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Use of Emamectin Benzoate in the Canadian FinFish Aquaculture Industry: A Review of Environmental Fate and Effects

Abstract

UMA Engineering Ltd. was requested by Environment Canada (EC) to prepare a literature review pertaining to the use of emamectin benzoate (EB) for sea lice control in coastal finfish aquaculture in Canada. EB is used under the trade name Slice®, developed by Schering Plough Animal Health. The review will consider:

  • EB’s use patterns and characteristics of application;
  • analytical methods and detection limits for EB and its desmethyl metabolite;
  • physicochemical properties, environmental fate and transport, aquatic toxicity and effects of EB and its desmethyl metabolite; and
  • the current relevant Canadian and international standards and regulations.

The review identifies specific knowledge gaps and provides recommendations on future research requirements including pre-requisites for any field studies.

The preferred chemotherapeutant for sea lice in Canada, at the present time, is “Slice ®”, which is a trade name for a product developed by Schering-Plough Animal Health (SPAH) that has EB ( CAS No. 155569-91-8, formerly 137512-74-4) as its active ingredient. Slice ® is not yet registered for use in Canada; however, it is available for limited use through Health Canada’s Emergency Drug Release program. It is undergoing the approval process by Health Canada for use in Canada. Internationally, Slice ® has been developed as an alternative to the use of other sea lice control products, including ivermectin, dichlorvos, azamethiphos, hydrogen peroxide, cypermethrin, teflubenzuron and diflubenzuron.

Emamectin belongs to the avermectin group, a family of closely related compounds produced by the fungus Streptomyces avermitilis, which share broad spectrum toxicity against nematodes, arthropods, and several other pest taxa. Slice ® is currently being used in British Columbia and Atlantic Canada under an “Emergency Drug Use” basis, for controlling sea lice at coastal finfish aquaculture operations. The recommended dosage of EB, administered as Slice ® is 50 µg kg -1 day -1 for a duration of 7 consecutive days.

In New Brunswick, treatment for sea lice is often initiated when infection rates reach > 5 pre-adult sealice per fish, or > 1 overigerous female per fish, depending on the water temperature and the season. Federally, the Feeds Act and Regulations require Canadian feed mills to maintain copies of records for prescriptions administered through feed at their manufacturing sites. In 1998 in Atlantic Canada, 4% of all manufactured fish feed was medicated, representing about 3,600 metric tonnes of feed. EB accounted for 38.1% of the prescriptions, while tetracyclines accounted for 52.4% and sulfonamides accounted for 9.5%. EB usage records were difficult to obtain for both the Pacific and Atlantic coasts of Canada. In British Columbia, it is estimated that use of EB as Slice® nearly quadrupled from the year 2000 (2.4 kg total quantity used) to 2002 (8.9 kg total quantity used), followed by a drop in 2003 to about 5 kg used. British Columbia Ministry of Agriculture, Fisheries and Food noted that the 7.35 kg of EB were prescribed in 2003 according to the data the Ministry collected from feed mills (Osborn pers. comm., 2005). The significance of this amount on the marine environment is unknown at this time.

Overall, there appears to be a strong dependence on the use of Slice® for sea lice control in finfish aquaculture in Canada and in Europe, and the available accounts suggest that multiple applications within grow out cycles may be the norm rather than the exception. Current information suggests that single applications of EB likely represent the norm among marine finfish farms in British Columbia (Osborn pers. comm., 2005). This is important, since previously completed environmental risk assessments for Slice® use in the marine environment have focused on predicted environmental concentrations base on a one-time rather than repeated applications at a site. In addition, some jurisdictions have recommended moving to a coordinated application of sea louse therapeutants across all farm sites in a single region, for a more integrated pest management approach. This practice, if implemented, might have negative consequences for non-target organisms in light of short-term EB concentrations associated with releases from multiple sites.

The strong lipophilicity of EB (log K OW = 5) suggests that the major portion of environmental releases will partition to, or remain in, suspended and settled particles. The potential for dissociation of some functional groups on the EB molecule, however, at a pH typical of seawater may result in greater tendency to partition into water than would be expected based on examination of the octanol- water partition co-efficient in isolation. The water solubility is expected to be in the range of 5 to 24 mg/L depending on salinity, and solubility limits are not expected to impose restrictions on leaching of EB or its metabolites from medicated feed or faecal pellets into the water column or sediment interstitial water.

Scientific data on concentrations of EB in the Canadian aquatic/marine environment are extremely sparse. Limited data may become available shortly based on studies in progress. There are significant knowledge gaps about expected or documented concentrations of EB and its metabolites in the environment on a global basis, and this imposes perhaps the greatest limitation on the ability of scientists and managers to accurately assess environmental risks from the use of Slice® at this time.

There is a reasonable amount of data on the short-term toxicity of EB to crustaceans and other aquatic organisms; however, substantial knowledge gaps were noted for: (i) data on chronic (as opposed to acute) toxicity, ii) ecologically relevant effects other than mortality, (iii) endocrine disruption effects (e.g., altered moulting and reproduction in lobsters exposed to EB); and (iv) toxicity data for benthic meiofauna such as nematodes which are potentially sensitive and ecologically important indicator species.

Recommendations for follow-up studies include:

  • Determining representative chemical concentrations in the Canadian coastal environment (i.e. water, sediment and biota) for both EB and related compounds such as the desmethyl metabolite, and
  • Conducting ecotoxicity studies on sensitive Canadian indigenous species under ‘real world’ conditions for a range of toxic effects including chronic and sub-lethal end-points.

Copy of report available by request at georgiabasin@ec.gc.ca

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