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Chapter: Aquaculture Principles and Practices: Selection of Species for Culture

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Introduction of exotic species

The advantages of limiting the number of aquaculture species and the scarcity of really domesticated species for culture have been referred.

Introduction of exotic species

The advantages of limiting the number of aquaculture species and the scarcity of really domesticated species for culture have been referred. The economic and market considerations that create interest in the introduction of exotic species, have also been mentioned in the previous section. Considering the natural geographic ranges of distribution of proven species, there is a strong argument for the introduction and transplantation of exotic species where necessary. However, the problem very often is how to decide whether it is necessary and, if so, what procedures and precautions should be taken to prevent possible undesirable consequences. History reveals that several indiscriminate introductions and transplantations have been made in the past for establishing sport and commercial fisheries, for ornamental purposes and for biological control. Some of them have had detrimental effects on the local fauna and have contributed to the spread of communicable diseases. There is no gainsaying the need for preventing such consequences by following appropriate procedures and effective national regulations. However, expanding aquaculture may find it very difficult to avoid the introduction or transplantation of species, or selected strains of local species, for experimentation or commercial production. Munro (1986) lists some of the aquaculture species that have already colonized outside their historical distributional range: tilapia species, cyprinids (common carp, Chinese carps), rainbow trout, walking catfish, Japanese and European oysters and fresh-water crayfish (Pacifastacus sp.). The majority of them have been introduced for valid reasons, but it is most doubtful whether any of these or other successful introductions have been preceded by detailed screening procedures. To this can be added the more recent introductions of several penaied shrimps (especially Penaeus monodon and P. (= Litopenaeus)vannamei) and the giant freshwater prawn(Macrobrachium rosenbergii) of proven performance in various tropical and semi-tropical countries. Atlantic salmon, an exotic, has established itself so well in cage farming in Chile that the farmed production of the species in 2001 (501000 tons) exceeds that of Norway.

 

Turner (1949) suggested criteria to be considered in introducing new species. The species should:

 

1.     fill a need, because of the absence of a similar desirable species in the locality of transplantation;

2.     not compete with valuable native species to the extent of contributing to their decline;

 

3.     not cross with native species and produce undesirable hybrids;

 

4.     not be accompanied by pests, parasites or diseases which might attack native species; and

5.     live and reproduce in equilibrium with its new environment.

 

The basic logic of these criteria is still valid and organizations such as the American Fisheries Society (Anonymous, 1973) and the Inter-national Council for the Exploration of the Sea (ICES, 1972 and 1979) have tried to strengthen the arguments for critical evaluation and propose methods of obtaining basic data for predicting the consequences of introduction. A close scrutiny of the rationale for introductions and the advantages and disadvantages of the candidate species has to be followed by a pre-liminary assessment of impacts before a decision is made to introduce the species for testing. If it is decided to proceed, thorough experimental studies should be carried out and results critically evaluated to make the final decision for general introduction or transplantation. ICES (1979) recommends the following procedures for the investigations:

 

1.     A brood stock should be established in an approved quarantine environment. The first progeny of the introduced species, not the original import, can be transplanted to the natural environment if no diseases or parasites become evident. The quarantine period will be used to provide an opportunity for the observation of disease and parasites. In fish, brood stock should be developed from stocks imported preferably as eggs or possibly juveniles, to allow sufficient time for observation in quarantine.

 

2.     All effluents from quarantine units are to be disinfected in an approved manner.

 

3.     A continued study should be made of the introduced species in its new environment.

 

While it is relatively less difficult to determine whether the imported stock brings in parasites or diseases, prediction of ecological effects based on controlled experiments has many limitations. It is therefore important that monitoring of the effects of introductions should be carried out on a long-term basis, in order to adopt necessary measures as early as possible.

There have been several recent initiatives by international organisations in controlling trans-boundary transmission of aquatic animal diseases (APEC/FAO/NACA/SEMARNAP, 2001; FAO, 2002a) which can be helpful, especially now, when aquaculture is expanding fast, involving more species and a greater volume of production.

Though efforts to control introductions started several decades ago, guidelines proposed by regional and inter-regional organizations have not succeeded in the general application of these measures for various reasons. The devastations caused by the spread of epizootic ulcerative disease syndrome (EUS) among finfishes in Asia and of the white spot syndrome virus (WSSV) among shrimps in shrimpfarming countries globally indicates the need for work-able codes and guidelines for the control of the transborder spread of diseases. The stringent nature of the measures proposed and the lack of legislative support are believed to be the major reasons for the lack of general accept-ance of the proposed guidelines. In order to be effective the guidelines have to be flexible and followed not only nationally but also regionally and inter-regionally, and to cover the introduction and transfer of all live animal trade, which is increasing with advances in transport within and between countries. Many sport fisheries are sustained by import or export of fertilized ova and seedlings transported over long distances for farming or stock enhancement.

Considerable controversies exist over protection of native biodiversity and fortuitous spread of pathogens. Because of these, the Fish Disease Commission of the Office International des Epizooties (OIE) developed recommendations and protocols for preventing international spread of aquatic animal diseases as a part of its International Aquatic AnimalHealth Code.These cover health surveillance of domestic and internationally traded animals. The guidelines provided are for reducing the risks associated with the introduction and transfer of species. Health status of aquatic animals is assessed for pathogens transfer on the basis of health certificate and quarantine measures. It is believed that some amount of risk is inherent in the introduction and transfer of aquatic animals and so health management measures should aim at being practical and cost-effective, and able to be implemented within the available administrative structure. Quarantines have to be developed to prevent the transfer of disease agents along with live aquatic animal movements at the exporting and importing points. Capacity for competence tosupervise and implement regulatory measures has to be built up in exporting and importing countries. Live aquatic quarantine facilities have to be available at all importing and exporting points. Regulations should define responsibility, which should be clearly assigned to a competent authority for health certification after proper inspections. The main health management task is to define the zones of specific disease occurrence after appropriate certification and quarantine action. It is essential that the authorities concerned in national government and trade have a consensus and willing-ness to employ the guidelines for them to be effective and for the regulations to be credible.


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