U.S. Dept Commerce/NOAA/NMFS/NWFSC/Tech Memos
NOAA-NMFS-NWFSC TM-29: Estuarine and Ocean Survival of Northeastern Pacific Salmon


EARLY OCEAN SURVIVAL OF SALMON OFF BRITISH COLUMBIA
AND IMPACTS OF THE 1983 AND 1991-95 EL NIÑO EVENTS

Brent N. Hargreaves

Canadian Department of Fisheries and Oceans
Biological Sciences Branch
Pacific Biological Station
Nanaimo, BC Canada V9R 5K6

Chinook salmon (Oncorhynchus tshawytscha) from the west coast of Vancouver Island (WCVI) have become a major contributor to commercial troll and recreational fisheries in British Columbia and southeast Alaska. The Somass River/Robertson Creek chinook stock is the most important WCVI stock, with total production exceeding 400,000 adults as recently as 1991. However, marine survival rates of this stock have varied by over two orders of magnitude.

Chinook salmon produced by this stock in a given year can return as 2-, 3-, 4-, 5-, or 6-year-old adults. Although only a small percentage return as 2-year-old males (jacks), once returns from these younger fish are known, the total return for that brood year for all years can be reliably forecast. This indicates that marine survival patterns are predominantly determined during the first year of marine life, before the first jacks return.

Variations in ocean conditions appear to strongly affect marine survival of WCVI chinook salmon stocks. The lowest marine survival rates observed previously for the Somass stock (0.1% for the 1983 brood year) coincided with the 1983 El Niño event. This effect was relatively modest, however, since only one brood year was affected. More recent El Niño events in 1991-95 have affected survival for at least three consecutive brood years (1990 to 1992), and this has had a much greater impact. The returns of WCVI chinook salmon in 1995 were low, and returns in 1996 and 1997 are forecast to be extremely poor. Predation on juvenile chinook salmon during the early marine period was likely a major cause of the poor survivals of the 1983 and 1991-93 brood years. The cause of the longer term pattern of declining survival rates which preceded both of these major El Niño events is not known.



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