From the point of view of the preservation of the genetic resources of fish, four main levels of concern and strategy can be identified (Table 1): (1) oceanic systems, (2) continental waters (both fresh and marine), (3) aquaculture and (4) stock enhancement programmes. The last two levels of concern are directly controlled by man, whereas the first two can be regarded as uncontrolled systems, although human activities can have major impacts on them.
There are a number of characteristics which clearly differentiate oceanic systems from continental waters as far as genetic resource preservation is concerned. Firstly, the much greater size of oceanic systems, in terms of both physical dimensions and biomasses involved, make them much more difficult to manage. It must be remarked, however, that a few of the larger river basins (e.g., in South America and Africa) and their exploited fish biomass approach oceanic dimensions.
The effects of meteorological and especially hydrological cycles are usually much more crucial in continental waters where they have a strong influence on fish populations - especially as far as reproductive migrations or strategies are concerned. The combination of these characteristics means that both natural and man-induced stresses have a relatively greater impact in continental systems. Included in this group are intertidal habitats, coral reef systems, brackish waters and fresh waters.
The scope of genetic resource preservation in fish
|Level||Potential Genetic Problems||Methods|
|Oceanic fisheries||Sub-population extinction||Tagging and monitoring|
|Continental habitats||Species extinction||Establish reserves|
|Genetic erosion||Scientific management of reserves|
|Aquaculture||Genetic erosion||Controlled breeding|
|Loss of fitness||Cryopreservation|
|Stock enhancement||Genetic erosion||Controlled breeding|
|Loss of fitness||Cryopreservation|
|Introgression with wild stock||Genetic sterilization of introduced stock|
Biologically, these conditions give rise to two further distinctions which are relevant to genetic resource conservation: (a) continental areas have very much higher numbers of endemic and locally distributed species and sub-populations than oceanic systems, and (b) the lower impact of stresses on pelagic oceanic species, and their high fecundity, make their extinction unlikely. At low abundance levels their exploitation will be uneconomical, whilst in enclosed continental systems many species could, and indeed have, disappeared.
In oceanic systems, the main changes are produced by the direct but selective exploitation through fisheries. Such exploitation can have two different effects on a considered species or population. One is a direct effect on the population structure of the target species by selective removal of a particular sector. The other is an indirect effect on non-target species, where the fishery may affect the food chain by, for example, upsetting established interactive relationships.
In continental waters the same effects can occur but often to a much greater degree and whole populations can become extinct, solely through over-fishing. Here, however, other important stresses occur, notably pollution, disruption of life history cycles and disruption of ecosystem structure. Examples include the introduction of foreign predators, competitors and pathogens.
Among the different kinds of pollution, air-borne pollution leading to acid precipitation probably has had the most devastating (and sometimes selective) effect on fish populations particularly in southern Scandinavia and the north-eastern and north-central U.S.A. The direct pollution of waters through the discharge of various chemicals (some of them mutagenic) as wastes or the run-off of agricultural chemicals is also of major importance. Land-use may also have highly undesirable effects such as the production of very acid soils (and run-off) and the silting of streams.
Another serious human impact is the disruption of biological (in most cases reproductive) cycles in fish populations as a consequence of building dams or other obstructions on rivers where migratory species occur. Dams may not only prevent the migration to upstream spawning grounds, but they may also change rivers into semi-lacustrine habitats quite unsuitable for stream species. Ecological barriers are also common and may be caused by zones of pollution in the lower reaches of rivers preventing the migration of various species.
The introduction of exotic species may also be included as a factor of importance to endemic fish populations (see Section 5.3). They may lead to the introduction of disease, higher levels of predation, or so affect the ecosystem (e.g., by competition for food) as to cause the extinction of local species. It is very rare to find introductions which have filled a totally empty niche, especially in the tropics, so introductions are highly likely to result in changes in the endemic populations.
Compared to oceanic and continental systems, aquacultural, hatchery and stock enhancement programmes are subject to an even greater degree of human control by definition. Within such systems, direct environmental and genetic management can and should be imposed and should only be limited by economic criteria and technological sophistication.
As shown in Table 1, a strategy for genetic resource preservation is largely dictated by the type of aquatic system. This section briefly outlines the approaches relevant to the various kinds of systems. Technical details are described in Section 5.
For any oceanic fish species, one of the first requirements for the formulation of a preservation policy must be a distribution map and the identification of any distinct sub-populations which there may be. Distribution data will probably be available from relevant fisheries, but the detection of sub-populations may have to be carried out by the use of one or more of the ‘labelling’ techniques available (e.g., tagging, the identification of meristic characters, electrophoresis, etc.). The importance of detecting sub-populations is that, although the danger of the extinction of oceanic species through overfishing is probably very small (the assumption being that the fishery will disappear before the species), it is perfectly possible that a unique sub-population could disappear. (Incidentally, the practice of a rushed, published description of a local population as a new sub-species, merely for the purpose of forestalling development, is a legalistic device that will probably prove counter-productive, since it is likely to be abused.)
Significant changes within an exploited population can only be detected through a continuing programme of monitoring designed to study the overall population structure, its composition and its change. The decline in any part of the population can probably only be controlled by regulation of the fishery, either through reduction of exploitation in space or time, or by altering the nature of the gear used.
The effect of fishing on the genetic diversity of oceanic populations is discussed below (Section 3.5). If there is evidence of undesirable genetic changes in any species because of selection imposed by the fishery, it would be ideal to try to reverse the trend by appropriate management tools.
One of the enormous problems related to smaller enclosed systems, especially in the tropics, is the identification of the species involved. In parts of Africa, Asia, and especially in South America, large numbers of endemic and very locally distributed fish species occur and many are undescribed. Thus an essential pre-requisite to any broad programme of genetic resource preservation is a proper taxonomic study of the fish species occurring in each area and a full check-list of these species, indicating the status of each, and, if possible, its significance in ecological, economic, scientific and social terms. Such lists would provide the basis of any international list of endangered species to be included in the Red Data Book published by the IUCN.
Because of the high probability of the loss of genetic diversity in any species taken into culture (Section 3.4), the best method of maintaining this diversity is to conserve self-maintaining populations in natural habitats. The normal procedure here would be to establish nature reserves (lakes, river basins, estuaries, coastal lagoons, coral reefs, etc.) which already contain one or more populations of the fish species or communities concerned. These reserves could well be multi-purpose areas, conserving several other types of habitat and communities in addition to the fish resource, e.g., National Park of Sabana Grande, Venezuela.
In some situations an individual species or its habitat may be so threatened that the only option is to collect stock and transfer this to an aquaculture system, or preferably, an alternative suitable location to create a new population. Ideally this new location should be within a nature reserve or in an area not liable to man-made pressures. The refuge so created can later be used to re-stock the original habitat if conditions there improve. This strategy is being used at present in both Canada and Scotland (Maitland, 1979) to preserve local populations of whitefish (Coregonus). Aquaculture techniques may be used on a short-term basis to enhance local stocks which are subject to temporary recruitment problems.
By definition, it is essential that man-induced stresses be strictly regulated in any established nature reserve. This includes control not only of fishing effort and gear, water quality degradation, barriers to migration, etc., but also of introduction of exotic fish or any other species likely to have a harmful impact on the ecosystem (see Section 3.8). The wild trout and charr watch concept, as recently proposed by Regier and Powers (1979), may prove to be one useful way of developing international monitoring schemes for important species or groups of fish.
Once aquatic reserves are established, scientific management to prevent degradation and loss of diversity is absolutely essential. In terrestrial ecosystems, management of nature reserves is a new and often contentious discipline, but the high rate of extinctions for vertebrate species in terrestrial reserves requires the immediate attention of managers and consultants (Frankel and Soule, 1981). Fishery and aquatic reserve managers would be well advised to consider this problem at the outset.