48. MODERN VIEWS OF FISH MIGRATION
by Dr P. A. Orkin
Centuries ago the goal-directed activities of men were ascribed to "reason", those of animals to "instinct". By instinct one meant an inborn, automatic, mental process directing external activity without the aid or accompaniment of reason or even of consciousness. Thus, when at the age of a year or more a young salmon makes its first seaward migration, this action was called instinctive. The proper time having come, some appropriate part of the brain began to tick over,' and the body obediently swam down to the sea. Researches have shown that such an interpretation is quite inadequate. In its first years, the young salmon, or parr, shows "territorial behaviour" - it remains in a circumscribed area from which it drives all smaller fishes. Like most river fishes, it is rheotactic, that is to say, it will not allow itself to be carried passively downstream; the sight of the bottom or bank drifting by brings about an immediate compensatory movement. By day territorial behaviour reinforced by rheotaxis, keeps it in one place; by night it rests on the bottom in shallow water.
As it becomes older it becomes photoperiodic - sensitive to progressive changes in the length of the day. The lengthening days of spring stimulate the pituitary gland lying at the base of the brain. The secretion of the pituitary in turn stimulates the thyroid gland. As with other animals, in creased thyroid secretion makes the salmon more active and restless. Its colour changes to a metallic silver, and the young salmon, now called a smolt, becomes gregarious and swims by night as well as by day. By day its rheotactic re flexes serve to keep it more or less in the same place, but in darkness its restless swimming inevitably results in its being carried downstream towards the river's mouth.
Meanwhile changes have been taking place in the smolt's excretory mechanism. In freshwater, water is continually diffusing into a fish and being excreted by the kidneys, whereas in the sea, water diffuses outward, and the fish compensates by drinking seawater and excreting the surplus salt through the gills. The gradual change-over from a fresh water to a marine type of excretory mechanism makes the salmon feel uncomfortable in the river so that when, at the river's mouth, it encounters a salinity gradient it tends to move ever farther offshore. Any move in the other direction will increase its discomfort and will soon be checked.
The salmon is certainly not "guided by instinct" in its seaward journey. It is not guided at all: when it can see its surroundings it endeavours to stay put, and its downstream movement is an unintended by-product of its aimless nocturnal activity. This in turn is a response to the increasing length of day which becomes effective only when the parr reaches a certain size. In the south of England the young salmon grows rapidly, and will reach this critical size and migrate at the age of one year; in the north of England it takes two years; in northern Scotland three years or even more. It is largely a question of average temperature and food supply. Once in the sea it swims aimlessly but, in a salinity gradient, finds one direction preferable to another. The story of the salmon as revealed by experimental analysis is, of course, much more complicated than this, and there is no room here to tell it all, but there appears to be no part of the salmon's activity which cannot be analysed into simple responses to external stimuli of the kind outlined above. Thus, after months or years in the sea, changes in the excretory mechanism are responsible for its movement first into less saline inshore waters and eventually into fresh water. It has been shown that the salmon recognises its natal river by smell, and that usually, though not invariably, it returns to the very streamlet whence it came. Once in the river, it ascends slowly, sometimes not at all, until the shortening days of autumn stimulate the pituitary gland to produce the gonaclotropic hormones which in turn stimulate the growth of the reproductive organs, and release the upstream migratory and spawning behaviour.
To sum up: in former times the salmon would have been compared to an automaton, controlled from within by a programmed type steadily unwinding regardless of external conditions. Today we would say that the salmon responds at every instant to external conditions and that, as these con ditions have remained constant for great ages, natural selection has preserved in the salmon those responses which together ensure a successful life-cycle.
(from Science Survey Vol. I. (Vista Books))