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Antibacterial agents in fish farming

Bellona, 09/01-2009

Like land-based forms of raising livestock, where large numbers of animals are placed in a very limited space, intensive fish farming provides various diseases and parasites ideal conditions to spread. Infectious diseases like furunculosis were previously treated with large quantities of antibiotics added to feed. In our discussion of the environmental impacts of antibacterial agents in fish farming, we would remind the reader that the use of antibiotics in the Norwegian fish farming industry has fallen from 50,000 kilograms to 500-600 kilograms in fifteen years (Grave et al., 2002). ). According the Norwegian Institute of Public Health statistics the antibiotic use in fish farming was at its lowest in 1999 with 591 kg. Since that time the use has increased to a top in of 1,428 kg in 2006 and a reduction to 649 kg in 2007.

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Current levels are low enough not to pose any environmental problem. Nevertheless, there are two reasons why we wish to focus on this issue. First, the use of antibiotics is still high in other countries where salmonids are farmed. This applies to Chile, for example, a country not as advanced in preventive fish health as Norway. There is no vaccine against the disease piscirickettsiosis, the disease causing the greatest losses in Chilean fish farming (Olsen, 1999). Antibiotic use in Chile is estimated at fifty tonnes (Jensen, 2002; Nutreco 2002). Second, we fear that consumption in Norway will rise if the farming of other species such as cod reaches significant production volumes. The reason for the rapid decline in Norwegian antibiotic use is primarily due to the availability of effective vaccines against the chief bacterial infections, as well a generally better fish health as a result of more suitable operation locations, less stress and improved hygiene against contagions (Poppe, 1999).

New farmed species will pose new health challenges, and the development of new vaccines may take time (Bleie, 2002). New salmon diseases and new variants of known diseases may also help to reverse the favourable trend in Norway. In 2002 consumption increased to about 1,000 kilograms, especially due to problems with winter ulcers (Norwegian Animal Health Authority, 2003; Rasmussen, 2002).

Environmental impacts of antibiotics in fish farming
Antibiotics are administered to fish through medicated feed that is mixed at feed plants. Antibiotics enter the marine environment when some of the feed is not eaten and sinks to the seabed or is eliminated in fish excretion (Smith, 1996). Three effects of the spread of antibiotics to the marine environment are highlighted:
- Antibiotic resistance
- Spread to wild fish
- Retarded decomposition of organic material

Antibiotic resistance
After intensive, repeated medication using the same antibacterial agent, reduced efficacy is often seen after a time against the disease in question. Before vaccination programs largely eliminated furunculosis outbreaks, oxolinic acid was chiefly used to treat it. After repeated treatments, it was noted that the efficacy was reduced, and in 1999 an investigation showed that 36% of the furunculosis bacteria were resistant against one or more antibiotics (Sørum, 1999). As a consequence of the emergence of resistance, treatments must be repeated or new agents employed. However, antibiotic resistance is not purely a problem of fish health. Resistance can develop in other bacteria in the marine sediments and in the water, thereby spreading to other organisms in the marine environment (Sandaa et al., 1992). The total burden of resistant bacteria people are subjected to is growing and may create new problems for human health.

Spread to wild fish
Antibiotics are also spread to wild fish directly when cod, for example, eat medicated feed that falls through the cages. This fish, in turn, may be caught and eaten by people, who thereby ingest limited doses of antibiotics (Røstvik, 1997).This is undesirable, when one considers the development of resistance in people. Since the use of antibacterial agents is currently so small, the quantity that wild fish ingest from feed spills will be practically equal to zero.

Retarded decomposition of organic material
Antibacterial agents, as the name indicates, are meant to kill bacteria. But as was mentioned above, parts of the doses of medication end up in the marine environment, especially in the sediments beneath and around the fish farm.The consequence of the spread is that the number of bacteria in the sediments is also reduced. Hansen et al. (1992) found that the total number of bacteria in the sediment after the addition of the antibacterial agents oxytetracycline, oxolinic acid and flumequine by 50-70% respectively. A fish farm has considerable discharges of organic material in the form of unutilised fish feed and excrement (see below in this chapter). A sharp decline in the bacterial flora around the farm retards the decomposition of the organic material and contributes to accumulation.

The biodegradation of antibacterial agents in sediments
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The biodegradation of antibiotics in the sediments goes relatively slowly (a half-life of up to 150 days in the topmost sediment layer, 0-1 cm), but varies substantially among the various agents and among types of sediment. A study in which samples were taken up from under various fish farms indicates that Florfenicol has a half-life a fraction of that of the other common agents (Hektoen et al., 1995)

Important to focus on antibiotics
The environmental impacts of high levels of antibiotic use in fish farming will be substantial and unwanted. Therefore, it is good news that the use of antibiotics in the Norwegian fish farming industry has nearly been eliminated in fifteen years, from 50 tonnes to about 1,000 kilograms. Nonetheless it is important to keep the focus on this issue in countries where the development of vaccines and general preventive health efforts have not come as far, and where considerable antibiotic use continues to be maintained.
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