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

Technical Summary

Oncorhynchus kisutch

Coho salmon
Saumon coho

Interior Fraser River watershed coho salmon
Fraser River watershed (BC) upstream of the Fraser canyon

Extent and Area Information

Extent of occurrence (EO)(km2)
(watershed areas are provided; water-covered areas may be more appropriate but have not been estimated)
South Thompson -17 814
North Thompson – 20 676
Lwr Thompson/Nicola -17 181
Fraser Canyon - ~1 000
Upper Fraser – (excluding Nechako, Stuart and above Pr. George) – 80 941

specify trend (decline, stable, increasing, unknown)


are there extreme fluctuations in
EO (> 1 order of magnitude)?


area of occupancy (AO) (km2)
(this presumably should be the water-covered areas minus areas of unsuitable habitat but these have not been estimated)

This has not been estimated but will be much less than the watershed areas provided as EO

specify trend (decline, stable, increasing, unknown)


are there extreme fluctuations in
AO (> 1 order magnitude)?


number of extant locations

> 75 spawning streams

specify trend in # locations (decline, stable, increasing, unknown)

Decline – as spawner numbers declined, they were seen in fewer streams

are there extreme fluctuations in # locations (>1 order of magnitude)?


habitat trend: specify declining, stable, increasing or unknown trend in area, extent or quality of habitat


Population Information

generation time (average age of parents in the population) (indicate years, months, days, etc.)
Approximately 3 yrs

number of mature individuals (capable of reproduction) in the Canadian population (or, specify a range of plausible values)

~24 000 of which ~20 000 are the result of natural spawning (mean of 1998-2000 estimates)

total population trend: specify declining, stable, increasing or unknown trend in number of mature individuals

Declining (based on data for the 2 largest populations, North and South Thompson)

if decline, % decline over the last/next 10 years or 3 generations, whichever is greater (or specify if for shorter time period)

~60% over 1990-2000 although 2001 data indicate an improvement.

are there extreme fluctuations in number of mature individuals (> 1 order of magnitude)?

Yes (for N. and S. Thompson at least)

is the total population severely fragmented (most individuals found within small and relatively isolated (geographically or otherwise) populations between which there is little exchange, i.e., ≤ 1 successful migrant / year)?

Total population is divided amongst 5 (sub)populations. Genetic exchange amongst populations not quantified but potentially is a concern.

list each population and the number of mature individuals in each

Total numbers for each (sub)population (wild fish in brackets) during 1998-2000:
South Thompson – ~3900 (~3900)
North Thompson - ~8400 (~7200)
Lower Thompson/Nicola – ~4000 (~1600)
Fraser Canyon – ~4100 (~4100)
Upper Fraser – ~3500 (~3500)

specify trend in number of populations (decline, stable, increasing, unknown)

There is no known change in number of populations. Spawners have been observed in fewer streams as escapements declined, but these are not considered to be separate populations.

are there extreme fluctuations in number of populations (>1 order of magnitude)?


Threats (actual or imminent threats to populations or habitats) [add rows as needed]

  • Interior Fraser coho underwent significant reductions in marine survivals which were probably related to climate change. Reductions in marine survivals may have been up to an order of magnitude as measured for other populations in southern BC.
  • Fishery exploitations were generally greater than the population could withstand during much of the period of declining natural marine survival (1989-1997), and as a result, coho numbers declined. Exploitations during 1999 and 2000 were low enough that populations were able to replace themselves. Marine survivals for fish returning in 1999 and 2000 also appear to be improved, although they are still low (~3%).
  • Habitat alterations also contributed to declines in interior Fraser coho salmon and this has been demonstrated for freshwater
  • Estuarine and marine habitat perturbations have presumably also played a role in population declines, but have not been quantified
  • Hatcheries have contributed significant numbers of coho to some watersheds and enhanced fish may be a threat to wild salmon in these instances

Rescue Effect (immigration from an outside source)

does species exist elsewhere (in Canada or outside)?

status of the outside population(s)?

Variable but many are depressed

is immigration known or possible?

Immigration is rare and is unlikely to have a rescue effect

would immigrants be adapted to survive here?

Perhaps but introductions from other areas not recommended

is there sufficient habitat for immigrants here?


Quantitative Analysis

Genetic analyses, various types of time series analyses