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Drugs used on B.C. salmon farms and effects
on the marine ecosystem
(Prepared for the David Suzuki Foundation by Sergio Paone, Ph.D.) A variety of chemicals, such as antibiotics, pesticides and fungicides are used on salmon farms to treat disease outbreaks. These drugs are often administered to the fishthrough their feed. Since salmon are mostly raised in open marine netcages, most of thedrug, or its metabolic byproducts, end up in the marine environment through uneaten feed orthe salmon's excrements 1. The distribution and environmental impact of these chemicals isa cause of great concern.
Antibiotics
The most common antibiotic used is oxytetracycline, with 6.4 metric tonnes used on B.C. salmon farms in 1998 2. Others include fluorfenicol and a class known as sulfonamides.
It has been shown that oxytetracycline is poorly absorbed by the intestinal tract of thes a l m o n 3. Consequently much of the drug is excreted unchanged into the marineenvironment, where it distributes itself between the sediment and water column, or isingested by wild sealife 4. Studies show that some antibiotics, including oxytetracycline andfluorfenicol, persist in the environment, and marine sediment acts as a long-term reservoir forthem 5.
Not surprisingly, investigators have shown that antibiotics can significantly alter the microbial community found in marine sediment. Not only can the total amount of bacteria bereduced, but also the relative abundance among the different species is altered. Sedimentdwelling bacteria provide a number of key services, in particular the cycling of nutrients suchas nitrogen, phosphorous and sulfur. Measurements reveal that antibiotics found in marinesediment near salmon farms lower the conversion rates for sulphates and nitrates.
What consequences may arise from an altered marine microbial community has not been studied. There is also the question of the possible effects of any chemicals producedwhen the fish metabolically convert the administered antibiotic. For example, when salmoningest florfenicol, the fish converts some of the antibiotic to florfenicol amine. There are nostudies on how this, or other antibiotic metabolites, can affect the natural marine community.
One area that has received a great deal of study, is the increase in antibiotic resistant bacteria in sediment under fish farms, in farmed salmon, and in wild organisms caught near salmon farms. The implications that this has for human health and is covered in some of ourother information sheets. With respect to the marine environment however, it should bementioned that increases in antibiotic resistant bacteria leads to increased use of antibioticson the salmon farm, increasing the environmental risks.
Pesticides
Sea lice infestations often cause problems at salmon farms, and are the primary reason that pesticides are used. The fish are treated with these chemicals in one of twoways. One is by using a tarpaulin to isolate the fish within the netcage and then adding thechemical in the seawater. After 30-60 minutes, the tarpaulin is removed and the solution isreleased to the marine environment 6. This method is used for pesticides such ascypremethrin , dichlorvos and azamethiphos. Other pesticides, such as ivermectin, areadministered within the feed.
The Canadian Pest Management Regulatory Agency has only approved azamethiphos for use against sea lice. Although not approved, the other pesticides are usedwith the permission of a veterinarian, often with little or no data available on their effects onthe marine environment.
In B.C., sealice are often treated using ivermectin. A high proportion of the administered chemical is excreted unchanged by the salmon, and accumulates in marinesediment beneath and in the vicinity of the fish farm. It can take from 90 - 240 days for justhalf of the chemical in the sediment to decompose. Recent analysis of sediment undersalmon farms has shown levels of ivermectin up to 6.8 milligrams (mg) per kilogram ofsediment. This concentration went down with distance from the farm, but in some samplesstill showed a concentration of 5.4 mg per kg of sediment 35 metres from the netcage. Theaccumulation of ivermectin was also expressed as amount per unit area. This gave figures of .675 mg per squaremetre (m-2) for under the Table 1: Studies which show lethal effects of ivermectin on neurological processes.
It also can bind to biological membranes,i n c r e a s i n g ions (a main componentof sea water) 7 . It potential to be toxic to awide variety of marine chemical to be very toxic to many species that live in or on the seabed. Table 1 lists some ofthe species that have been tested and the concentrations of ivermectin for which the speciesbegins to die off. It can be seen that the concentrations that are lethal to these organisms arein the range of what has been measured under and near salmon farms (see paragraphabove).
The lethal effect of ivermectin on the polychaetes is particularly interesting. This large class of marine worms is often a crucial part of many marine food chains. They also are keyto the decomposition of accumulated organic matter, such as fish feces and uneaten feedthat accumulates under salmon farms. The worms constantly turn over the marine sedimentallowing oxygenated water to reach aerobic decomposing bacteria. Without these worms,the marine sediment can become depleted in oxygen and proper decomposition cannotoccur.
Two of the other pesticides, cypermethrin and azamethiphos, used on salmon farms have also been shown to have toxic effects on marine organisms. A recent study showedthat cypermethrin is lethal to lobster larvae at concentrations of 0.06 to 0.16 micrograms perlitre of seawater 11. The same study showed that azamethiphos killed shrimp and adultlobster at concentrations of 50 micrograms per litre of seawater. Sub-lethal concentrations of azamethiphos at 5 and 10 micrograms per litre were also shown to reduce spawning infemale lobster.
The use of pesticides on salmon farms results in negative impacts on marine organisms found near the salmon farm. The effect on many more marine organisms needs tobe studies, as well as the effects of sub-lethal concentrations, which has received very littleattention.
References
1) Lunestad, B.T., 1992. Fate and effects of antibacterial agents in aquatic environments. In: Michel, C.M., Alderman, D.J. Eds. , Chemotherapy in Aquaculture: From Theory to Reality. OfficeInternationale des Epizooties, Paris, pp. 151–161.
2) See Farmed & Dangerous3) Samuelsen, 1994.
4) GESAMP 1997.
5) Samuelsen, 19946) Haya, K., et. al., Environmental impacts of chemical wastes produced by the salmon aquaculture industry. ICES Journal of Marine Science, 58, 492-496, 2001.
7) Davies, I.M., et. al., Environmental risk of ivermectin to sediment dwelling organisms. Aquaculture, 8) Davies, I.M., A review of the use of ivermectin as a treatment for sea lice [Lepeophtheirus salmonis (Kroyer) and Caligus elongatus Nordmann] infestation in farmed Atlantic salmon (Salmosalar L.). Aquaculture Research, 31: 869 - 883, 2000.
9) Thain, J., et. al. Acute Toxicity of ivermectin to the lugworm Arenicola marina. Aquaculture, 159: 10) Collier, L. An assessment of the acute impact of the sea lice treatment ivermectin on a benthic community. Journal of Experimental Marine Biology and Ecology, 230: 131-147, 1998.
11) Haya, K., et. al. Environmental impact of chemical wastes produced by the salmon aquaculture industry. ICES Journal of Marine Science, Vol. 58: pp. 492-496, 2001.

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