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Transmission Dynamics of Parasitic Sea Lice from Farm to Wild Salmon
Martin Krkošek 1, Mark A. Lewis 1, John P. Volpe 21 Centre for Mathematical Biology
2School of Environmental Studies, University of Victoria
Parasitic sea lice infestations of juvenile salmonids have been correlated with salmon farming and are concurrent with declines in affected populations, worldwide. However, these relationships are correlative only and their causal nature is unclear. Scientific understanding and consensus has largely been limited by an inability to track parasite movement from farmed to wild salmon.
This report looks at lice infestations on wild juvenile pink and chum salmon as they migrated past an isolated salmon farm down a long and narrow migration corridor in the Broughton Archipelago, British Columbia, Canada (see Box 1). Mathematical models were used to analyze these data (see Box 2) and revealed juvenile salmon were initially infected with sea lice originating from two host populations: farm salmon and naturally occurring hosts. The calculations suggest the infection pressure near the farm was approximately 70 times greater than natural ambient levels and exceeded ambient levels for 30 km of migration route (see Box 3). This amounts to a total direct contribution of sea lice from the farm that was approximately 30,000 times greater than the natural production of sea lice in the length of habitat occupied by the salmon farm.
The results also show that once lice were transmitted to wild juvenile salmon, they were transported down the migration route where they reproduced and re-infected the wild juvenile salmon. Inclusion of this second generation of lice raises the farm-origin infection pressure above natural levels for approximately 75 km of migration route resulting in a total direct and indirect contribution of sea lice that was 200,000 times greater than the natural production of sea lice in the length of habitat occupied by the salmon farm. This transport and reproduction of farm-origin lice raises the possibility for disease growth and spread to other wild salmon populations up and down the coast.
Sea Lice (Lepeophtheirus salmonis and Caligus clemensi)
Sea lice have two distinct phases in their lifecycles – planktonic and parasitic. Planktonic larvae float freely in the water and allow lice to be transmitted between wild and farmed salmon. During the second, parasitic phase, lice attach themselves to a host salmon and feed on the surface of the fish – leading to increased disease and sometimes death in their infected hosts (See Box 2).
The peer-reviewed primary scientific literature on sea lice interactions between wild and farmed salmon in British Columbia makes the following conclusions: (1) infection rates on wild juvenile pink salmon were greater in salmon farming regions than in regions without salmon farms 1; (2) within a salmon farming region most lice on wild juvenile pink and chum salmon originated from farmed salmon 2; and (3) transmission of lice from farmed to wild salmon leads to population growth and spread of lice in wild salmon populations. Salmon aquaculture likely has negative impacts on wild salmon populations and the next scientific challenge is to quantify this impact.
There is sufficient scientific evidence to compel precautionary action by regulators. The premise of industrial-scale open net aquaculture in wild salmon habitats needs to be reconsidered and the proposed expansion of the industry throughout BC must be halted until the full ecological costs of this industry are understood. There is a clear potential for severe and irreversible damages to be inflicted upon wild salmon populations and their dependent cultures, ecosystems, and economies.
1Morton, A., Routledge, R., Peet, C., & Ladwig, A. 2004 Sea lice (Lepeophtheirussalmonis) infection rates on juvenile pink (Oncorhynchus gorbuscha) and chum (O. Keta) salmon in the near shore marine environment of British Columbia, Canada. Canadian Journal of Fisheries and Aquatic Sciences. 61, 147-157.
2Krkošek, M., Lewis, M. A., & Volpe, J. P. 2005. Transmission dynamics of parasitic sea lice from farm to wild salmon. Proceedings of the Royal Society of London Series B.
Box 2. Louse Lifecycle and Transmission Dynamics