Calcium is part of salmon bouquet
A complex melange of sensations imprinted in the olfactory bulb, the part of the brain that receives signals of smell, seems to be responsible for the precise homing ability of salmon. Scientists have investigated the physiological basis for the salmon's remarkable skill in finding "nurse" lakes, and later returning to the very stream where it hatched and began to grow after years of roaming through the oceans. In experiments on the sockeye salmon (Oncorhyncus nerka) they have shown that this species has the most sensitive calcium detector yet discovered in the natural world.
The sockeye, unlike many other salmon, has a two-stage migration to the sea. The small "fingerling" fry first migrate to "nurse lakes" in which they spend two years, before finally heading out to sea. The current theories on the salmon's homing have recently shown that there are two navigation systems at work. One of these, the long range one, gets the fish to the right land mass. The second, used in the water currents around this land mass, gets the fish into the right river. Recent experiments have shown this second system to be mediated largely via the "smell" and taste of the home water. The Sockeye, though, has other problems. First, the river it must head up is not its birthplace, but the outlet of its nurse lake, which may smell quite differently. Secondly, it even has to find the nurse lake as a fry in the first place, and how does it know for instance whether to swim up or down stream?
The first question scientists had to ask themselves was, how does the fry get to the lake? Putting the fry in a Y-shaped maze, they gave them a straight choice between lake water and well-water. Fish reared up to that time in lake water chose lake water, fish reared in well-water showed no real preference. Totally naive fish from any river irrespective of the lake it drained showed a clear choice for lake waters.
It looks then as if all Sockeye fry instinctively prefer lake water when migration to the sea begins. That's fine for fish downstream of the lake--they simply sniff their way up. But what about fish in rivers draining into the lake? Work carried out some years ago showed that sockeyes normally swim against the current, but that this tendency is much less marked in non-lake water. So fishes above the lake who would always be in non-lake water would tend to end up in the nurse lake anyway.
Other experiments looked at the electrical responses in the olfactory bulb neurones which must underlie these behavioral events. Looking at the responses of individual cells, scientists found some very curious results indeed. Some neurones responded to water from one river source only, often one which the fish had never experienced. In other words the neurones seem to be pre-patterned to detect an entire olfactory "fingerprint" of any water, and somehow the neurone appropriate to the river in which the fish hatches eventually dominates the subsequent preferences. For the detection of lake water, it was found that neurones responded to lake water in a 1 to 100 dilation with distilled water. This highly tuned system showed no greatly enhanced response to the home water though, which seems to show that the imprinting of the smell of "home" takes place at much higher level than the olfactory bulb.
What ever it is in lake water which salmon find so appealing must have a physico-chemical basis. Research scientists have homed in on calcium and found in the process the most sensitive calcium detector yet discovered in nature. The Sockeyes could pick up calcium ions at well below the naturally occurring concentration range.
Is it the calcium level of home water then which eventually guides the fish home? As a pointer some Sockeyes were reared in Lake Washington water, and then run in the Y-maze with their home lake water in one arm and the same water with a tiny amount of calcium ions added in the other. All chose their unadulterated home water.
Scientists are not saying that calcium ions alone guide the fish home; they probably respond to a whole "bouquet" and may help both to guide fry to the lake, and adults back in from their years in the high seas.