J.Holcik
Laboratory of Fishery Research and Hydrobiology
Bratislava, Czechoslovakia
ABSTRACT
Introduction of fish species into waters that are foreign to them is a highly controversial practice. Poor success has been registered in many cases, particularly those carried out before 1945. Introductions, even if successful, cannot produce biomass in excess of that determined by the natural productivity of the water body concerned. However, by filling vacant trophic niches, appropriate introductions can ensure that more of the productivity accrues to the fish community. Theoretical considerations of the influence of the nature of fish communities on fishery potential and yields of lakes and reservoirs are presented and the consequences of the concept that there are fundamental differences in the adaptive capacity of lacustrine and riverine species for the management of fisheries are suggested.
RESUME
L'introduction d'espèces ichtyques dans des eaux qui leur sont étrangères est une pratique très controversée. Dans de nombreux cas, les résultats enregistrés ont été médiocres, notamment dans le cas des expériences effectuées avant 1945. Cette introduction, même si elle est réussie, ne peut pas produire plus de biomasse que celle provenant de la productivité naturelle dans les plans d'eau intéressés. Cependant en comblant les niches trophiques vacantes, des introductions d'espèces appropriées peuvent assurer une meilleur productivité et accroître les communautés de poisson. On présente des considérations théoriques sur les effets que les communautés de poisson peuvent avoir sur le potentiel et les rendements ichtyques des lacs et des réservoirs et on décrit les conséquences que les différences fondamentales dans la capacité adaptative des espèces lacustres et fluviales peuvent avoir pour l'aménagement des pêches.
The practice of introducing fish species into waters from which they were previously absent has spread widely during the last 40 years. According to the list of international transfers of inland fish species compiled recently by Welcomme (1981), which is by no means definitive and remains to be completed, no less than 170 inland fish species belonging to 33 families have been transferred worldwide for various purposes to-date. Along with the increasing intensity of introductions, there is a growing tendency to question the wisdom and benefits of such transplants, particularly in the case of natural ecosystems. Generally speaking, there are two opposing attitudes to introductions by fishery management. The first, based on the adverse affects of introduced exotics on the native fish fauna, results in partial or total prohibition of introduction of some or all species of fish, as is the case of some western countries (Zenny, 1969). The opposite attitude, which emphasizes the positive effects of introductions, encourages transfers on a massive scale, often supported by government, as in the case of some other countries, for example the U.S.S.R., where there are many research institutions and executive organizations directly engaged in the practice. While there is no doubt as to the positive influence of introduced fishes in fish culture, where the processes leading to production can be easily managed, the situation is much more complicated in natural water bodies. Although there is a vast amount of literature dealing with the results of introduction of exotics into natural waters, there is a lack of any serious, impartial and generally valid evaluation of transplantation experiments based on thorough ecological analysis. No wonder, therefore, that controversy arises even in the same ecosystem as may be seen from the recent discussion between Markevich (1978) and Malyutin (1980) on the results of introductions into Lakes Aral, Balkhash and Sevan and, indeed, on the appropriateness of introductions at all. Without doubt, the huge amount of data and information dealing with transplantations which are frequently of different value (e.g., one is unable to allow a judgement to be made as to whether any increased catch of fish in some water body is due to introduction or due to changes in fishing effort). Thus, the evaluation of a particular introduction or even the role of exotic species is beyond the ability of one person. I will, therefore, only discuss some problems associated with introductions into the natural ecosystems. In agreement with Regier (1968) by the term “exotic” I mean any species not native to the community under consideration.
Almost all introductions into natural ecosystems made prior to the second world war may be considered to a certain degree as a gamble. The situation improved after the war as a greater awareness of ecology developed and introduction experiments started to be planned on a scientific basis. Here the leading role has been played by the Soviet Union, where the theory and practice of introductions and the acclimatization of fish is based on ecological principles (Zenkevich, 1940; Driagin, 1954; Burmakin, 1961, 1963; Karpevich, 1960, 1965, 1975; Rass, 1965). At the present time almost all intentional introductions are more or less carefully planned. Despite this, there are a surprisingly large number of failures. According to Burmakin (1963) up to 1957 1 398 water bodies in the U.S.S.R. of which 87 percent were lakes, 7 percent reservoirs and 6 percent rivers, were planted with 51 species of freshwater fish belonging to 12 families. However, the naturalization of the transplanted species was attained only in 12 percent of these water bodies. Furthermore, the catch of the acclimated fish lagged considerably behind the effort exerted for their introductions. From the data presented by Karpevich et al. (1975) and Burmakin and Shimanovskaya (1975) one can calculate that between the periods 1957–63 and 1964–71 the mean annual number of plantings of exotic fishes rose by 123 percent, the number of water bodies stocked with exotics by 164 percent and the quantity of fishes (eggs, fry, young and adults) introduced by 229 percent. However, the catch of introduced fishes in the same period increased only by 56 percent and its percentage share to the total inland fish catch only by 61 percent. According to the last evaluation of Lifshic and Belousov (1979) only 3 percent of all introductions realized in the U.S.S.R. up to 1978 gave a commercial benefit. The discrepancy between the effort and result of introductions is explained mostly by the lack of biological justification for the introductions, insufficient analysis and generalization of experience obtained, by the slow development of the theory of acclimatization as well as by shortcomings in the actual practice and organization of introductions (Karpevich et al., 1975; Lifshic and Belousov, 1979).
As well as the lack of apparent benefit, there are also cases of adverse or unexpected effects arising from introductions. For instance it was observed that in both natural and manmade lakes stocked with the grass carp (Ctenopharyngodon idella) the species composition of both higher plants as well as phytoplankton changed dramatically and the total catch of fish declined significantly (Vinogradov and Zolotova, 1974; Kogan, 1974; Radziej and Krzywosz, 1979; Krzywosz et al., 1980). Decrease of population density of Salmo ischchan, an endemic trout of the Sevan Lake is attributable to the introduction of the whitefish Coregonus lavaretus s. lato (Reshetnikov, 1980). Even in such a large lake as the Aral Sea (area 66 458 km2) the adverse effects of acclimated foreign species of fish on the native ones have been observed. Due to this and also other reasons, the total annual catch of fish in this lake has decreased by some 10 000 t (Karpevich, 1975; Markevich, 1978). The negative effects of common carp (Cyprinus carpio), walking catfish (Clarias batrachus) and many other exotic species upon the ichthyofauna of the U.S.A. (Lachner et al., 1970) is also well known.
It is often thought that the introduction of both invertebrates and fish may increase productivity in a water body and thus also the yield of fish, a point of view often expressed in scientific papers (e.g., Karpevich, 1975; Karpevich et al., 1975; Ioffe, 1963, 1972). However, it is clear that as the productivity of an ecosystem depends wholly on an amount of energy entering it, any introduction of a new organism into a developed ecosystem may only increase the number of consumers of energy but not the productivity. This implies that the introduction of new fish species and/or invertebrates into an already developed ecosystem cannot lead to the increase of the total yield of fish and will only result in a change in catch composition. This postulate may be demonstrated in the case of the Balkhash Lake in Kazakhstan. This large (area 17–19 thousand km2), shallow (maximum depth 26.5 m) and endorheic lake in central Asia contained originally only five species of endemic fish (three cyprinids - Schizothorax argentatus, S. pseudaksaiensis and Phoxinus poljakowi, one percid - Perca schrenki and one cobitid - Noemacheilus strauchi). Since 1905 when the wild form of the common carp (Cyprinus carpio) escaped from a pond near Alma-Ata and entered the lake, a further 27 species of fish have been introduced, of which only eight were planned while several other species were injected illegally or by chance. Twenty species naturalized fully and six of them became so abundant that they are now the subject of commercial fishing (C. carpio, Abramis brama orientalis, Rutilus rutilus, Stizostedion lucioperca, Silurus glanis and Aspius aspius). Moreover, 17 species of invertebrates, mostly of Caspian origin, were also transplanted in the period 1957–62, ten of them quickly reproduced and spread over the whole lake (Serov, 1968, 1975; Cyba, 1974, Strelnikov and Dikanskij, 1975; Karpevich, 1975; Bashunov, 1977; Bashunova, 1976). Evaluating the results of these introductions it is claimed that the fish yield increased by 25–30 percent (Karpevich, 1975) or that it even doubled (Kozlov, 1978). However, in fact the catch of fish remained on the same level for according to the data of Karpevich (1975), Bashunova and Bashunov (1976) and Cyba (1975) one can calculate that in the period 1932–49, i.e., prior to the massive introduction of fish and invertebrates, the mean annual catch of fish in Lake Balkhash was 12.5 thousand t, while in the period 1950–73 it was 11.3 thousand t. Only the composition of the fish catch changed substantially, as the average share of native fish, which was about 32 percent in the first period, decreased to only 8 percent and even more recently these fishes are no longer found in the commercial catches (Bashunova and Bashunov, 1976). One suspects, however, that in the former period the potential fish yield in the Balkhash Lake was probably higher. At that time fishing effort was certainly lower than later on, when a trawl fishery started both here and in other large lakes (Kuderskij, 1974). Obviously, the introduction of exotics does not increase either the productivity of the lake or its fish yield and the estimates of biomass of fish are the same. Such changes in catch structure should be considered significant in economic terms in industrially developed countries but are less significant in countries of the third world, where the need of cheap fish protein is much greater (Fernando, 1977, 1980, 1980a).
Although the productivity of a lake cannot be increased by the introduction of exotics it does not mean that in some cases the yield of fish cannot be enlarged in this manner. To explain this apparent paradox let me discuss the results of investigations performed by Professor Fernando and myself during the last two years.
It is known that in many South Asian lakes and reservoirs the generally low yields based on native fish species have increased significantly following the introduction of some African cichlids, mainly Sarotherodon mossambicus. This fact has led Fernando (1965, 1980, 1980a) to propose the hypothesis that the high positive correlation between the increased yield and introduction of this cichlid is due to its better adaptation to lacustrine conditions than the original fish species which are of riverine origin. Further analysis (Fernando and Holcik, 1982), based on lakes and reservoirs, has shown that a distinction should be made between the fishes of riverine and lacustrine origin, as their characteristics and ability to inhabit the lacustrine environment are quite different. In geologically young lakes (mainly the lakes of glacial origin) and in reservoirs one deals with fishes of purely riverine origin which are not adapted to spread over the whole area. In most of these water bodies the fish are concentrated mostly along the littoral and the density of fish decreases markedly with increasing distance offshore and increasing depth. In large, deep lakes the pelagial and profundal parts are practically without fish. The nearshore range of fish in these lakes is limited by the isobath of about 25 m. Quite another situation exists in geologically old lakes with relatively longestablished lacustrine environments (e.g., Tanganyika, Titicaca, Lanao, Baikal). Here, in spite of their extensive area and depth fishes occur not only over their whole area but also within the whole water column. In these lakes, however, beside the fishes occurring in the neighbouring river basins, there are also endemic, purely lacustrine species which are unable to inhabit the lotic environment and can be regarded to be stenoecious in this respect. Specific adaptations have evolved in these species which enable them to utilize all existing niches. The pelagial zone of such lakes is inhabited by few species only which are short-lived and have short food chains (phyto and zooplankton eaters). These species are notable for their relatively high population stability due to which high yields are reached despite high fishing pressure. It is remarkable that from about 120 families of fish inhabiting fresh waters, there are only about 12 which can be considered preadapted for lacustrine conditions and of these only five families are purely lacustrine. The majority of endemic lacustrine species comes from the families Cichlidae and Clupeidae, lower numbers are contributed by the Salmonidae, Cyprinidae and Cyprinodontidae. Exclusively lacustrine families are monotypical Indostomidae, Adrianichthyidae and Chauduriidae and also Comephoridae and Cottocomephoridae, the last two with only two species each. However, the majority of fish inhabiting recent standing water bodies is of riverine origin. Due to the constantly changing environmental conditions and the great diversity of habitats riverine fishes are able to inhabit both lentic and lotic environments, i.e., they are euryecious in comparison with true lacustrine species, although lakes and reservoirs can be considered to be only a temporary habitat for them (in the evolutionary sense of the word) to which they are not fully adapted. As there are no specialized pelagial and profundal species among them (due to the relatively small size and depth of rivers) riverine species which enter natural or manmade lakes are able to inhabit only a relatively narrow littoral belt and the pelagial zone is entered only when shallow. In support of this hypothesis I refer to the echosounding observations by Marchal and Laurent (1977) as a means of estimating ichthyomass in Lac Leman, a lake of glacial origin (area 510 km2, maximum depth 310 m). Their echo-integration survey has shown that 84 percent of the total ichthyomass is concentrated along the shores at less than 100 m depth. Fish biomass is reduced as distance from the shores increases; at depths between 100 and 200 m only 11 percent and at 200 m only 5 percent of the total biomass was present. The echosounding observations made by Balon (1974) in the Lake Kariba, the large reservoir (area 5 364 km2, maximum depth 93 m) built on the Zambezi River in Central Africa demonstrated that only about 38 percent of its total area was inhabited by fishes which were confined to shallow coves and a relatively narrow belt along the shores limited by the 25-m isobath. The pelagial zone of this reservoir became inhabited by fishes only after the introduction of the clupeid fish Limnothrissa miodon which was transferred there from the Lake Tanganyika. These observations indicate that in geologically young lakes and in reservoirs populated by riverine species of fish there are some free ecological niches and these water bodies may be classified as undeveloped ecosystems when compared with geologically old lakes occupied by true lacustrine fishes.
It is therefore suggested that when deciding to introduce new fish species into a lake or reservoir its riverine or lacustrine origin should be considered along with the characteristics of the water body under consideration. In such cases, one has to consider not only as vacant trophic niche but also the spatial distribution of fishes and their ability to occupy particular spatial strata and other specific properties leading to the delimitation of a niche. The example of Lake Balkhash, where few native species were able to produce the same fish yield as a number of exotics, indicates that the native fish community occupied both trophic and spatial ecological niches of this lake. This example also indicates that the term “niche” embodies many variables, i.e., it is “the N-dimensional hypervolume” as defined by Hutchinson (1965). In the practice of introductions, however, the term “niche” is usually narrowed to the trophic niche (e.g., Karpevich, 1975).
Experience accumulated during experiments with introduction of exotic species of fish indicates that, in spite of increasing knowledge in the ecology both of a particular species and of the whole fish communities, we are still unable to formulate generally valid conclusions and to construct a definitive, practical model of introductions. We are still “somewhere between total ignorance (no model at all) and a knowledge that permits decisions to be derived by simple deduction (well specified, broadly accepted model)” as emphasized 14 years ago by Regier (1968). In my opinion, there are still considerable gaps in our knowledge on the main interspecific relationships among fishes in any water body on the ecological variability and the ability of fishes to adapt themselves to new environments, on their possible impact upon the other species and on the responses of native fish to the introduced species as well as in our knowledge on the functional mechanisms of the whole ecosystem. Although the concept of Fernando and Holcik (1982) described in section 5 on the nature of fish community and on the consequences for the management of fisheries may be considered a step toward the improvement of the practice of introductions, it should be used only with strict precautions and the utmost care as each lake may be considered a unique entity. I agree with Magnuson (1976) that natural lakes are like islands; they are isolated, small and relatively young, and that the number of species in a lake is the result of an equilibrium between rates of immigration of new species and extinction of existing species. Human activity may accelerate the rate of immigration and extinction and may induce greater instability in species structure thereby adding greater uncertainty into the results of fisheries management. By choosing exotics for introduction man still takes into consideration his own advantage or immediate benefits but pays little attention to what these species can do to the whole ecosystem (Regier, 1968; Lifshic and Belousov, 1978).
For the foregoing reasons one can only outline some very rough concepts applicable in the management of fisheries. In my opinion natural lakes where the native fish community is still flourishing should be stocked with exotics only after very careful considerations and every introduction must be based on the results of detailed limnological analysis. Only stenoecious species from families preadapted for lacustrine habitat should be chosen. One has to remember of course that here the species introduced need not reproduce and establish the self-sustaining population and therefore its population density has to be maintained only by regular stocking. Geologically old lakes inhabited by endemics should not be subject to introductions in order to preserve their invaluable intrinsic value. Much more simple seems to be the situation in lakes where the fish community is periodically destroyed by the winter fish-kills (as is the case of many shallow Siberian lakes) or when the fish fauna of a lake is composed of a few widely distributed riverine species. The many, mostly small lakes in northern and eastern Europe are regularly stocked with coregonines (mainly Coregonus peled) in combination with the common carp often after the total or partial eradication of the original fish community by poisoning or by complete fishing out. Their fish yields frequently exceed those obtained prior to this practice (Kuderskij, 1974), which could be recommended, although in these cases stocking has to be regularly repeated and may be financially burdensome. Reservoirs, which usually depend on stocking, should be stocked both by euryecious riverine species of the local origin and by stenoecious lacustrine exotics. Rivers also may require a regular stocking with native species in order to reach their potential.
I wish to thank Mr. S. Ziegler, Laboratory of Fishery Research and Hydrobiology, Bratislava, for his assistance and help with the bibliography.
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