HOMING MIGRATION OF SOCKEYE SALMON , ONCORHYNCHUS NERKA , TO

Adult sockeye salmon, Oncorhynchus nerka, return to the Fraser River via either of two routes: a northern route through Queen Charlotte Strait. Johnstone Strait, and the Strait of Georgia between the mainland and Vancouver Island, and a southern route along the west coast of Vancouver Island and through Juan de Fuca Strait. The proportions of the total run of sockeye salmon using the two routes varies substantially from year to year. Understanding the factors influencing the migratory routes of Fraser River sockeye salmon provides a basis for forecasting the coastal migrations of salmon as they make the transition between oceanic and riverine environments. Our analysis ofwest coast troll catch and high seas tag-recovery data indicates that the salmon make landfall in different coastal regions from year to year. If the majority of Fraser sockeye approach the coast of Vancouver Island, then most will migrate via the Strait of Juan de Fuca. However. when landfall occurs north of Vancouver Island in the Queen Charlotte Sound area. most homeward migrating Fraser sockeye will travel through Johnstone Strait. Northern diversion rates ofFraser River sockeye salmon for the period 1953-77 were positively correlated with Fraser River discharge. For the period 1978-85 a strong positive correlation was evident with sea surface temperature (SST) along the northwest coast of Vancouver Island (Kains Island lighthouse). We conclude that Fraser River discharge and SST in the vicinity of Kains Island do not guide sockeye salmon in any direct way during their coastal approach, but that they reflect oceanographic conditions that affect salmon migrations directly or indirectly by acting on the feeding distribution. distance. or direction they must travel to reach home. The Fraser River in British Columbia, Canada, is among the most important producers of sockeye salmon, Oncorhynchus nerka, in North America. Forty to sixty separate stocks, inhabiting the different lakes of its watershed, produce 2 to 20 million adults yearly (IPSFC 1954-1985). Sockeye salmon from the Fraser River system generally spend 1 year in nursery lakes after emergence and then migrate to sea as smolts. Most spend two winters in the ocean, returning to spawn in their home river as 4-yr-olds. To reach the Fraser River from their ocean feeding grounds they can take either of two routes around Vancouver Island (Fig. 1). From 1953 until 1977, the majority homed via the southern route through the Strait of Juan de Fuca (average 84%, range 65-98%). Since 1978, a larger proportion of sockeye have migrated via the northern route through Johnstone Strait (average through Juan de Fuca Strait 56%, range 20-78%) (IPSFC 1954-1986). lDepartment of Fisheries and Oceans, Fisheries Research Branch, Pacific Biological Station, Nanaimo, British Columbia V9R 5K6, Canada. 2University of British Columbia, Department of Oceanography. Vancouver, B.C. V6T 1Y4, Canada; present address: School of Fisheries WH-I0, University of Washington. Seattle, WA 98195. Manuscript accepted April 1981. FISHERY BULLETIN: VOL. 85, NO.3. 1981. In 1958 a relatively high proportion (35%) of Fraser River sockeye salmon returned via the northern route. A large number of fish did not make landfall off the west coast of Vancouver Island but rather arrived in the more northerly Queen Charlotte Sound area (Tully et al. 1960) (Fig. 1). This coincided with anomalously high water temperatures off the coast of British Columbia. Tully et al. (1960) and Royal and Tully (1961) suggested that intrusion of warm water from the south in 1958 directed the homing sockeye salmon northward and closer to the mainland. Moreover, the fish appeared 10 days later in the fishery around Vancouver Island and over a longer period than usual, suggesting that they might have detoured around the area of warm water and made their coastal approach in the cooler nearshore waters. Alternatively, they might have initiated their homeward migration later or from a more distant area than usual. Favorite (1961), on the other hand, took the view that the unusual extent ofdilute seawater of Fraser River origin offshore from Queen Charlotte Sound in 1958 determined the location where the migrating sockeye entered coastal water. He assumed that homeward migrating salmon are attracted to dilute seawater contain-