National Geographic : 1968 Aug
Later Bob and I watched Drs. Horrall and Stasko slip a duplicate cylinder-7/8 of an inch thick and 31/4 inches long-down a sock eye's gullet (page 208). The fish, released, re mained inactive for an hour or so while we hunched over our receiving equipment in a skiff. Then, about noon, the steady note being picked up through the directional hydrophone in the water turned into a "wowing" sound. "That happens when the fish swims," said Ross, springing to life. "Muscle movement varies the pressure against the transmitter, modifying the sound waves." Aivars started the outboard motor and we began our pursuit. The transmitter in the salmon had a range of about a mile, and we fol lowed the fish as it approached East Sound, which cuts Orcas Island nearly in two. The shore there curves north-as if pointing the shortest route to the salmon's Fraser River spawning ground. Yet it forms a cul-de-sac, and if the sockeye had entered, it would have had to turn around and come out again. Without hesitation the salmon ignored the false promise of East Sound and headed for Peavine Pass, the route that would lead most directly toward the Fraser River. Bob and I left then, but Ross and Aivars trailed the fish for nine hours. They learned that it tended to take the direct route rather than follow shore line indentations-another bit added to the knowledge of salmon migrations at sea. Its Nose Knows Its Old Home If the mechanism of ocean migration is still mysterious, scientists are on surer ground when it comes to explaining how the salmon gets from river mouth to spawning site. It follows its nose. As Dr. Hasler at the Univer sity of Wisconsin puts it: "The fish smells its way home from the coastline of the sea, track ing a familiar scent like a foxhound." That scent may be produced by dissolved organic material from plants in the tributary and may vary from one tributary to another only in the slightest degree. Downstream it is rapidly reduced to extremely low concentra tions. To detect such faint traces, a fish must have fantastic ability to pick up odors and to tell them apart. The salmon has such a sense. It can per ceive dilutions of one part in a billion. If a quart of water in which a human hand or the paw of a bear has been dipped for a minute is poured into a stream, it can cause migrating salmon to stop climbing a fish ladder a hun dred yards away. Other fish have similar acuity. Laboratory experiments show that the 206 Head in a tagging hood, a pink salmon fry is wired for sound. A tiny stub of hair-thin wire slips through a hollow needle, right, into the insensitive cartilage of the head of a fry held in a biologist's fingers. The magnetized wire, which the fish wears for life, later triggers the "beep" of a sensing device checking fish at a cannery or as they swim upriver for spawning. Retrieved and examined, the color-coded wire tells when and where the fish began its travels. Tattooed seafarers, coho sprayed with fluo rescent pigment shimmer under ultraviolet light. Such spots, which last several months, help biologists trace paths of migration.