WANDERING FISHES

Otago Witness, Issue 3886, 4 September 1928, Page 76

WANDERING FISHES

By

Professor J. Arthur Thomson.

It is a pity to spoil the word “ migration ” by applying it to other kinds of mass-movements. True migration is best illustrated among birds; it is a wellestablished seasonal swing between summer quarters, the nesting and- breeding place, and winter quarters, the resting and feeding place. True migration is a regular ebb and flow. But this sort of mass-movement is quite different from a shifting of shoals of fishes from one part of , the sea to another during a period of the year when there is no question of breeding. They are following the movements of the animals on which they feed, and these are influenced by changes in the physical .conditions of the water, such as temperature. Or again when the Scandinavian lemmings have devoured everything edible they go on the march in search of food, and a small band soon becomes a great army.' But this is not migration. It is comparable to what has often taken place in human history, when the setting in of aridity, or something equally disquieting, forced the people to trek to new countries. In some cases, we admit, it is difficult to know’ what word to use. Ums, a swarm of locusts is often no more than a mass-movement spurred by hunger: the hordes have eaten up every green thing, and they, must press on, first on foot, and afterwards.. on wing. But there are countries where the locusts appear regularly at a certain time of year, and after doing terrible damage to vegetation disappear again, to the great relief of the inhabitants. thus, in Algiers the locusts appear in vast numbers in the early months of the year, coming from the Sahara, and return to near the Equator in winter. In the same way there are several kinds of monkeys in India that ascend the Himalayas in summer, to a height sometimes of 10,000 feet, and descend to the lew ground in the winter. This is a periodic mass-movement, but it has no close connection with the birth of the young ones. It seems, then, that we may distinguish true migrations, as in birds and fur-seals, which are regular, seasonal, and connected with breeding, from regular movements that have iw such connection, and also from irregular irruptions or dispersals which are due to over-population and lack of food, or to some change for the worse in the physical conditions. Leaving birds by themselves, let us picture some of the different kinds of movements among other creatures.

Fish-eating turtles pass most of the year in the open sea, but they are egglaying reptiles, and the eggs must be deposited on land. For the turtles are land-reptiles that have taken to the sea as a second string. So it comes about that when the time for egg-laying draws near the turtles make for certain sandy beaches, on islands or on the mainland, and there they bury their eggs, leaving them to be hatched by the warmth of the sun. When the young turtles break out of the egg-shell they make straight for -the sea—even if obstacles are put in their way. The whole story illustrates true migration.

Or, to take another reptile, there are some of the sea-snakes which come to the. shore-rocks to bring forth their young. They are not egg-layers like the turtles, they bring forth their young ones as little snakes, but it is known of certain kinds that the mother- stays with her family for some time before returning to the open sea. This also is true migration. But whereas in birds the migration is wrapped up with the difference in the climates of the nesting place anu the resting place—birds always nest in the colder part of their migratory range—the migration of turtles. is associated with the fact that the eggs would be lost in the sea, while the migration of certain sea-snakes has

’ do with the fact that it is risky for a lung-breathing animal to be born in the open waters.

When the salmon have become large and lusty in the sea, they make for the rivers, prompted by the urge to breed. They press up against the stream, often surmounting low waterfalls and overcoming strong rapids; and they are fasting all the time, notwithstanding their great exertions. To put it in slightly metaphorical words, they are moved by “ love ” rather than by “ hunger.” They continue their journey till they come to suitable gravelly beds, where the female makes a furrow and deposits the eggs, which the male fertilises with the milt. After a slow development, which does not concern us here, the eggs develop into alevins, which become fry, which become parr, which become smelts, which in their second or third year become restless and make for the sea. This, again, is true migration.

Flounders are often found far up rivers, a dozen miles or more from the shore, but they have to go down .to the sea to spawn, and the young flounders must remain for a long time in the salt water—a life-history just the converse of the salmon’s.

The lamprey, which is at a lower level than a fish, is interesting in its movements. There are some kinds—brook lampreys—that remain always in fresh water, as many trout do; but the large lamprey, which grows to be a yard long and as thick as one’s wrist, feeds and grow’s in the Bea, but comes up the rivers to spawn. * Now, the large lamprey’s story differs from that of the salmon in this, that both parents die after spawning. Yet we cannot exclude the marine- lamprey’s

movements from the rank of migration on the ground that the adults do not survive to make a return journey to the sea. The same remark applies to the movements of the common eel, which are the converse of those of the sea lamprey. The eels grow large in rivers and ponds; they spawn and die in dark, deepish water far out in the Atlantic; the larvae, after about three years of changeful development in the open sea, ascend the rivers as elvers. The adults never share in the return journey, but the story as a whole illustrates true migration. Take, on the other hand, the movements of the “ army-worm,” which have often excited great astonishment. The “ army-worms ” are the maggots of a fly, and they hatch out in damp woods among putrefying vegetable debris. There they feed and grow, but if very dry weather sets in and the floor of the wood ceases to be damp, the “ armyworms ” must march. Different companies meet one another and combine into regiments, and these into armies—sometimes ten feet long and six inches broad—an extraordinary instance of the abundance of life. In most cases the maggot army soon comes to naught: the birds see to that. But this is no migration; it is a compulsory trekking with hunger and drought as spurs. Very interesting is the mass-move-ment of small spiders, so often seen in autumn, but sometimes occurring at other seasons. Several kinds of small spiders climb on to tall plants on a breezy morning and, standing with their heads to the wind, pay out three or four threads of silk—the gossamer. When these are long enough the wind tugs at them; the spiders let go and are borne through the air, sometimes for miles. The details do not concern us here, but the spiders eventually sink to earth and their ballooning threads, that have served their purpose, are entangled on the surface of the fields and meadows. Our point is that the significance of these aerial journeys is to transport the spiders to a new area, from a crowded region, it may be, to one with more elbow room. It is an exploration rather than a migration, but it is a mass-movement often involving thousands at a time. The fact that the -wind may occasionally blow half of them out to sea raises no difficulty. No arrangements are perfect. There are other creatures besides mice and men whose ways go “ agley.” To sum up: (1) When vast numbers of dragon-flies, butterflies, spiders, and so on, up to rats and lemmings, suddenly appear, but not regularly, in a region where they are not common, this indicates a dispersal and an incursion prompted by hunger, either due to overpopulation or to some unpropit ions change in the physical conditions. (2) When troops of reindeer, or bands of monkeys, or schools of cetaceans, or shoals of fishes regularly’ come and go from certain places to certain places at different times of year, this indicates an answer-back to seasonal changes. Such mass-movements might be called periodic wanderings. (3) When the robber crabs of Christmas Island leave the coco-trees and make for the shore, where they spawn and whence they return, followed long afterwards by the surviving members of their families, this is a true migration, like that of the fur-seals to their rookeries, like that of the turtles to their sandy island, like that of the salmon to their river, and- like that of the birds who “ change their season in a night, and wail their way from cloud to cloud down the long wind.” But we recognise, of course, that the three grades are, as it were, on an inclined plane, fading into one another, and that all the mass-move-inents are expressions of life’s indomitable insuigenee.—John o’ London’s Weekly.