Coho salmon (Oncorhynchus kisutch) COSEWIC assessment and status report: chapter 6
Distribution
Coho salmon occur naturally only within the Pacific Ocean and its tributary drainage (Scott and Crossman 1973). Within North America, naturally spawning coho salmon occur in streams and rivers from California north through BC to Alaska. Their distribution extends across the Bering Sea through Kamchatka to Sakhalin Island and rarely as far south as Peter the Great Bay (Fig. 5, Sandercock 19912). In addition, coho have been introduced to many locations including the Great Lakes.
Figure 5: Approximate Distribution of Naturally Spawning Coho Salmon Globally
From Sandercock 1991, reproduced with permission of C. Groot.
Coho salmon spawn and rear in most coastal streams and rivers of BC (Fig. 5). The marine distribution of many populations is reasonably well understood from results obtained through the Mark Recovery Program (MRP) operated by Fisheries and Oceans Canada (DFO). Magnetic coded-wire tags (CWTs) are inserted into large numbers of young salmon annually as these fish leave freshwater. Recoveries of these binary coded tags from fish sampled in the various fisheries provide information on fishery exploitation rates and apparent marine distributions.
CWTs from coho salmon that were spawned in the interior Fraser River have been recovered in fisheries from Alaska to Oregon. Most were gathered during troll and sport fisheries off the West Coast of Vancouver Island and in the Strait of Georgia (Fig. 3). Recent catch distributions of coho from streams draining into the Strait of Georgia including the Fraser have been dominated by dramatic swings between fisheries inside and outside of the Strait of Georgia. Prior to 1991, large numbers of coho remained inside Georgia Strait each year and supported major sport and troll fisheries. In 1991, and during 1995-2000, the majority of coho appeared to leave the Strait of Georgia and spend most of their adult lives off the West coast of Vancouver Island. Marine conditions including ENSO events and climate change are known to affect the marine distribution of coho salmon (Pearcy 1992; Beamish et al. 1999a).
The Fraser River is the largest river in BC and produces more salmon than any other river in the world (Northcote and Larkin 1989). The interior Fraser watershed is part of the Southern Mountain COSEWIC Ecological Area. Coho salmon are widespread throughout the Thompson River system, the largest watershed within the Fraser River system. However, their distribution in non-Thompson Fraser tributaries is not well known. Coho salmon occur at least as far upstream as the Nechako River, but there are several major upper Fraser watersheds where coho probably occur but their presence has not been confirmed (Fig. 3).
For the North and South Thompson drainages where the spawner survey data are the most reliable, there is evidence that coho are spawning in fewer streams than they were when coho were more abundant. Bradford (1998) noted that 32% of streams that had fish observed in them in 1988 had reached ‘none-observed’ status in 1997 (i.e. 3 generations later). This proportion was reduced to 18% in 1999. In a preliminary assessment of the possibility of using stream occupancy to assess the status of Thompson coho, Bradford and Irvine (2000a) found a non-linear reduction in stream occupancy with declining coho abundance. Reductions in stream occupancy began to occur when the overall abundance was reduced by about 75% from peak abundance.
Footnotes
2 Sandercock (1991) reports that coho have been found in rivers in Hokkaido, northern Japan, but these fish probably were not coho, or if they were coho they were strays; coho spawning naturally in Japan have not been confirmed (J. R. Irvine, unpub.).
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