Aquaculture

Chemical Usage in Aquaculture:
Implications for Residues in Market Products
Publ: Greenpeace Research Laboratories, Department of Biological Sciences,University of Exeter EX4 4PS, UK Introduction
The culture of finfishes and shellfishes now accounts for some 30 million tonnes ofproduction worldwide. The widespread decline (through overfishing) of many speciestargeted in capture fisheries continues to drive expansion of the aquaculture industry.
Diseases occur in both natural and cultured animals and can be categorised broadly intotwo groups: 1) Infectious diseases where the causative agent is bacterial, viral or parasitic.
2) Non-infectious diseases caused by toxic substances, improper nutrition, poor water quality, physical damage or genetics.
Infectious diseases are a major concern in aquaculture both in terms of the potentialnegative impacts on production and the potential for disease impacts on wild populations.
Outbreaks of disease are typically caused by widely distributed, opportunistic pathogens.
In natural systems they have a low prevalence and low intensity of infection, but in finfishcultures or shellfish hatcheries where stresses lower resistance and stocking densityfacilitates transmission of disease, impacts of disease outbreaks can be severe.
Total prevention of disease in aquacultural systems is likely to be unattainable in practiceeven though pathogen free broodstock for e.g. shrimp aquaculture is available. Diseasemanagement, therefore, depends upon good culture practice in combination withchemotherapeutic agents. Some agents may be administered (often in feed) on aprophylactic basis, although in many countries, the US for example, this is forbidden.
Parasiticides, anaesthetics, spawning hormones, oxidants, disinfectants and herbicides areall routinely used. In the case of antibiotics, the development of vaccines has led to a sharpreduction in use in many of the finfish cultures in Europe and North America.
Even though chemical usage is widespread in the aquaculture industry, and the genericchemicals in use are known, accurate statistics on useage are hard to come by. As pointedout in a recent comprehensive review of chemical use in the south-east Asian sector(Gräslund & Bengtsson, 2001), despite the importance of this industry, documentation ofthe quality and quantity of chemicals and biological products used is scarce. The importindustry is a substantial one. Nearly 27,000 tonnes of prawns and shrimps with a value ofaround GBP 125 million were imported into the UK alone in 1997. While themicrobiological quality of these is closely monitored, chemical residues have receivedlittle attention.
The issue of chemical residues present in seafood, particularly antibiotics, has been throwninto sharp relief by reports of residues of nitrofuran antibacterials being detected inshrimps imported into the EU from Vietnam and from Thailand. Nitrofuran antibacterialsare veterinary drugs whose use in food producing animals and fish is banned in the EUbecause of health concerns, including a possible increase in cancer risk in humans.
Chloramphenicol has been detected in shrimps imported from Myanmar, Burma.
Chloramphenicol, a broad spectrum antibiotic which has latterly been associated withaplastic anaemia in humans, has been banned in the EU for use in food producing animalsand fish. This is a drug of last resort in human medicine for Salmonella typhimuriuminfections. The EC has responded to these findings by imposing a requirement on memberstates to monitor imported shrimp from these areas for the specific residues. (Commission Decisions: 2002/249/EC; 2002/250/EC; 2002/251/EC, CEC 2002a,b,c). At the same time,there are relatively few constraints on chemical usage in aquaculture and many antibioticsare widely available.
Results of current literature review
The identification of antibiotics in imported shrimp has led to extensive coverage of theissue in the international media and, as noted above, led to EC imposition of monitoring.
In relation to coverage of this and related issues in the scientific literature, interrogation ofthe in-house Science Unit literature database and on-line British Library resourcesgenerated a general review (cited above) on classes and types of chemicals used inaquaculture. No quantitative data were included in this review (Gräslund & Bengtsson2001).
Antibiotic residues have been detected in a small proportion (8-9%) of tiger prawnstested in the UK (Willis et al. 1999). Of 204 prawns tested in 1994, one containeddetectable oxolinic acid (a quinolone antibacterial), one contained sulphonamides and 16showed the presence of oxytetracycline. A more recent evaluation using 98 samples from17 producers spread over 3 countries showed that of these, 23 showed antibiotic activity.
18 contained residues of the antibiotic trimethoprim and 3 of these also containedgentamicin. The agents responsible for the activity in the remaining seven samples werenot identified. Chloramphenicol and oxytetracycline were not found. This issue wasflagged by the authors as requiring ongoing surveillance.
Various other articles in the specialist journals have dealt with the issue of antibioticresistance (e.g. Sorum 1999 & 2000) and the impact of vaccines on reducing drug use isacknowledged for the Scandinavian sector. An extensive descriptive literature exists onthe various diseases which can impact diverse aquaculture operations while others focuson the treatment regimes derived for specific problems. While many these papers containinformation of interest, they do not address the issue of residues of chemicals present inthe marketed products Some articles in the literature provide quantitative data on antibiotic usage in aquaculture.
These have already been referred to in Johnston et al. (1998). Such data have primarilybeen derived from the Scandinavian and North American aquaculture sector. GESAMP(1997) also provide information on the broad classes of chemicals in use in aquaculture,but provide no quantitative data or information on usage patterns.
With respect to the implications of residues in marketed products, while many papersmake generic reference to this as established fact and, therefore, of general concern, thereare few data in the literature. Even more general searching of internet resources simplyretrieved reporting of the chloramphenicol and nitrofuran issue, together with manyresources documenting the identity of chemicals in use in the sector. Other resources detailproducts registered for use in various legislatures.
In relation to detection of bacteria/viruses in marketed product, little if any informationexists, although antibiotic resistance in sediment bacteria impacted by aquacultureactivities is now well documented. In addition, the focus of the little research carried out todate has been on antibiotic residues. No data were found on other chemicals such as pesticides and antifoulants known to be used in the sector. Finally, no data were found onresidues of synthetic chemicals such as dioxins and PCBs transferred via feedstuffs in usein aquaculture beyond information already reported in EC Reports (see: e.g. EC 2000),largely from regulatory surveillance programmes.
Overall, the coverage of this issue in the technical literature suggests that, while thepotential for problems is high, it remains poorly researched. A similar conclusion wasreached by Gräslund and Bengtsson (2001) in relation to shrimp aquaculture:- “Theoretically, chemicals other than antibiotics that are added to the shrimp ponds, orby-products from the applied substances, that have a bioaccumulation potential, could befound as residues in the shrimps. …However, little attention has been paid to the risk ofresidues other than antibiotics in farmed shrimps, and no data from such investigationshave been found”. Point of departure for further campaign discussion
Some detailed background on the various chemicals and groups of chemicals used inaquaculture, together with a broad outline of the environmental problems they can cause,is already provided in the 1998 Greenpeace Report on the World’s Oceans (Johnston et al.
1998). Following on from the additional concerns raised above regarding residues carriedover into seafood, this document outlines (in Table 1) those chemicals reportedly used inaquaculture (shrimp and finfish) world-wide which, by virtue of their intrinsic properties,may render them of concern with regard to consumer health.
This is not intended to provide an exhaustive review of their toxicologies, nor to attempt toestablish relative levels of hazard; indeed, there is largely insufficient information onwhich to base such judgments. Rather, Table 1 provides a condensed overview of thosechemical agents which may be of greatest concern on the basis of their toxicity andlikelihood to persist and/or accumulate in biological tissues, as a starting point foridentification of priorities for further research (including, as practicable and necessary,analytical determinations).
Table 1: Summary of chemicals used in the culture of finfish and/or shrimps which are, or may be, of concern with respect to residues in
the final seafood product based on their intrinsic properties and/or reported detection of residues in foodstuffs.
Chemical
Trade names
Chemical type
Use Category
References
(penicillin-type)antibioticsDevelopment ofresistant strains sealice in Europeanand North Americansalmon culture Philippines, but nowbanned. Use in othercountries notdocumented S. America, and inbottom oyster culturein NW USA (1996), GESAMP(1997) Graslund &Bengtsson (2001) produce oestrogenicresidues (Tyler et al.,2000) “hazardous”.
Breakdown product,endosulfan sulphate,is more persistent andexhibits similar toxictraits yellowtail in Japanand in SE Asiashrimp hatcheries [MSDS available athttp://ntp-server.niehs.nih.gov/htdocs/CHEM_H&S/NTP_Chem9/Radian91-53-2.html] GESAMP (1997),Graslund andBengtsson (2001) used in shrimphatcheries in thePhilippines(Baticados etal.1990) infections(and never forchildren or pregnantwomen) resistant strainsResidues resistant todegradation thyroid in humansPotentialreproductive toxin pyrimidine to yieldbroad spectrumantibacterial action difluoro-benzamide)CAS: 83121-18-0Tricaine methane- aminobenzoate,methanesulfonic acidsalt)CAS: 886-86-2 bream, though alsohas application inshrimp culture in SEAsia Residues accumulatein fish cultured intreated ponds andpersist for manymonths References
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