What can you use? What can't you use? Dr David Alderman of the Centre for Environment, Fisheries and Aquaculture Science (CEFAS) Weymouth Laboratory, clears up some of the confusions arising from EU statutory requirements.
Pressures, whether due to consumer concerns or to increased regulation arising from European market harmonisation, have led to a steady change in the way chemicals are used in aquaculture over recent years. In particular, the UK Veterinary Medicines Directorate (VMD) non-statutory residue monitoring programme's reports on residues of malachite green and ivermectin in farmed fish, and the resultant problems, caused the aquaculture industry to examine the position regarding the use of treatments very carefully.
Council Directive 96123/EC provides for the addition of fish meat to EU Member States' statutory residue monitoring programmes - this will apply from 1998 in the UK.
The outcome is that fish farmers see their traditional chemical remedies increasingly curtailed and it is, of course, in the hatchery and fry production stages that the majority of such remedies have been used.
Whilst the use of antibiotics has been closely controlled for many years by the Medicines Act and related legislation, little or no restraint has been applied to other "traditional" chemicals, despite the fact that in many cases their use could undoubtedly have been regarded as a veterinary medicinal use. Such traditionals are effectively "grey" products, not the property of any single manufacturer or supplier. This makes it uneconomic for any company to develop and generate the data needed to "legitimise" them as licensed mainstream medicines.
Malachite green
Controls existed on the discharge of such chemicals from fish farms, but, in general, the limits on discharge consents did not present a practical problem. The most obvious example was malachite green. The most critical need for malachite green has always been in the hatchery to control overgrowth. Of fungus on incubating eggs.
Recent results from the VMD surveillance of trout suggest that hatchery use of malachite green, despite the time, plus the enormous growth dilution factor from egg to table fish, could result in residue problems in table fish. As a result of this, the British Trout Association has advised members that the use of malachite green is restricted to eggs only and that any fry resulting from treated eggs must be positively released - in other words, they must be checked and found to be free of residues of malachite green before being sold on.
Malachite green can give residue problems because the physical layout of many farms with the hatchery at the top of the farm and the growth-out stock in the outlet channels means that anything used in the hatchery and therefore in its discharge water will affect ready-for-market fish. Of course this "down farm" contamination is a potential problem with any treatments used in a hatchery on such a farm.
Although a major study of malachite green has now been started by the Food and Drugs Administration (FDA) in the USA, results from this study will not be available in less than five years. Even assuming the results are favourable, its persistence in fish tissues makes it unlikely that malachite green will ever be used again in the extensive way that it was, given the recent BTA advice.
Not Annex IV
The presence of malachite green residues in market-ready fish is clearly unacceptable because no Maximum Residue Limit (MRL) has been set but malachite green has NOT been placed in Annex IV (or indeed any other Annex to regulation 2377/90/EEC) so that its use remains in the grey "unauthorised" area, as with most traditional remedies. Annex IV of that Regulation is a list of substances which specifically must not be used in food animal species.
Formalin and hydrogen peroxide are the only current useable alternatives to malachite green in the hatchery which do not create residue problems even if discharged into the rest of the farm. They do however present the greater user hazards of potential respiratory toxicity and inflammability respectively, whilst being less effective in use.
A number of other organic substances have sometimes been used as malachite green alternatives, perhaps more with ornamental fish than food fish. These produce similar problems to malachite green, and, indeed, available information indicates that they should never be considered even for hatchery use.
Copper sulphate is also to be found in lists of traditional hatchery remedies. It is of doubtful efficacy and in the quantities which would be needed to produce any therapeutic effect, would certainly breach any discharge consent.
There has been some confusion on the regulatory status of other traditionally used compounds such as Chloramine T. It can be clearly stated that Chloramine T is NOT in Annex IV of regulation 2377/90/EEC and indeed the manufacturer of that product is currently actively discussing with the EU regulators the possibility of applying for Chloramine T to be placed in Annex II for fish use. This would be required before it could be given a Marketing Authorisation for its use in aquaculture.
Whilst we are in the process of losing many of the traditional chemical remedies, the increasing cost of introducing new veterinary drugs for aquaculture means that companies are decreasingly likely to fund the necessary development. The newly started MAFF LINK project to develop a malachite green substitute, in which pharmaceutical companies, the fish farming industry and government are working together, plus the recent news about Chloramine T are among the few current examples of positive developments in this field.
The fry production stage is, if anything, more vulnerable than egg production to the lack of available medicines. Fish eggs are relatively tough and can withstand the use of aggressive products such as iodophores, formalin or hydrogen peroxide. With older fish, vaccines are available to control the effects of most bacterial diseases, and viral vaccines are beginning to appear, but fry are not fully immunocompetent. Also, the most effective vaccines are injectable and not a practical proposition for many diseases in very young fish. The control of diseases such as RTFS awaits the development of effective bath or oral vaccines.
Land farmers have a wide range of disinfectants available to them and approval systems exist for disease control and hygiene - in the dairy, for instance. Again, for the most part, aquaculture gets the "leavings" and uses other people's disinfectants, the only specialised product in this area being aquaculture-specific buffered iodophores. For the most part the available "second-hand" disinfectants are quite effective for hatchery hygiene, disinfection of equipment and so on. However, tougher bacterial pathogens such as Flavobacterium psychrophilum, the cause of RTFS, are not easily killed by most disinfectants. In the case of RTFS, pathogenic strains of the organism possess a thick protective coat which is not penetrated by the disinfectant, plus an ability to survive beneath the inevitable scaling and encrustations which build up on fish tanks and troughs in use.
Something "special" is needed for disinfection in hatchery sites infected with this type of pathogen, but although the market is small, a few potential products have so far shown major promise in the continuing studies conducted in the MAFF-BTA project on RTFS at CEFAS Weymouth and the Institute of Aquaculture in Stirling. Although legislative controls may further restrict what chemicals can be used in aquaculture, new products are being developed, which if successful will increase the range of approved treatments.
(from Fish Farmer International File, March/April 1997)
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