Science Advisory Report 2013/049

Potential exposure and associated biological effects from aquaculture pest and pathogen treatments: anti-sea lice pesticides (part II)

Summary

• During anti-sea lice pesticide treatments in southwest New Brunswick, fluorescein dye was mixed with the pesticide in order to trace the transport and dispersal of the pesticide.  In situ water samples were analyzed for both the pesticide and fluorescein dye concentrations.  This data shows a good relationship between in situ fluorescein dye concentration and in situ pesticide concentration indicating that the concentration of fluorescein dye is a good surrogate for field pesticide concentrations, over time periods of a few hours.
• Comparison of the horizontal spread of dye released from tarp pesticide applications to the Okubo model (1971, 1974) indicates that the Okubo model underestimates pesticide dilution from net-pens over the first few hours after release.  However, a modified Okubo model based-approach that takes into consideration the initial enhancement of the horizontal spread (assumed to be due to cage infrastructure) gives a more accurate estimation of dye plume size and dilution (concentration).
• The characteristics of the immediate discharge from well-boats seem to be reasonably well represented by jet dynamic steady state theory.  Based on this theory, entrainment of water into the effluent should result in pesticide dilution up to a factor of 40 at a distance of 100 m from the discharge point, based on a 0.5 m discharge pipe diameter.
• Estimates of effects thresholds for American lobsters and other indigenous invertebrates, based upon the predicted exposure profile, the prescribed treatment concentrations and the concentration that is lethal to 50% of the test organisms (LC₅₀) after 1 hour pesticide exposure, indicate that the potential magnitude of effects increases from Interox-Paramove® 50 (active ingredient: hydrogen peroxide) to Salmosan® 50WP (active ingredient: azamethiphos) to Excis® (active ingredient: cypermethrin) to AlphaMax® (active ingredient: deltamethrin).  Sensitivity is species and life stage dependent for all four pesticides tested.
• Of the four pesticides studied, there is considerably more toxicology data available for Salmosan® 50WP.
• Sublethal and delayed effects were observed in adult American lobsters after repeated pulse exposures to Salmosan® 50WP at 10% and 5% of prescribed treatment concentrations, but no sublethal or delayed effects were observed at 1% or 0.1% of prescribed treatment concentration.
• In order to give an indication of the potential risks associated with exposure to each of the four anti-sea lice pesticides at in situ concentration and exposure durations, risk quotients (RQ) were calculated using the ratio of the exposure concentration to the 1-hour LC₅₀ concentration.  Based on the most sensitive non-target species tested, the pesticide that has the lowest risk quotient at the tarped net-pen treatment concentration (i.e., the concentration initially released into the environment) is: Paramove® 50 (RQ = 1.2), followed by Salmosan® 50WP (3.1), Excis® (151) and AlphaMax® (588).
• Risk quotients calculated for the concentration at discharge after well-boat treatments, for the most sensitive species tested, based on 1-hour LC₅₀s, ranged from: 0.46 for Paramove® 50, to 1.16 for Salmosan® 50WP, and 221 for AlphaMax®.
• Risk quotients calculated at 10 m from the discharge pipe (following well-boat bath treatments), based on 1-hour LC₅₀s, ranged from: 0.1 for Paramove® 50, 0.3 for Salmosan® 50WP, and 55 for AlphaMax®.  Similarly, the estimated risk quotient at 100 m from the point of well-boat discharge ranged from 0.01 for Paramove® 50, 0.03 for Salmosan® 50WP and 6 for AlphaMax®.
• Based on the relative calculated risk quotients, the predicted pelagic area of influence (i.e., the predicted plume area at or above LC₅₀ following release of the pesticide) following well-boat treatment is estimated to be smaller than the area of influence following tarp treatment (i.e., 1100 m² for Salmosan® 50WP to be diluted to LC₅₀, following tarp treatment, and 50,000 m² for AlphaMax®).
• Further validation and refinement to enhance the models would be beneficial, particularly with respect to vertical mixing and exposure of benthic environments, sub-lethal or no effect level (NOEC) interactions, and population and lifestage considerations for other non-target species.  This is particularly important for application of the approach to other salmon producing areas in Canada.
• Although the field work was conducted in southwest New Brunswick, the general principles regarding transport and dispersal and the orders of magnitude of dilution are expected to apply elsewhere since the Okubo relationship is based on data collected from many places around the world.  The modified Okubo-model should be tested to validate the effect of farm infrastructure on horizontal spread and dilution rates.  To apply the Okubo models elsewhere and to improve their predictability, local oceanographic and environmental data are required for model parameterization.
• Site-specific differences including local hydrography, bathymetry, treatment procedures, local stratification and current regime will influence the depth to which pesticides mix and the direction and magnitude of the pesticide transport.  The bathymetry within the zones of influence will also influence whether the benthic habitat will be exposed.
• While the risk quotient approach to estimating the potential exposure concentrations of pesticides to non-target organisms can be applied across all salmon farming regions in Canada, characterization of the effects requires local knowledge of the biology, ecology, and population dynamics of non-target species.
• The approach developed predicts the potential for effects to individuals but does not address population scale effects.

This Science Advisory Report is from the March 13-15, 2013 assessment of Guidelines on Defining Potential Exposure and Associated Biological Effects from Aquaculture Pest and Pathogen Treatments: Anti-Sea Lice Bath Treatments (Part II).  Additional publications from this meeting will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.

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