Against the current

When salmon hatch they have an innate sense of up and down, and they automatically move downward into the spaces between the gravel. This response to gravity (geotaxis) allows alevins to remain in the gravel and continue developing without being washed down river. About a month after hatching alevins become more active and increase lateral movements within the gravel. During this phase the alevins begin to orientate themselves toward the current (positive rheotaxis) and spend more time in the lighted region of the gravel. Once salmon emerge from the gravel as fry, their built-in guidance system, rheotaxis, directs them to face into the flow. Instead of spending enormous amounts of energy looking for food, they will swim in place and wait for plankton and other food to come to them. It’s kind of like a moving buffet with a bit of competition.