National Geographic : 1968 Aug
eel can detect alcohol in water in proportions equal to a teaspoonful in Lake Superior. Dr. Hasler's theory suggests that salmon become "imprinted" with the particular com plex smell of their home water during their residence as eggs, fry, or fingerlings. This experience, he believes, is powerful enough to cause the fish to "remember" the odor from one to seven years later when it is swimming upstream as an adult. Similar imprinting at the river's mouth, where fingerlings play for a time before going to sea, may provide a second remembered odor. To test his theory, Dr. Hasler's team col lected coho salmon that had climbed sepa rate branches of Issaquah Creek near Seattle, Washington. The fish were taken downstream and released just below the creek's fork some with their nostrils plugged by cotton soaked in petroleum jelly. Most of the fish with sense of smell unimpaired found their way back to the original collecting point. But the ones with nose plugs were unable to choose their own stream with certainty. For some salmon, the trip upriver is short. Pinks and chums usually spawn close to the sea, sometimes right in the intertidal zone. Other species travel hundreds of miles inland; some chinooks battle upstream for months, journeying as far as 2,000 miles from the coast. Salmon Shun Food on Homeward Trip The inland migration is a herculean ordeal. The salmon arrive at the river mouth in prime condition, their flesh often tinged red from carotenoids of the shrimplike crustaceans on which they have fed at sea, and oil-laden from a diet of herring and other fatty fishes. But once headed upstream, they stop eat ing altogether. The jaws of the male begin to hook grotesquely; eventually he cannot close his mouth. The stomachs of both sexes shrivel. Through their long struggles against current and waterfall, the fish live on body-stored fat alone, becoming mere carriers for the sex prod ucts they will deposit before they die. Commercial fishermen lie in wait for the returning salmon hordes. Gill-netters take EKTACHROME(BELOW)ANDKODACHROME BY NATIONALGEOGRAPHICPHOTOGRAPHER ROBERTF. SISSON N.G.S.