The Encyclopedia Americana (1920)/Fishes, Geographical Distribution of

FISHES, Geographical Distribution of. The laws governing the distribution of animals are reducible to three very simple propositions. Each species of animal is found in every part of the earth having conditions suitable for its maintenance, unless:

(a) Its individuals have been unable to reach this region through barriers of some sort; or,

(b) Having reached it, the species is unable to maintain itself, through lack of capacity for adaptation, through severity of competition with other forms, or through destructive conditions of environment; or else,

(c) Having entered and maintained itself, it has become so altered in the process of adaptation as to become a species distinct from the original type.

Under the first head, numerous illustrations may be given. The absence of loaches, Nile fishes (Mormyrus) in America and of

suckers and mooneyes in Europe may serve as examples.

Of species under (b), those that have crossed the seas and not found lodgment, there is, in the nature of things, no record. Of the existence of multitudes of estrays there is abundant evidence. Now and then one among thousands establishes itself permanently, and by such means a species from another region will be added to the fauna. The rest disappear and leave no trace. A knowledge of the currents of the sea and their influence is essential to any detailed study of the dispersion of fishes.

In the third class, that of species changed in the process of adaptation, most insular forms belong. As a matter of fact, at some time or another almost every species must be in this category, for isolation furnishes the most potent elements in the initiation and intensification of the minor differences which separate related species. It is not the preservation of the most useful features, but of those which actually existed in the ancestral individuals, which distinguished such species. In many cases the persistence of characters rests not on any special usefulness or fitness, but on the fact that individuals possessing these characters have, at one time or another, invaded a certain area and populated it. In other words over and beyond the &ldquo;Survival of the Fittest,&rdquo; we have the &ldquo;Survival of the Existing.&rdquo; The former element is competitive, producing adaptation to conditions. The latter is non-competitive, maintaining hereditary traits, not necessarily useful in themselves.

Barriers Checking Movement of Fishes.— The limits of the distribution of individual species or genera must be found in some sort of barrier, past or present. The chief barriers which limit marine fishes are the presence of land, the existence of great oceans, the differences of temperature arising from differences in latitude, the nature of the sea-bottom and the direction of oceanic currents. That which is a barrier to one species may be an agent in distribution to another. The common shore-fishes would perish in deep waters almost as surely as on land, while the open Pacific is a broad highway to the albacore or the swordfish.

Again, that which is a barrier to rapid distribution may become an agent in the slow extension of the range of a species. The vast continent of Asia is undoubtedly one of the greatest of barriers to the wide movement of species of fish, yet its long shore-line enables species to creep, as it were, from bay to bay, or from rock to rock; till, in many cases, the same species is found in the Red Sea and in the tide-pools or sand-reaches of Japan, or even in the brooks or coral pools of Tahiti or Samoa. In the North Pacific the presence of a range of half-submerged volcanoes, known as the Aleut ai ia n and the Kurile islands, has greatly aided the slow movement of the fishes of the tide-pools and the kelp. To a school of mackerel or of flying fishes such rough islands might form an insuperable barrier.

Temperature the Central Fact.— It has long been recognized that the matter of temperature is the central fact in all problems of geographical distribution. Few species in any group freely cross the frost-line, and except as borne by oceanic currents, few species extend their range far into waters colder than those in

which the species is distinctively at home. Knowing the average temperature of the water in a given region, we know in general the types of fishes which must inhabit it. It is the similarity in temperature and physical conditions, not the former absence of barriers, which chiefly explains the analogy of the Japanese fauna to that of the Mediterranean or the Antilles. This fact alone must explain the resemblance of the Arctic and Antarctic faunæ. Like forms lodge in like places.

Ocean Currents.— We may consider again for a moment the movements of the great currents in the Pacific as agencies in the distribution of species.

A great current sets to the eastward, crossing the ocean just south of the tropic of Cancer. It extends between the Gilbert and the Marshall islands and passes on nearly to the coast of Mexico, touching the Galapagos Islands, Clipperton Island, and especially the Revillagigedos. This may account for the number of Polynesian species found on these islands, about which they are freely mixed with immigrants from the mainland of Mexico.

From the Revillagigedos the current moves northward, passing the Hawaiian Islands and thence onward to the Ladrones. The absence in Hawaii of many of the characteristic fishes of the Samoan Islands and the Gilbert Islands is perhaps due to the long detour made by these currents, as the conditions of life in these groups of islands are not very different. Northeast of Hawaii is a great spiral current, moving with the hands of the watch, forming what is called Fleurieu's Whirlpool. This does not reach the coast of California. This fact may account for the almost complete distinction in the shore-fishes of Hawaii and California.

It is, of course, not necessary that the movements of a species in an oceanic current should coincide with the direction of the current. Young fishes, or fresh-water fishes, would be borne along with the water. Those that dwell within floating bodies of seaweed would go whither the waters carry the drifting mass. But free-swimming fishes, as the mackerel or flying-fishes, might as readily choose the reverse direction. To a free-swimming fish, the temperature of the water would be the only consideration. It is thus evident that a current which to certain forms would prove a barrier to distribution, to others would be a mere convenience in movement.

Centres of Distribution.— We may assume in regard to any species, that it has had its origin in or near that region in which it is most abundant and characteristic. Such an assumption must involve a certain percentage of error or of doubt, but in considering the mass of species, it would represent essential truth. In the same fashion, we may regard a genus as being autochthonous or first developed in the region where it shows the greatest range or variety of species. Those regions where the greatest number of genera are thus autochthonous may be regarded as centres of distribution. So far as the marine fishes are concerned, the most important of these centres are found in the Pacific Ocean. First of these in importance is the East Indian Archipelago, with the neighboring shores of India. Next would come the Arctic Pacific and its bounding islands, from Japan to British Columbia. Third in importance in this regard

is Australia. Important centres are also found in temperate Japan, in California, the Panama region and in New Zealand, Chile and Patagonia. The fauna of Polynesia is almost entirely derived from the East Indies; and the shore fauna of the Red Sea, the Bay of Bengal and Madagascar, so far as genera are concerned, seems to be not really separable from the Indian fauna generally.

In the Atlantic the chief centre of distribution is the West Indies; the second is the Mediterranean. On the shores to the northward or southward of these regions occasional genera have found their origin. This is true especially of the New England region, the North Sea, the Gulf of Guinea and the coast of Argentina. The fish-fauna of the North Atlantic is derived mainly from the North Pacific, the differences lying mainly in the relative poverty of the North Atlantic. But, in certain groups common to the two regions, the migration must have been in the opposite direction; exceptions that prove the rule.

Realms of Fresh-Water Distribution.— If we consider the fresh-water fishes alone we may divide the land areas of the earth into districts and zones, fairly agreeing with those marked out for mammals and birds. The river-basin, bounded by its water-sheds and the sea at its mouth, shows many resemblances, from the point of view of a fish, to an island considered as the home of a mammal. It is evident that with fishes the differences in latitude outweigh those of continental areas, and a primary division into Old World and New World would not be tenable.

Distribution of Fresh-Water Fishes.— As to their distribution in the streams, the fresh-water fishes may be subdivided as follows:

1. Lowland fishes; as the bowfin, pirate-perch, large-mouthed black bass, sun-fishes and some catfishes.

2. Channel fishes; as the channel catfish, the mooneye, the goldeye, garpike, buffalo fishes and drum.

3. Upland fishes; as many of the darters, shiners and suckers, and the small-mouthed black bass.

4. Mountain fishes; as the brook-trout, and many of the darters and minnows.

To these we may add, (5) the more or less distinct classes of lake fishes, inhabiting only the waters which are deep, clear and cold, as the various species of whitefish, lake herring, cisco and the Great-Lake trout; (6) anadromous fishes, or those which run up from the sea to spawn in fresh waters, as the salmon, sturgeon, shad and striped bass; (7) catadromous fishes, like the eel, which pass down to spawn in the sea; and (8) brackish-water fishes, which thrive best in the debatable waters of the river-mouths, as most of the sticklebacks and the killifishes.

As regards the range of species, we have every possible gradation from those which seem to be confined to a single river, and are rare even in their restricted habitat, to those which are in a measure cosmopolitan, ranging everywhere in suitable waters.

Barriers to River Fishes.— The existence of boundaries to the range of species implies the existence of barriers to their diffusion. We may now consider these barriers and, in the

same connection, the degree to which they may be overcome.

Least important of these are the barriers which may exist within the limits of any single basin, and which tend to prevent a free diffusion through its waters of species inhabiting any portion of it. In streams flowing southward, or across different parallels of latitude, the difference in climate becomes a matter of importance. The distribution of species is governed very largely by the temperature of the water. Each species has its range in this respect — the free-swimming fishes, notably the trout, being most affected by it; the mud-loving or bottom fishes, like the catfishes, least. The latter can reach the cool bottoms in hot weather, or the warm bottoms in cold weather, thus keeping their own temperature more even than that of the surface of the water. Although water communication is perfectly free for most of the length of the Mississippi, there is a material difference between the faunas of the streams in Minnesota and in Louisiana. This difference is caused chiefly by the different temperature occupying the difference in latitude. That a similar difference in longitude, with free water communication, has no appreciable importance, is shown by the almost absolute identity of the fish-faunas of Lake Winnebago and Lake Champlain. While many large fishes range freely up and down the Mississippi, a majority of the species do not do so, and the fauna of the upper Mississippi has more in common with that of the tributaries of Lake Michigan than it has with that of the Red River or the Arkansas. The influence of climate is again shown in the paucity of the fauna of the cold waters of Lake Superior, as compared with that of Lake Michigan. The majority of our species cannot endure the cold. In general, therefore, cold or northern waters contain fewer species than southern waters do, though the number of individuals of any one kind may be greater. This is shown in all waters, fresh or salt. The fisheries of the northern seas are more extensive than those of the tropics. There are more fishes there, but they are far less varied in kind.

But in most streams the difference in latitude is insignificant, and the chief differences in temperature come from differences in elevation, or from the distance of the waters from the colder source. Often the lowland waters are so different in character as to produce a marked change in the quality of their faunas. These lowland waters may form a barrier to the free movements of upland fishes; but that this barrier is not impassable is shown by the identity of the fishes in the streams on the uplands of middle Tennessee with those of the Holston and French Broad. Again, streams of the Ozark Mountains, similar in character to the rivers of East Tennessee, have an essentially similar fish-fauna, although between the Ozarks and the Cumberland Range lies an area of lowland bayous, into which such fishes are never known to penetrate. We can, however, imagine that these upland fishes may be sometimes swept down from one side or the other into the Mississippi, from which they might ascend on the other side. But such transfers certainly do not often happen. This is apparent from the fact that the two faunas are not quite identical, and

in some cases the same species are represented by perceptibly different varieties on one side and the other. The time of commingling of these faunas is perhaps now past, and it may have occurred only when the climate of the intervening regions was colder than at present.

The effects of waterfalls and cascades as barriers to the diffusion of most species is self-evident; but the importance of such obstacles is less, in the course of time, than might be expected. In one way or another very many species have passed these barriers. The falls of the Cumberland limit the range of most of the larger fishes of the river, but the streams above it have their quota of darters and minnows. It is evident that the past history of the stream must enter as a factor into this discussion, but this past history it is not always possible to trace. Dams and artificial waterfalls now check the free movement of many species, especially those of migratory habits; while, conversely, numerous other species have extended their range through the agency of canals.

Every year fishes are swept down the rivers by the winter floods; and in the spring, as the spawning season approaches, almost every species is found working its way up the stream. In some cases, notably that of the quinnat-salmon and the red salmon, the length of those migrations is surprisingly great. To some species rapids and shallows have proved a sufficient barrier, and other kinds have been kept back by unfavorable conditions of various sorts. Streams whose waters are charged with silt or sediment, as the Missouri, Arkansas or Brazos, do not invite fishes; and even the occasional floods of red mud, such as disfigure otherwise clear streams, like the Red River or the Colorado (of Texas), are unfavorable. Extremely unfavorable also is the condition which obtains in many rivers of the Southwest; as for example, the Red River, the Sabine and the Trinity, which are full from bank to bank in winter and spring, and which dwindle to mere rivulets in the autumn droughts.

In general, those streams which have conditions most favorable to fish-life will be found to contain the greatest number of species.

There can be no doubt that the general tendency is for each species to extend its range more and more widely until all localities suitable for its growth are included. The various agencies of dispersal which have existed in the past are still in operation. There is apparently no limit to their action. It is probable that new &ldquo;colonies&rdquo; of one species or another may be planted each year in waters not heretofore inhabited by such species. But such colonies become permanent only where the conditions are so favorable that the species can hold its own in the struggle for food and subsistence. That various modifications in the habitat of certain species have been caused by human agencies is of course too well known to need discussion here.

Of watersheds in the United States the most important and most effective is unquestionably that of the main chain of the Rocky Mountains. This is due in part to its great height, still more to its great breadth, and most of all, perhaps, to the fact that (Two Ocean Pass excepted) it is nowhere broken by the passage of a river. In the few cases when species have crossed this barrier, some break in the chain (as the Two

Ocean Pass in Wyoming connecting the Snake River with the Yellowstone) has now been recorded.

Habitat of Species.— Each species finds its habitat fitted to its life, and then in turn is forced to adapt itself to this habitat. Any other kind of habitat then appears as a barrier to its distribution. Thus to a fish of the ripples a stretch of still water becomes a barrier. A species adapted to sandy bottoms will seldom force its way through swift waters or among weeds or rocks.

The stream that has the greatest variety of animals in it would be one (1) connected with a large river; (2) in a warm climate; (3) with clear water, and (4) little fluctuation from winter to summer; (5) with little change in the clearness of the water; (6) a gravelly bottom; (7) preferably of limestone, and (8) covered in its quiet reaches and its ripples with water-weeds. These conditions are best realized in the United States in the tributaries of the Ohio, Cumberland, Tennessee and Ozark rivers among American streams, and it is in them that the greatest number of species of fresh-water animals (fishes, crayfishes, mussels, etc.) has been recorded. These streams approach most nearly to the ideal homes for animals of the fresh waters. The streams of Wisconsin, Michigan and the Columbia region have many advantages, but are too cold. Those of Illinois, Iowa, northern Missouri and Kansas are too sluggish, and sometimes run muddy. Those of Texas and California shrink too much in the summer, and are too isolated. The streams of the Atlantic Coast are less isolated, but none connects with a great basin, and those of New England run too cold for the great mass of the species. For similar reasons the fresh-water animal life of Europe is relatively scanty, that of the Danube and Volga being richest. The animal life of the fresh water of South America centres in the Amazon, and that of Africa in the Nile, the Niger and the Kongo. The great rivers of Siberia, like the Yukon in Alaska and the Mackenzie River in British America, have but few species of fresh-water animals, though the kinds fitted for life in cold, clear water exist in great abundance. See