).  Can. J. Fish. Aquat. Sci. 65: 1635-1648.

Leave a Reply