All but the most inastute among us are aware of the seasonal migrations of birds. The movements southward in autumn and northward in spring impress us with the numbers of birds flying, and the temporal and spatial predictability of these migrations. While they are less in the public eye, the migrations of Pacific salmon are at least as impressive.
Despite population declines due to loss of habitat, pollution, and tremendous fishing pressure, tens of millions of adult salmon swim from the open North Pacific Ocean to specific coastal regions, and then swim upstream (often hundreds of miles) to the stream in which their parents spawned and died years before. A sizable body of observational and experimental evidence, much of it the work of Dr. Arthur Hasler of the University of Wisconsin and his co-workers, indicates that the homing ability of salmon in fresh water is based largely on the memory of odors unique to each stream.
It is generally agreed, however, that the movements of salmon on the open Pacific Ocean are not guided by odors, but that a more complex navigational system may be operating. Unfortunately, it is both expensive and logistically very difficult to carry out experiments with adult salmon on the high seas.
Scientists interested in finding out how the salmon find their way across large bodies of open water are fortunate, because one species, the sockeye or red salmon, makes extensive lake migrations as juveniles. Other species either remain in rivers (chinook and coho salmon) or swim directly to the ocean (chum and pink salmon) after they emerge from their incubation areas in streams. Sockeye, on the other hand, swim to a lake and disperse to feed for a year, migrating out of the lake and down to the ocean a year later. Scientists have been able to gain some insights into the ocean migration patterns of adult salmon by studying the migratory behavior of young sockeye in large lakes.
In the early 1960s, Dr. Case Groot determined that sockeye salmon migrating to the outlet of Babine Lake, a narrow 100-mile-long body in British Columbia, did not swim randomly to reach the outlet, but moved in a rapid, oriented fashion. In controlled experiments, he was able to demonstrate that the small ( 4-inch) salmon can get directional information from the sun's position in the sky.They can also orient using the polarization patterns of sunlight passing through the earth's atmosphere (a sense unknown to humans). When Dr. Groot eliminated these two sources of directional information, however, the salmon could still orient their movements using another, unknown sensory system.
In the early 1970s, Dr. Ernest Brannon of the University of Washington showed that the baby sockeye, known as fry, have a very complex guidance mechanism for finding their "nursery" lake. He also showed that they have compass-directional preferences which, he hypothesized, would facilitate their distribution in their lake.
To understand why the fish need such abilities, we must realize that tens of millions of fish in a small area of a lake would quickly eat all the food, and would also attract the attention of predators. Dr. Brannon felt that natural selection had favored fish that moved away from the river mouth and dispersed in the lake. His work supported Dr. Groot's evidence that the movements of small salmon in large lakes is a directed, not a random, process, but the guidance mechanism had still not been discovered. The fish often swim at night, and in deep water, where the sun's position would not be a useful orienting cue.
In the late 1960s and early '70s, evidence began to mount that migrating birds and homing pigeons could use the earth's magnetic field as a navigational aid. Evidence for sun and star orientation was more generally accepted, but the perception of magnetic fields had been formerly dismissed as impossible. Experiments carried out with migratory birds in Germany by Dr. Wolfgang Wiltschko and his co-workers and tests with homing pigeons by the late Dr. William Keeton of Cornell University and others indicated that magnetic fields were part of the repertoire of orientation cues available to the birds.
While a variety of animals were eventually shown to detect magnetic fields, no successful experiments were carried out with Pacific salmon until 1979. Lakeward-migrating sockeye fry from three river systems were placed in four-armed tanks. Fish from each system tended to move in the compass direction that corresponded to the direction that they would have to swim upon entrance into their lake. Thus one population headed north- northwest, one headed northeast, and one headed south-southeast. To determine if the fry were using the stars or the sun as sources of directional information, the tests were replicated with covers over the tanks. The fish still oriented properly.
To test the hypothesis that magnetic fields are used by the fish, a cube-shaped coil of copper wire was placed around the tank. A direct current running through the coil caused the magnetic field inside to be altered. North was effectively in the west, with the whole field rotated 90 degrees counterclockwise. This redirection of the magnetic field was accompanied by a similar redirection of the movement patterns of the fish in the tanks.
Interestingly, the migratory directional preferences of each population appear to be inherited, not learned. But, one may ask, if the fish are all sockeye salmon, how can they have genetic differences? To answer this, we must recall that adult salmon almost always return to spawn in the river where they were incubated as eggs. Thus individual populations are able to evolve specializations to enhance their fitness for particular river systems. One such specialization appears to be migratory directional orientation.
The next question posed was: Are magnetic fields more important than the sun or nighttime celestial cues? What is the hierarchy of navigation systems? The answer to this question, like so many about salmon, is that it depends on the population. The night migrating fry that swim north in Lake Washington by Seattle were oriented to the altered magnetic field, even if they could see the night sky. However, day-migrating fry from the Chilko River in British Columbia ignored the altered magnetic field as long as they could see the daytime sky. In a covered tank, though, they appeared to use the magnetic field for directional information.
What initially emerges from this kind of research is a bewildering assortment of directional preferences being cued by various sensory inputs and displayed at various times of the day by fish from various rivers. But the overall pattern is relatively clear. Tens of millions of baby sockeye salmon must find a lake in order to avoid predation and find food. They will use olfaction and a response to water currents (known as rheotaxis) to find the lake. This may mean swimming downstream, upstream, or downstream and then upstream, depending on the geography of the system. Once the fry reach the lake, their migrations in it are guided by a variety of cues. Since no cue is completely reliable, natural selection has favored fish that can use more than one. A year later, the fish are stimulated to migrate from the lake, and they employ guided compass responses to accomplish this. While a great deal of work still needs to be done , research indicates that the migrations represent remarkably complex interactions between environmental inputs and inherited response patterns.
The experiments described here all dealt with orientation by small sockeye salmon in large lakes. Still unanswered are several major questions: Do sockeye use magnetic fields to help guide their estuarine and oceanic movements? Do other species of salmon sense magnetic fields? How do salmon detect magnetic fields? Will such information aid fisheries managers in predicting migration routes and timing? Only time and more research will tell, but prospects appear good for greater understanding of the migrations of these remarkable fish.