Fish Farms and Sea Lice Infestations of Wild Juvenile Salmon in the Broughton Archipelago—A Rebuttal to

Factors influencing the long-term dynamics of larval sea lice density at east and west coast locations in Scotland

Sea lice (Copepoda: Caligidae) are marine copepods that parasitize finfish, and in cases of high infestation can result in severe epithelial damage and mortality. In Scotland, 2 species of sea louse, Lepeophtheirus salmonis and Caligus elongatus, pose a significant economic burden to the marine Atlantic salmon aquaculture industry and potentially impact wild salmonids. The purpose of this study was to determine how the density of pelagic sea lice is affected by external variables, in order to improve our understanding of sea lice dynamics. Long-term data from 2 sampling sites on the east and west coasts of Scotland were modelled independently in conjunction with environmental and anthropogenic variables. Statistical analysis identified that at the east coast site, the most influential factor affecting lice density was salinity. On the west coast, salinity, rainfall and farmed salmon production year were most influential. Molecular and morphological techniques also showed that the individuals recorded on the east coast were C. elongatus, a generalist copepod parasite, whereas only the salmonid-specific L. salmonis were found on the west. These results reiterate the role of environmental factors in influencing sea lice dynamics, and that salmonids are the primary hosts of sea lice on the west coast, but there could be non-salmonid host species as well as salmonid species influencing east coast sea lice densities.

Allee effect from parasite spill-back

The exchange of native pathogens between wild and domesticated animals can lead to novel disease threats to wildlife. However, the dynamics of wild host-parasite systems exposed to a reservoir of domesticated hosts are not well understood. A simple mathematical model reveals that the spill-back of native parasites from domestic to wild hosts may cause a demographic Allee effect in the wild host population. A second model is tailored to the particulars of pink salmon (Oncorhynchus gorbuscha) and salmon lice (Lepeophtheirus salmonis), for which parasite spill-back is a conservation and fishery concern. In both models, parasite spill-back weakens the coupling of parasite and wild host abundance-particularly at low host abundance-causing parasites per host to increase as a wild host population declines. These findings show that parasites shared across host populations have effects analogous to those of generalist predators and can similarly cause an unstable equilibrium in a focal host population that separates persistence and extirpation. Allee effects in wildlife arising from parasite spill-back are likely to be most pronounced in systems where the magnitude of transmission from domestic to wild host populations is high because of high parasite abundance in domestic hosts, prolonged sympatry of domestic and wild hosts, a high transmission coefficient for parasites, long-lived parasite larvae, and proximity of domesticated populations to wildlife migration corridors.

How sea lice from salmon farms may cause wild salmonid declines in Europe and North America and be a threat to fishes elsewhere

Fishes farmed in sea pens may become infested by parasites from wild fishes and in turn become point sources for parasites. Sea lice, copepods of the family Caligidae, are the best-studied example of this risk. Sea lice are the most significant parasitic pathogen in salmon farming in Europe and the Americas, are estimated to cost the world industry euro300 million a year and may also be pathogenic to wild fishes under natural conditions. Epizootics, characteristically dominated by juvenile (copepodite and chalimus) stages, have repeatedly occurred on juvenile wild salmonids in areas where farms have sea lice infestations, but have not been recorded elsewhere. This paper synthesizes the literature, including modelling studies, to provide an understanding of how one species, the salmon louse, Lepeophtheirus salmonis, can infest wild salmonids from farm sources. Three-dimensional hydrographic models predicted the distribution of the planktonic salmon lice larvae best when they accounted for wind-driven surface currents and larval behaviour. Caligus species can also cause problems on farms and transfer from farms to wild fishes, and this genus is cosmopolitan. Sea lice thus threaten finfish farming worldwide, but with the possible exception of L. salmonis, their host relationships and transmission adaptations are unknown. The increasing evidence that lice from farms can be a significant cause of mortality on nearby wild fish populations provides an additional challenge to controlling lice on the farms and also raises conservation, economic and political issues about how to balance aquaculture and fisheries resource management.

Modelling sea lice dispersion under varying environmental forcing in a Scottish sea loch

The spread of infectious larval sea lice, Lepeophtheirus salmonis (Krøyer, 1838), between wild salmonids and farmed Atlantic salmon, Salmo salar, remains a contentious area of uncertainty. However, as laboratory and field experiments increase our knowledge of sea lice behaviour under environmental forcing, numerical modelling tools can be used to predict the spread of infectious sea louse larvae from a point source. A three-dimensional numerical model has been developed and recently validated within Loch Torridon, a fjordic sea loch on the west coast of Scotland. Output from the numerical model is used to drive a particle tracking model which follows statistical representations of sea lice through the planktonic stages of a louse life cycle. By including maturation and mortality, the models can be used to predict the dispersion and transport of infectious sea lice from a point source and can be used to produce maps of infectivity under varying environmental conditions. Results highlight the importance of the wind-driven circulation for larval lice transport and suggest that local environmental conditions have considerable impact on the probability of sea lice infection spreading between wild and farmed fish populations.

Fish louseArgulus funduli(Crustacea: Branchiura) ectoparasites of the euryhaline teleost host,Fundulus heteroclitus, damage the ion-transport capacity of the opercular epithelium

Killifish ( Fundulus heteroclitus (L., 1766)) collected in the wild and kept in full-strength seawater were naturally parasitized by the ectoparasite Argulus funduli Krøyer, 1863, a copepod fish louse that creates inflamed skin lesions on the opercular epithelium and host gills. We assessed the damage done by lesions by counting the density of mitochondria-rich cells by fluorescence microscopy and by measuring Cl–secretion rate electrophysiologically using control (no lesions) and affected isolated opercular epithelia, often as paired left and right membranes from a single fish. Epithelia with lesions had a significantly reduced Cl–secretion rate, and in the lesions, the density of chloride cells was near zero. Contralateral membranes without lesions from infested fish had transport rates not significantly different from membranes taken from uninfected control animals, indicating no overcompensation on the contralateral membranes. Healthy control and infested animals were transferred to hypersaline conditions (twice seawater). Infested and control animals all survived transfer and had elevated plasma Na+and plasma osmolality. Infested animals failed to significantly elevate Imto the same level as healthy animals and there was a difference in hematocrit. Happily, the hypersaline challenge also resulted in detachment and death of adult A. funduli. We conclude that Argulus lesions impair salt transport in affected membranes but do not significantly affect survival on hypersaline challenge, and that hypersaline exposure is a successful treatment for A. funduli infestation in these strongly euryhaline teleosts.

Life cycle considerations for improving sustainability assessments in seafood awareness campaigns

It is widely accepted that improving the sustainability of seafood production requires efforts to reverse declines in global fisheries due to overfishing and to reduce the impacts to host ecosystems from fishing and aquaculture production technologies. Reflective of on-going dialogue amongst participants in an international research project applying Life Cycle Assessment to better understand and manage global salmon production systems, we argue here that such efforts must also address the wider range of biophysical, ecological, and socioeconomic impacts stemming from the material and energetic throughput associated with these industries. This is of particular relevance given the interconnectivity of global environmental change, ocean health, and the viability of seafood production in both fisheries and aquaculture. Although the growing popularity of numerous ecolabeling, certification, and consumer education programs may be making headway in influencing Western consumer perceptions of the relative sustainability of alternative seafood products, we also posit that the efficacy of these initiatives in furthering sustainability objectives is compromised by the use of incomplete criteria. An emerging body of Life Cycle Assessment research of fisheries and aquaculture provides valuable insights into the biophysical dimensions of environmental performance in alternative seafood production and consumption systems, and should be used to inform a more holistic approach to labeling, certifying, and educating for sustainability in seafood production. More research, however, must be undertaken to develop novel techniques for incorporating other critical dimensions, in particular, socioeconomic considerations, into our sustainability decision-making.