Herrings appear at the surface of the water in vast shoals in the spring, when the water is full of the floating eggs and larvae of fishes and invertebrates. On this food they gorge themselves, and rapidly become fat, so that by the month of May they are in the "mattie" condition. In summer and the early part of autumn there are innumerable myriads of minute Crustacea and other forms of microscopic life at the surface of the water, so that its colour is often quite changed by them. The West Coast is probably indebted for some portion of its extreme abundance of embryonic and larval existence in spring to the influence of the south-westerly winds and the oceanic current beating up against it. The fat herring or mattie fishery is confined to the West Coast, and in the North Sea the shoals are not nearly so soon attracted from their winter quarters in the depths.

In the food problem we have the chief explanation of the migrations of the herring. During the winter months the surface of the sea (though it is instinct with life in spring and summer) is comparatively untenanted. If we ply the towing-net we shall find that it comes up comparatively void at certain times, while in the same places at other times it is filled with the evidences of a teeming fauna. Unfortunately, however, this mode of explaining the movements of the herring carries us only a very little way, and then leaves us confronted with a large number of fresh problems, each more difficult to all appearance than that with which we started. By resolving the laws which determine the movements of the herring into a question mainly of food, we only raise the question of the conditions regulating the food supply; and if the present state of knowledge as to the herring's mode of life is imperfect, incomparably more imperfect is the state of knowledge with regard to the natural history of the numerous minute animals that enter into its bill of fare from day to day. We know very well, by the practical test of the towing-net, that the supply of living organisms at the surface of the sea is extremely variable in quantity according to the seasons. We know also that, when the surface is comparatively vacant, an abundance of living things is sometimes to be found by the same instrument when it is applied a few fathoms down. The great majority of inhabitants of the ocean are free-swimming creatures, to be found at different depths or altitudes according to circumstances. Rough weather appears to cause the entire surface fauna to descend some distance for refuge. There are also seasonal changes and diurnal oscillations, which appear to be of great importance in relation to the present subject.

As regards the seasonal changes in the surface fauna, we can point only to the general fact that certain species are prominently visible at certain periods of the year, and are quite invisible at others, for in this country there has not yet been such systematic and exact research carried on by numerous observers daily all through the year, aided by suitable boats or steam vessels, appliances, and mechanical power, as is necessary in order to arrive at anything like a creditable knowledge of this domain of nature. Researches of the right kind have been carried on for several years at the Naples Zoological Station, and on a very limited scale at some of the seaside laboratories that are now being established in Europe, while much good work has been done in America. The importance of this work in relation to the herring fisheries cannot be over-estimated; for until it has been carried out in earnest, and on a sufficient scale, we shall continue to be working in the dark, when perseverance and a moderate outlay by Government would give us invaluable light. The fact is familiar enough that large patches of the sea around these islands are occasionally seen quite alive with Entomostraca and other minute organisms with or without the phenomenon of phosphorescence. A similar appearance is seen in the southern hemisphere, and known as the "Pasture of the Whales," and has been described by Darwin and others. In the more northerly Scandinavian fiords enormous masses of a small crustacean called Thysomopoda inermis make their appearance every spring, and are the occasion of a great whale fishery. Similar creatures are the main staple of the herring's food. At Naples the exact investigation of the seasonal changes in the surface fauna has led to some important discoveries. Dr Carl Chun, in a monograph on "Ctenophora" issued among the reports of that station, records how this order of Actinozoa are abundant in spring, disappear almost entirely in summer, and return again in autumn. Where they go to in summer is not left in mystery, for Dr Chun has caught them deep in the water during the period of their absence from the surface. Why they retire to the deep water is not clearly explained, but for one thing they appear to feed on microscopic crustacea—such as are the main staple of herring food—which are in the deeps at the same time. Some very valuable observations bearing on these vertical migrations of the small invertebrates that are so supremely important in the economy of fish life, have been made at some of the great fresh-water lakes of Europe within the last few years. These observations, which have yielded very similar results in different places, and in the hands of different observers, are so far confirmatory of those carried out at the Bay of Naples. One of the investigators who has been working at this subject (Dr Weissmann) found during his researches in the Lake of Constance clear evidence of an annual periodicity manifested in the free-swimming crustacean forms, his conclusion being that they disappear altogether at certain seasons (different for different species), when they are represented only by eggs. [Nature, xvii. 92.]

The diurnal oscillations are not less distinctly marked than the seasonal. Many species swim close to the surface at night and descend into deeper water on the return of day, the whole fauna generally keeping together in these diurnal movements. Dr Weissmann ascertained that the mass of small crustaceans descended to a depth of about 25 fathoms; and M. Forel found, exceptionally, in the Lake of Geneva, that one species was to be met with at twice or thrice that depth. The ascent and descent are slowly accomplished, the migrants appearing first, and lingering longest, in regions shaded from the sun. M. Forel, in discussing this question, to the elucidation of which he is one of the first contributors, points to the evident analogies between the lacustrine and the marine "pelagic" fauna, maintaining that though the scale is different, the general laws are the same. [Nature, ibid.] There is, indeed, no doubt about the broad fact that the crustaceans come to the surface in the evening, and descend some distance at sunrise. The herring shoals do likewise. It is at night that the Scotch seas are intersected by hundreds of miles of netting suspended from the surface; when daybreak returns the crustaceans and the herrings disappear together, and the nets are withdrawn.

The importance of these facts in relation to the local and general movements of the herring is not as yet sufficiently appreciated. A Swedish writer, M. Widegren, writing a few years ago on the management of the Baltic fishery, observed that during the winter the herring lives in the deep water, outside the coasts where it has its spawning places, but that it moves about in winter as in summer, and may be caught under the ice at depths of from 5 to 25 fathoms, and not in outside waters only, but also in bays and inlets. The food of the herring, M. Widegren goes on to say, consists of small crustaceans almost invisible to the naked eye, which are found in large quantities both in shallow and deep water. [U.S. Commission Report for 1878, pp. 127, 128.] By towing in the sea water with a net made of fine gauze, large numbers of these little animals may be caught. They are more or less plentiful at different times, under different conditions of weather, and at different depths. This may possibly explain to some extent the fact that the herrings are not always found at one and the same depth. In summer these small crustaceans are found nearer the surface, and the herrings at this time likewise go nearer the surface."* Such was the point to which the question was carried some years ago ; and the tendency of all recent researches has been to connect the migrations of herrings more and more with the supply of herring-food, though not. excluding the spawning function as a factor in the case. And it is becoming increasingly probable that the scientific investigation of the problem requires to be directed to vertical fully as much as to horizontal movements of the shoals. The fisheries in the sea-lochs of the west coast diminish towards vanishing point as autumn changes into winter. Yet herrings are known to be in these lochs all the year round. The shoals only descend into those lower altitudes of the deep water which from a biological point of view are still unexplored.

Nevertheless, it is well known that great masses of herrings do assemble at the surface of the outer sea on the west as on the east coast; and also that particular lochs are from time to time almost completely deserted. It is, therefore, necessary to deal with changes of locality as well as of altitude in the water, as an important part of the general problem. Many fanciful theories have been started in order to account for these changes of locality. We need not, however, go far afield in quest of theories, nor is there any reason why we should attribute to a shoal of herrings such a high order of intellectual powers as would be implied in a deliberate resolve to migrate to a particular area many leagues away. All far-fetched theories about herring movements are utterly worthless, and we need consider only the operation of visible and tangible causes. There need be no hesitation in fixing upon food as the principal cause of the horizontal as well as of the vertical migrations. A second and probably powerful cause, especially as regards the sea-lochs and other narrow waters, is to be found in the presence in great force of the herring's enemies. A third and more doubtful cause, upon which, however, much stress has been laid, is the spawning function.

At the spawning period herrings, like other fishes, consume very little food, for which reason large and dense shoals are able to subsist without much reference to the quantity of food to be found about the spawning place. After spawning, however, the shoals immediately break up, so that "shotten" fish are not to be found in anything like the dense masses in which "fulls" are often met with. The calls of hunger are now dominant; and as the maximum spawning period is towards the end of summer, there is no longer the plethora of food at or near the surface that there was earlier in the season. It seems, therefore, to be a necessity of their existence that the herrings should spread themselves over a large area of sea, so as to mitigate competition in the pursuit of food. A very moderate day's journey multiplied by a large number of days will cover a wide expanse of ground. Herring fry are to be found sometimes in great masses near the land, and large numbers of them are sometimes cast ashore in storms; but as these fry get beyond the whitebait stage, and into that stage in which they are apt to be mistaken for sprats, they require more food, and have to go farther afield in search of it. This leads them out into the open sea. Herrings of the same class—fry, sprat-herring, mat-ties, "fulls," and "spents"—are respectively gregarious, numbers of each class being found together in shoals or "schools." In autumn and winter, when the Copepod crustaceans and the larvae of fishes and invertebrates inhabiting the water at or near the surface are comparatively scarce, the movement of the herrings must, on the whole, be away from the coast. They have to roam far and wide in search of food. The coast-line is a rigid barrier, but those minor shoals (for by this time the shoals all tend to subdivide and become minor) which put out to sea find no barrier in their way. The direction in which they go is probably very much a fortuitous matter, but some will go outward at first, and more and more will follow as they are repulsed by the coast. Far out at sea there is a numerous pelagic fauna. In spring this fauna is increased by immense accessions of larvae, which in calm weather keep well to the surface, though roughness sends them a little lower down. At the surface they are acted upon by the prevailing winds, or rather by the currents to which these winds give rise, and chiefly by the great northeasterly current known as the Gulf Stream. On the western side of Scotland this would tend to accumulate herring food in the direction of the land, and thus we have a probable explanation of how it comes to pass that the matties are found to journey prevailingly landward. This food brought them to the condition of matties or fat herring, and all the time it has been leading them on the whole towards the fishermen. Apart from the influence of this current, and of temporary cross-currents caused by temporary winds, the surface fauna of the spring and early summer has its greatest profusion near the land. Flat fishes, which do not descend to the Atlantic abysses, flourish in the shallow plateau extending out a hundred miles or so beyond the Hebrides. Their ova float high in the water while the process of incubation is going on, and the young remain for a time near the surface. In the vicinity of land also the oscillatoriae, or minute vegetable organisms that are the food of the "herring food," abound. Larvae of the molluscs, of the higher crustaceans, and of numerous other invertebrates, as well as of fishes, are all most plentiful in their season—that season being spring and early summer—within a moderate distance of the land. Thus we have a reasonable and probable explanation, of the general landward movement of the matties, just as the comparative scarcity of food at another period of the year serves to explain why a large proportion of the herrings leave the shore waters.

All the herrings of the shore waters, however, do not leave them. There is sustenance for a certain proportion, hence the outward migration is far from being a migration en masse. Numbers of herrings, as has been said, often remain in the lochs all the year round. In this connection we must consider briefly the influence of enemies upon the movements of the herring. The shoals are haunted wherever they go by a motley gang of voracious foes. Whales, porpoises, and sharks, insatiable cod and gluttonous mackerel, carry on a remorseless and never-ending warfare with the gregarious and defenceless herring. The lochs and other inshore waters sometimes harbour great packs of dogfish and considerable shoals of mackerel. Seals have a keen eye for herring, and clouds of sea-birds hover above the spring shoals, and find the struggle for existence easy for the time. In the lochs, where space is restricted, the havoc committed by such depredators as dogfish and mackerel must be much greater than where there is more sea-room. The herrings, in confined waters, are never far from the whole force of their foes, and these foes may so multiply as to destroy the balance of nature. Loch Fyne, for instance, has sometimes a great abundance of mackerel, and this fact alone goes far to account for the occasional unsatisfactoriness of the herring fishery in that loch. Very many herrings are destroyed from day to day by their enemies, and a shoal pursued by mackerel or dogfish may be driven out of the loch on what may be called an involuntary migration. I am not aware that there is positive evidence of herrings having been thus driven away, but they may have been; and it is certain that when they are long in any circumscribed locality they come to be very much harassed by the gathering multitude of their enemies, so that migration to some quieter locality, or to the open sea, if not actually forced upon them, becomes a highly expedient step, and even a necessity, if extermination is to be prevented.

Of spawning as a cause of migration, the only evidence is the fact that, so far as is known, the spawn is deposited upon hard shingly or shelly ground at a moderate depth, and often where there is vegetation. The best known spawning place is the Ballantrae Bank, where immense quantities of spawn are deposited towards the end of winter. That this and other banks are frequented with much regularity certainly points to a migratory movement on the part of the herrings; but so imperfect is the state of information (except as to a few localities) on the question of where they do and where they do not spawn, that no certain conclusion can be formed as to the extent or even the real nature of the migration. So far as is known, the depositation always takes place in comparatively shallow water, and it certainly takes place at or very near the bottom. The surface-swimming habits of the herring would lead us to expect that its spawning ground would be at no great depth; and as the eggs of most fishes and other animals of the sea-floor float in the water, while those of herrings adhere to the ground, it is probable that the shallowness of the water where the herring has its nativity is of importance in relation to the vivifying influence of the sun's rays. For, as is well known, light as well as heat has a great influence on organic life, and there is no well-authenticated instance of herring spawn having been brought up from depths beyond the reach of sunlight.