THE BIOLOGY OF FRESHWATERS
By PROFESSOR C. M. YONGE
IN this country the study of freshwater has lagged far behind that of the sea. There are abundant reasons, both purely scientific and also economic, why this should have been so. The sea is the home not only of an infinitely more numerous life, but also of a much greater variety of animal forms. It is indeed the ideal medium for life, in which this probably originated and then evolved for long ages before it became capable of passing elsewhere. The sea is a most constant medium, varying little throughout the year or from place to place in its chemical and physical properties. It still remains the only possible medium for many forms of life. A good example is the group which includes the starfish and the sea urchins, which have never acquired the means of emancipating themselves from dependence on the marine environment.
The wealth of the sea is such that its more important products—food fishes, whales and shellfish—have become the raw materials of great industries. There has thus been a double urge to study the inhabitants of the sea—the curiosity of the biologist anxious to determine their nature and means of sustenance and continuance, and the less dis- interested desire of fishermen and merchants to extend the scope and efficiency of the industry in which they are engaged.
When a typical marine animal is placed in freshwater it rapidly swells because water is drawn in by the more saline blood and body fluids, and death follows. Hence whenever a marine animal attempts to pass the estuarine barrier, success depends on the capacity to prevent this. Having success- fully solved this initial problem the animal finds itself in very different surroundings from those it has left behind. Freshwater is just as varied from place to place and from season to season as the sea is constant. Exceptional powers of resistance and of adaptation alone make life possible. In addition both space and food are restricted.
In consequence of all this freshwater is inhabited by much smaller and much less conspicuous fauna than is the sea. Salmon, which feed exclusively in the sea after their initial departure from the rivers in the form of smolts, are the only really large animals which normally inhabit our rivers. The majority of other freshwater fishes are small, while the invertebrate animals are usually inconspicuous.
Thus it has come about that in this country with its extensive and varied coastline and prolific fishing-grounds the urge from both science and industry led to initial con- centration on the sea. The establishment of the Marine Biological Association of the United Kingdom in 1884 and of its Laboratory at Plymouth four years later may bc. regarded as the crucial dates in the history of marine biological investigations in this country. It was otherwise in countries with limited coastlines or with vast areas remote from the sea, such as those of Central Europe or North America. There interest in freshwater biology was early aroused, owing in part to the importance of certain inland fisheries, such as those of the Great Lakes of America and the carp fisheries of Europe, but also to the interest of biologists for whom the sea was too remote to be a distracting influence.
Of recent years there has been a welcome growth of interest in the freshwaters of England. It existed earlier in Scotland, where salmon fisheries are important, and fresh- water lochs are too numerous not to have invited some attention. On this side of the Border an initial stimulus was the ever-widening effects of pollution, which have rendered rivers desolate, devoid of life and an offence to the eye and often to the health of the community. The Salmon and Trout Association did a national service in 1921 when it approached the Minister for Agriculture and Fisheries on this matter. The scientific staff of the Ministry later conducted comparative studies on pure and polluted waters and were able to show the devastating effect of many forms of industrial effluents, notably those of beet-sugar factories. A survey of the Tees, both in its unpolluted upper reaches and in the estuarine waters, which receive the untreated sewage from a population of over a quarter of a million as well as many trade effluents, was later undertaken under the direction of the Water Pollution Board.
The most widespread effect of pollution was found to be the lowering of the oxygen content of the water, in some cases to zero, owing to the decomposition of the organic matter passed into it. Active fish, such as trout, can only live in highly oxygenated water and so are directly affected by such pollution, as well as by the destruction of the fresh- water shrimps and insect larvae on which they feed. Specific poisons were also found, in particular lead effluents and cyanides. Much still remains to be done before the problems of river pollution are solved, but these preliminary studies have already made possible the removal or treatment of certain effluents and have indicated the desirability and nature of other measures.
It was thus in an atmosphere of increasing interest in the problems of freshwater biology that in 1929 there was founded at a small meeting held in the rooms of the Linnaean Society the Freshwater Biological Association of the British Empire. The object of this body was to found a biological station on the shores of some British lake where permanent and long-continued researches could be carried out on fundamental problems. Money was scarce, but the necessity for building was fortunately avoided when the Council of the National Trust agreed to rent to the Association a portion of Wray Castle, near Ambleside, on the shores of Windermere.
In this admirably situated but structurally grotesque nineteenth-century castle a small staff began work late in 1931. With the country in the depth of a financial crisis times could hardly have been less propitious for the initiation of work of potentially unlimited scope and supported by an annual income of a few hundred pounds. But each succeeding year has seen developments both of work and income, and the laboratory has attracted increasing support from Universities, Fishery Boards, water undertakings and Government Departments.
A great advantage of work in the Lake District is the wide variation in conditions in the numerous lakes, because in this variety lies the fascination of freshwater biology. Changes occur not only from lake to lake and from season to season but even from one part of a lake to another. Closely allied animals live only in restricted regions defined by the nature of the bottom, the type of vegetation and other factors. Finally, lakes evolve. Material is carried into young lakes by rivers and local drainage and so as time passes more and more organic matter tends to accumulate on the bottom, providing a reservoir of nitrogen which continually passes into solution in the form of nitrogenous salts. On the abundance of these nutrient substances depends the crop of microscopic plant life, and on this in turn the abundance of animal life, either directly or, as in the case of fishes, by way of smaller animals With increasing age the productivity of lakes therefore increases, which may be an advantage or the reverse, according to the use to which the water is to be put. If fish are required, the more productive the better, but if the lake is to be used as a reservoir heavy growths of plant life are a disadvantage. Certain lakes, such as Thirlmere, never tend to accumulate organic matter and form ideal reservoirs, but the majority are becoming increasingly pro- ductive. Various remedies are suggested, such as the removal of organic matter before or after it enters from the rivers, but this is expensive, and the most feasible scheme appears to be natural cropping by fish which could then be caught, each fish representing the removal of so much organic matter. Fishing in such reservoirs should, therefore, be encouraged, and in the event both fishermen and water boards would benefit.
It is general conclusions such as these which indicate the stage reached in the study of our freshwaters and also the far-reaching value of the work which is developing so rapidly at our only freshwater biological laboratory.
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