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COSEWIC assessment and status report on the coho salmon (Interior Fraser population) in Canada

Biology

General

Most coho salmon return to freshwater in the fall and spawn during fall and early winter. All fish die after spawning. Fry emerge from the gravel the following spring and usually reside in freshwater for a year before migrating to sea as smolts. Most coho spend 18 months at sea before returning to freshwater and therefore have a 3-year life cycle. Variations on this general life cycle include juveniles that emigrate to sea immediately upon emergence, juveniles that emigrate as 2-year-old smolts, and precocious male coho that return to spawn after only 6 months at sea (jacks). More detailed general descriptions are provided in Scott and Crossman (1973) and Sandercock (1991).


Interior Fraser Coho

Biological characteristics were reviewed in Irvine et al. (1999a) and only a brief update and summary is provided here. Female coho are larger than males in most interior Fraser systems, but less abundant (~45% of returns), traits characteristic of many coho populations. Interior Fraser coho are smaller than most similar aged coho documented by Sandercock (1991) and Weitkamp et al. (1995). Mean post-orbital hypural lengths3 (cm) for coho from the North, South, and lower Thompson drainages (sample sizes in brackets) were 42.3 (7149), 45.7 (256), and 44.0 (1853) respectively. Temporal patterns in fish size have not been found for interior Fraser coho, although Weitkamp et al. (1995) documented declines in fish size over time for many populations of coho salmon. Fecundities for interior coho are highly variable, and generally less than for coho returning to the lower Fraser or provincial averages (Irvine et al. 1999a), as expected given the generally smaller sizes of coho in the interior Fraser.

Most (93%) interior Fraser coho went to sea in their second year (i.e. European age 1._), with a small proportion (7%) remaining in freshwater for one or two more years (n= 2274 adult coho aged with scale analysis). Almost all fish from this sample returned to freshwater after the normal 1 winter at sea; only 2 fish were aged as jacks and 6 as having spent more than 1 winter at sea.




Footnotes

3 Measured from the hind margin of the eye to the posterior end of the hypural plate.