LACUSTRINE SOCKEYE-SALMON RETURN STRAIGHT TO THEIR NATAL AREA FROM OPEN WATER USING BOTH VISUAL AND OLFACTORY CUES

Mechanisms of the amazing ability of salmon to migrate a long distance from open water ro natal streams for spawning are still unknown. Lacu strine sockeye salmon (Oncorhynchus nerka) in Lake Toya offers an exce llent model system for studying the orientation mechanism in open wate r, because mature fish return to the natal area with a high degree of accuracy. First we examined the percentage of fish returning to the na tal area after they were released 7 km south of the natal area. Forty percent of control male mature fish and 25% of the fish blinded by inj ection of a mixture of carbon toner and corn oil into the eyeball were captured in the natal area within 5 days. Forty-four percent of fish with brass rings (control) and 31% of fish with NdFe magnetic rings wh ich interfere with the magnetic cue were captured in the natal area wi thin 3 days. These experiments suggested that, although the number of blinded fish captured in the natal area was less than that of the cont rols, the difference was not statistically significant. In the fish ca ptured in the natal area within 3 or 5 days, fish which found the nata l area using their olfactory cue after random swimming for a long time and returned to that area may be included. Hence we tracked fish tele metrically using an ultrasonic tracking system, and found that mature males released at a long distance (3.6 or 6.8 km) from the natal area swam straight to the vicinity of the natal area. Interference of the m agnetic cue by the attachment of a magnetic ring did not affect their direct return. Blockage of the visual cue caused them to move randomly . These data suggest that lacustrine sockeye salmon return straight to the vicinity of the natal area using their visual cue and finally rea ch the exact homing point using their olfactory cue.