A Scientific Review of the Potential Environmental Effects of Aquaculture in Aquatic Ecosystems - Volume 3

Environmental Fate and Effect of Chemicals Associated with Canadian Freshwater Aquaculture

Robert J. Scott
Department of Biology, The University of Western Ontario, London, Ontario

Executive Summary

The freshwater aquaculture industry in Canada is growing and with this growth comes the potential use of various chemical agents to treat water, fish or pathogens (e.g. fungicides, disinfectants, anesthetics, pigments, hormones and antibiotics). Despite the broad range of chemicals used in aquaculture around the world, only a subset is licensed for sale in Canada. This review considers chemotherapeutants actively used in Canadian freshwater quaculture. Sixteen databases were searched for scientific publications on the environmental fate and effect of aquaculture chemotherapeutants. The majority of literature concerns marine systems, with few studies on freshwater aquaculture and only two directly examining freshwater aquaculture in Canada.

There are seven chemicals approved for sale when labeled for food fish use in Canada, including four antibiotic drugs (oxytetracycline, florfenicol, sulfadimethoxine plus ormetoprim, sulfadiazine plus trimethoprim), one anaesthetic (tricaine methanesulphonate) and two fungicides/disinfectants (formaldehyde and hydrogen peroxide) (Health Canada 2001a). Oxolinic acid has been included in this paper, although it is not currently used on Canadian aquaculture farms. However, this chemical is very widely used in salmonid culture outside of Canada, including the United States, and off-label prescription potential exists where veterinarians can legally prescribe it. In addition, oxolinic acid provides a wide degree of information regarding fate and effect data which could be relevant to other antibiotics.

Many studies have been published examining the fate and effect of antibiotics in marine systems, but few have been published with regard to the same issues in freshwater systems. As with all intensive animal husbandry, aquaculture practices create an opportunity for the proliferation and spread of pathogens that can lead to significant mortality of stock and subsequent loss of revenue (Dixon 1994). Antibiotics can be administered directly by injection or by releasing feed containing antibiotics directly into the aquatic ecosystem. Unconsumed medicated feed is available to wild animals. In addition, antibiotic-containing feed can accumulate in the sediments or unabsorbed antibiotics can be released in fish feces or urinary waste (Bjorklund and Bylund 1990, 1991), subsequently influencing the natural bacterial flora, an important component of ecological food webs. Thorpe et al. (1990) estimated that 1.4 to 40.5% of fish feed passed uneaten through an Atlantic salmon sea-cage. However, this may be a conservative estimate since diseased fish feed poorly (Bjorklund et al. 1990), and the majority of active form antibiotic passes unabsorbed through the gastrointestinal tract of fish (Cravedi et al. 1987; Bjorklund and Bylund 1991; Plakas et al. 1998). On the other hand, advances in feeding technology (e.g. underwater video; Foster et al. 1995) and alternative methods of incorporating antibiotic into feed (Duis et al. 1994) can affect the amount of antibiotic reaching the environment.

Nitrosomonas spp. and Nitrobacter spp. are important bacteria for nutrient cycling in freshwater trophic webs converting ammonia (toxic) to nitrate (non-toxic) (Ricklefs and Miller 2000), but in laboratory microcosms, oxytetracycline greatly inhibited the processing of ammonia (Klaver and Mathews 1994 ). During disease outbreaks in catfish ponds, the use of antibiotics cured the disease, but reduced bacterial conversion of toxic ammonia to nitrate, allowing ammonia to build up in pond sediments (Klaver and Mathews 1994).

The evolution of drug resistant strains of pathogenic bacteria is perhaps the most important implication of antibiotic use in aquaculture. Resistance to antibiotics is present in bacterial populations naturally (McPhearson et al. 1991; Johnson and Adams 1992; Spanggaard et al. 1993) and antibiotic use gives resistant strains the opportunity to proliferate and spread. Studies that examined antibiotic resistance following drug therapy at fish farms (Bjorklund et al. 1990, 1991; McPhearson et al. 1991; Nygaard et al. 1992; Samuelsen et al. 1992a; Spanggaard et al. 1993; Ervik et al. 1994; Kerry et al. 1996a; Herwig et al. 1997; Guardabassi et al. 2000) and in microcosms (Kerry et al. 1996; Herwig and Gray 1997; O'Reilly and Smith 2000) show an increased frequency of resistance to several drugs across a variety of bacterial species. However, Kapetanaki et al. (1995) and Vaughan et al. (1996) suggest that increased levels of bacterial drug resistance can arise independently of the presence of a drug (through sterile fish feed, sediments added to microcosm studies, uneaten fish food) and confound studies.

No published studies directly examined the environmental fate and effect of fungicides, disinfectants and anaesthetics within the scope of this review, but several studies have examined tissue deposition, toxicity and stress responses in fish in order to determine appropriate use rates for these chemicals in aquaculture practice (Xu and Rodgers 1993; Howe et al. 1995; Schreier et al. 1996; Rach et al. 1997a, b, 1998; Gaikowski et al. 1998, 1999; Keene et al. 1998; Jung et al. 2001).

In addition to the chemicals discussed above, carotenoid pigments (astaxanthin and canthaxanthin) are added to aquaculture feed to enhance flesh colour in cultured salmonids (Guillou et al. 1995; Metusalach et al. 1997). No studies have been published on the environmental fate and effect of carotenoid pigments introduced in fish feed. Carotenoid pigments could build-up in sediments since the molecules are non-water soluble and stable in the absence of light. Finally, salmonid production can be enhanced by culturing females only, a condition that manipulates the sex phenotype by exposing juvenile fish to 17 -alph-methyltestosterone either through immersion or incorporation in feed. No studies are available on the environmental fate or effect of this hormone within the scope of this review.

Knowledge Gaps

• Research is needed on the fate and effect of therapeutants in freshwater systems.
• Research is needed to identify the causal factors controlling the distribution, accumulation, and persistence of chemicals in freshwater.
• Research is needed regarding the factors affecting microbial resistance to antibiotics in freshwater.
• Research is needed to examine the chronic toxicity of antibiotics and other chemotherapeutants to fish and other freshwater organisms.
• There is a need to develop standard sampling design and analytical protocols in aquaculture science.
• There is a need for an inventory of therapeutant usage patterns that includes reports of what is used, where and in what amount.

References

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Bjorklund, H.V. and G. Bylund. 1991. Comparative pharmokinetics and bioavailability of oxolinic acid and oxytetracycline in rainbow trout (Oncorhynchus mykiss). Xenobiotica 21: 1511-1520.

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