GROOT and QUINN: HOMING MIGRATION OF SOCKEYE SALMON 



opposite to the increasing saltwater preference 

 documented by Baggerman (1960) and 

 Mclnerney (1964) for juvenile salmon during the 

 period of seaward migration. Mclnerney (1964) 

 argued that the shift in salinity preference over 

 time could gradually lead fish along the gradient 

 of salinities found in coastal areas toward the 

 open ocean. 



Salinity measurements during the 1959 cruise 

 (S.I.O.U.C 1965) mentioned previously showed 

 that from the middle of the Gulf of Alaska to the 

 tip of Vancouver Island, sea surface salinities de- 

 creased from 32.73 to 32.46%o over a distance of 

 about 1,757 km (Table 4). Fish swimming at a 

 rate of 48 km/day would have met changes aver- 

 aging 0.00015%c/km or 0.0074%c/day when ap- 

 proaching the coast. The threshold for recognition 

 of salinity differences by sockeye salmon is un- 

 known. However, if it is similar to that of min- 

 nows iPhoxinus phoxinus) of 0.003S?f (Glaser 

 1966), then it is about 20 times higher than the 

 average difference that wil be encountered during 

 a km of travel. 



Table 4. — Salinity at depth, recorded by the RV Brown 

 Bear. 16-24 July 1959 (data from S.I.O.U.C. 1965) 



iQata not Included In calculations of gradients. 



Moreover, salinity changes towards the coast 

 do not occur in a smooth gradient (Table 4). Water 

 masses of different salinities and temperatures 

 form a dynamic patchwork that is continuously 

 changing under the influence of wind and cur- 

 rents (Tabata 1984). We therefore consider it un- 

 likely that a general preference for lower salinity 

 water determines the approach direction of hom- 

 ing Fraser River sockeye migration. Smith (1985) 

 concluded that for fishes in general ". . .there is 

 little evidence that salinity is a guiding mecha- 

 nism." 



Second, the sockeye could react to home odors 

 from the Fraser River in the offshore waters as 

 suggested by Favorite (1961) and Wickett (1977). 

 The sensitivity of salmonids to certain odors is 

 high: 10"^ M for morpholine and 10"^ M for free 



amino acids (Brett and Groot 1963; Hara et al. 

 1984). However, it is questionable that, given the 

 extensive mixing in the Fraser River and in the 

 ocean, the already low concentrations of odors 

 from the different nursery lakes would be above 

 threshold. Moreover, odors generally act as re- 

 leasers and not as directors of responses ( Johnsen 

 and Hasler 1980). It is difficult to understand how 

 salmon could change their migration routes far 

 offshore in the ocean, even if they could sense the 

 aroma of their home water. We therefore conclude 

 that the relationship between Fraser River dis- 

 charge and diversion rate of sockeye salmon re- 

 turning via the northern route is not a direct, but 

 probably an indirect one. 



CONCLUSION 



We hj^othesize that Fraser River discharge 

 (1953-77) and SST at Kains Island (1978-85) pri- 

 marily reflect certain atmospheric and related 

 oceanographic conditions, which affect Fraser 

 River sockeye salmon winter distribution and/or 

 migration in the ocean. The weather conditions in 

 the Gulf of Alaska are controlled by the locations 

 and intensities of two major semipermanent at- 

 mospheric pressure cells; the Aleutian low and 

 the North Pacific high (Favorite et al. 1976; 

 Thomson 1981; Emery and Hamilton 1985). The 

 interannual variations of these pressure cells af- 

 fect precipitation and the extent of the snow pack 

 during the winter, as well as temperature, salin- 

 ity, and circulation patterns in the ocean (Favor- 

 ite et al. 1976; Thomson 1981; Emery and Hamil- 

 ton 1985). 



Anomalous temperature conditions in the 

 ocean, resulting from varying atmospheric condi- 

 tions, may affect salmon migrations directly or 

 indirectly by acting on their feeding distribution 

 or on the distance or direction they must travel to 

 reach home. When ocean conditions are warmer 

 than usual, sockeye salmon tend to encounter the 

 coast of British Columbia at the north of Vancou- 

 ver Island. In such cases their approach to the 

 Fraser River will be primarily through Johnstone 

 Strait. Following cold winter conditions in the 

 Gulf of Alaska, landfall usually occurs along the 

 west coast of Vancouver Island and migration to 

 the home river is primarily via the Strait of Juan 

 de Fuca. 



ACKNOWLEDGMENTS 



We thank Lawrence Mysak, Kevin Hamilton, 



467 



