Research Document - 2015/064
Transport and dispersal of sea lice bath therapeutants from salmon farm net-pens and well-boats
By F.H. Page, R. Losier, S. Haigh, J. Bakker, B.D. Chang, P. McCurdy, M. Beattie, K. Haughn, B. Thorpe, J. Fife, S. Scouten, D. Greenberg, W. Ernst, D. Wong, and G. Bartlett
Abstract
Salmon aquaculture sea lice bath treatments result in the release of the bath water containing the pesticide into the ambient environment. The consequence of these releases to non-target organisms in the receiving environment depends upon the dilution and toxicity of the therapeutant, and whether the non-target organisms get exposed to the released therapeutant. This report summarizes work conducted during 2010 in relation to measuring and modelling the transport and dispersal of sea lice pesticide effluents from sea lice treatments associated with commercial net-pen salmon farming in the southwest New Brunswick area of the Bay of Fundy in eastern Canada. The work involved commercial tarp and skirt based treatments of net-pens as well as the use of well-boats. Field studies were conducted on local salmon farms during an active and particularly severe outbreak of sea lice. In all cases fluorescein dye, either alone or in combination with a pesticide, was added into the treatment volume that contained commercial quantities of salmon. The concentration of dye and, at times, pesticide was measured within the treatment volume prior to its release into the receiving environment and for several hours after its release. Treatments were accompanied by measurements of the temporal evolution of the horizontal and vertical distributions of the dye concentration, and by measurements of water currents using moored current meters and near-surface GPS drifters.
The purpose of this report is to summarize the dilution and transport of the released dye gathered from the treatment field studies. The report is organised into five main sections: Section one presents a brief context for the work; Section two describes the net-pen treatment process and the work conducted in association with tarped and skirted net-pens treatments; Section three describes the well-boat based treatment process and the work conducted in association with well-boat based treatments; Section four describes results of chemical analyses of water samples taken during the treatment studies and, in addition, on laboratory experiments involving pesticides; and finally, Section five summarizes the work and presents some preliminary conclusions.
The process of conducting a tarpaulin or skirt bath treatment of a net-pen involves several steps. Initially, the outside perimeter of the fish cage net is shallowed to a depth of about 3-4 meters. Next, either a tarpaulin is deployed around and under the net or a skirt is deployed around the net. Subsequently, the pesticide is pumped into the treatment volume and is assumed to become mixed throughout the enclosed volume by the movements of the fish and the water movement generated by the oxygenation system. Finally, at the end of the treatment period, the tarpaulin or skirt is removed, the cage net is allowed to drop to its normal depth, and the pesticide begins to leave the cage and advect into the ambient receiving waters. During the field studies, dye was added to tarped and skirted pens during treatment to allow visualization of the treatment water both within and outside of the treatment volume. Both horizontal and vertical distribution of the dye was measured after release from the treated pens. The data indicate that the rate of horizontal dispersion initially exceeded that estimated by Okubo’s empirical relationship for the temporal spread of a conservative substance in the coastal ocean but at approximately one hour post-release the evolution of the area occupied by the released substance evolved at a rate similar to that predicted by Okubo. The concentrations of the dye and pesticide as a function of time after release were measured: the released material is diluted by approximately a factor of 10 after 30 minutes, a factor of 100 after 1 hour and a factor of 1000 after 3 hours. A passive particle tracking model based on an implementation of the FVCOM water circulation model for the local area was used to predict the advection and dispersion of the treatment water in the ambient waters. The model gives a reasonable estimate of the observed horizontal distributions of released dye but did not perform well in the vertical.
In southwest New Brunswick well-boats are used to treat salmon for sea lice. A well-boat is a 100-200 foot long vessel in which the cargo holds can be filled with water. The holds are called the wells and the water inside them is mechanically recirculated and aerated. During sea lice treatment, fish are pumped into the wells where they are treated with pesticides. When the water is discharged from the wells it is pumped through one or more pipes (each ~12-14" in diameter) that exit the side or bottom of the boat. The pumping rate varies among vessels. During the study, dye was added to the treatment water along with the pesticide in order to observe the temporal evolution of dye within the well of the well-boat and to measure the temporal evolution of concentration, horizontal area and vertical distribution of dye within the discharge jet generated by well-boat treatments. The data indicate that discharges from well-boat treatments are quantitatively consistent with jet dynamics and are diluted more rapidly than from net-pen treatments. It was observed that the discharge dynamics vary between well-boats, which is due to each well-boat having unique structural features and different discharge characteristics. A particle tracking model based on the FVCOM generated circulation to predict the dispersion of treatment water into the receiving waters was explored but it was determined that, in its present state, the model is unable to give a reasonable estimation of the horizontal trajectory of the dye plume.
Although the purpose of the work was to gain insight into the distribution, mixing, transport and dispersal of pesticides, the results summarized above pertain to the distribution, mixing, transport and dispersal of the fluorescein dye. Analyses of water samples obtained from the released plumes of dye and pesticide as well as laboratory experiment results indicate that there was a linear correspondence between pesticide (azamethiphos and deltamethrin) and dye concentrations but that the temporal decay of hydrogen peroxide is very slow and negligible on time scales of a few hours.
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