A study of the variability of the return migration route of Fraser River sockeye salmon (Oncorhynchus nerka)

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Abstract
This paper discusses the interannual variability observed in the fraction of the Fraser River sockeye spawning run that returns around the north end of Vancouver Island [Canada] and then through Johnstone Strait (the so-called "northern diversion"). Estimates of the northern diversion have been produced by the International Pacific Salmon Fisheries Commission for each year from 1953 through 1983. In the present study commercial catch data were used to estimate the diversion rates for the years 1906-1952. The extended time series 1906-1983 was then employed in an examination of the relation between the diversion rates in each year and (i) the phase of the familiar 4-year cycle of the Fraser sockeye, (ii) the size and timing of the spawning run, and (iii) environmental factors such as ocean temperature and Fraser River discharge. The results obtained suggest that there is at most a weak relationship between the diversion rates and the 4-year cycle. There is a strong correlation between the nothern diversion and the duration of the spawning run; the sockeye run tends to be spread out over a longer period in years with anomalously large returns through Johnstone Strait. A very significant correlation was found between the diversion rate and the temperature change in the coastal waters off Vancouver Island over the last 18 months of the salmon''s ocean residence. Strong, persistent warming trends in these coastal waters are known to accompany most large tropical El Nino events; thus it often happens that large northern diversion of the Fraser sockeye occur after major Los Ninos (most notably in 1915, 1926, 1958, and 1983).

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