Agriculture and Environment: Salmon

Environmental Impacts of Production: Use of Chemical Inputs

In addition to the antifoulants discussed earlier, chemical inputs in salmon farming include antibiotics and insecticides such as organophosphates and synthetic pyrethroids.
Therapeutic chemicals may be applied as a bath treatment or administered in feed, but in both cases the chemicals eventually make their way outside the salmon cage into the wider marine environment.

The effects of chemicals on the greater marine environment are not well known. The ecological impacts resulting from the use of antibiotics in salmon farming have not been studied. It is conceivable that antibiotics could accumulate in the tissue of wild fish and invertebrates, while also leading to resistance in target pathogens and other microbial species.

The strictest country, yet over 50 different chemicals permitted
Scotland is known for being the strictest country when giving out permits to salmon farmers. Their typical discharge consent, however, allows the use of over 50 different chemicals. The number of drugs permitted for use by the Veterinary Medicines Directorate has increased from 3 to 40 from 1989 to 2002 (Staniford 202).

In short, "the global advance of intensive salmon farming has meant that farmed fish have become agents of pollution rather than biological indicators of pollution" (Staniford 2002).

Fewer use of drugs and chemicals in salmon production
Several different drugs and chemicals are used to combat diseases and parasites in the production of salmon. Over time the industry has learned how to produce more salmon using fewer drugs and chemicals. However, the learning curve has tended to be repeated in each new area of culture.

For example, from 1985-87, antibiotic use in salmon farms in Norway increased from 17 to 48 metric tons per year, more than the combined use of all antibiotics for humans and terrestrial animals in the country (Weber 1997). In 1999 in the United Kingdom, 4 metric tons of antibiotics were used in salmon farming compared to 11 metric tons in cattle rearing and less than 1 metric ton with sheep (Berry and Davison 1999). As vaccines have been developed and as management systems have been improved, these levels have declined drastically.

Usage varies by country
In Chile, however, the reduction in the use of antibiotics has bee slower, even though most of the major investors are Norwegian. In 1990 the salmon industry used 13 metric tons of antibiotics, by 1995 usage had increased to 65 metric tons, and by 1998 it was 100 metric tons. In 1993 Chile used 75 times more antibiotics per kilogram of salmon produced than Norway (Claude and Oporto 2000).

In the early years, most antibiotics were put in the manufactured feed, and as late as 1999 medicated feed was still common in Chile (Claude and Oporto 2000). At least three-quarters of antibiotics in feed are lost to the environment, whether the feed is eaten or not (Weber 1997). Little is known about the impact of these drugs on ecosystems in general or an individual species in particular.

Disadvantages of prophylactic use of drugs
The prophylactic use of drugs can lead to growth of drug-resistant strains of pathogens in both wild and cultivated fish populations. The abuse of antibiotics through prophylactic use can also build up pathogenic resistance in humans. In 1991, 50% of the bacteria responsible for the fish disease furunculosis were resistant to two compounds used to treat the disease.

Scientists disagree about the extent to which resistance has developed, but they agree that resistance will increase as antibiotic use increases - and that this resistance can be passed on to human pathogens. In addition, there are a limited number of compounds that are effective on aquatic pathogens, which means there will be even graver consequences if resistance develops.

The chemicals are not always even appropriate. For example, the chemicals used to treat sea lice have largely been developed for terrestrial use, and little research has bee done on their use in the marine environment.

In Scotland salmon producers used a chemical delousing agent called dichlorvos to reduce infestation of salmon by sea lice. Later research suggested that this chemical killed oysters, mussels, and other shellfish and crustaceans within 75 metres of the salmon cages (Weber 1997).


Extracts from "World Agriculture & Environment" by Jason Clay - buy the book online from Island Press

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