Sea lice, in particular the salmon louse Lepeophtheirus salmonis, are considered the most damaging parasites of marine farmed salmonids, costing the global aquaculture industry over €300 million per year, with the Scottish industry alone losing €33 million annually, corresponding to 5% to 6% of the production value. In addition to the direct costs of sea lice control there are production losses, due to downgrading, and reduced growth costs as well as the consequences of immune suppression, which can lead to infection by other pathogens.
These factors in turn have detrimental outcomes in terms of fish welfare and profitability. Additional costs, connected with the possibility of lice derived from farmed salmon infecting local wild salmon stocks, also have to be considered, according to the farming context.4 Due to increasing drug resistance in sea lice, which affects susceptibility to many available treatments such as organophosphates, pyrethroids, avermectins and topical disinfectants, much research has been dedicated to the development of new control methods and the understanding of the sea louse life-cycle.9,
These include management practices such as integrated pest management, which encourages synchronized fallowing and lice treatment at different farms in a particular system. Other control strategies being investigated for marine copepod parasites include the development of new therapeutants and vaccines and the use of controls involving aspects of chemical ecology.
These strategies, however, are not likely to be commercially available within the short to medium term. Encouraging results have also been recently obtained with respect to breeding programmes for genetic resistance to sea lice in commercial Atlantic salmon, Salmo salar, populations, with heritability of up to 0.3 reported. Differences in susceptibility to sea lice infection and abundance of sea lice on wild and farmed strains of Atlantic salmon have been investigated, with as much as 70% variation between the highest and lowest infected family strains, showing the potential of selective breeding and family selection.
It was reported, that susceptibility to louse infectioncould be dependent on a Major Histocompatibility Complex (MHC) genotype, although a major effect is unlikely. The effect may be due to variation in quantitative trait loci associated with MHC class II regions through linkage disequilibrium. It is, however, untested that QTLs (quality trait loci) moderate the effect of MHC genes, but the discovery of a genetic element that conclusively affects susceptibility to lice is an exciting development. However, resultsfrom studies of salmon susceptibility to bothLepeophtheirus salmonis and the branchiuran Argulus coregoni suggest that there are many interactingfactors that contribute to the extent of a louse infection on an individual, thus the role of genetic factors may be small.
Selection for salmon resistance, however, is a long-term goal and only reduced sensitivity to sea lice is likely to be achieved. Finally, another relevant strategy is the use of cleaner fish, such as wrasse (Ctenolabrus and Labrusspp.) in polyculture. In line with the principles of integrated pest management, the best solutions to the sea lice problem are likely to involve the coordinated use of a broad range of these and other strategies. In recent years there has been a dramatic increase in the production of farmed Atlantic salmon, raising concerns about the environmental impact of these activities.
One particular area of concern is escapees, which have been documented to cause genetic changes in native populations as a result of interbreeding. Although considerable technological advances have been made in the design of cages, escapes through natural disaster, human error or mechanical failure are small, but inevitable risks and salmon being on-grown in the marine environment are reproductively competent. For these reasons, the production of sterile fish to mitigate the environmental impact of escapees and potential inter-breeding with wild stocks, is receiving ever-increasing attention.
The use of triploid salmon in commercial Atlantic salmon aquaculture is the only commercially acceptable means of sterility to address the environmental impacts of escapees. Differences in performance, physiology, behaviour and morphology between triploid and diploid fish are well described, and these differences could conceivably contribute to differential susceptibility to sea lice infection. A range of factors contribute to the susceptibility of Atlantic salmon to infection and may signal host suitability to the parasite, such as nutritional condition and size/morphology.
Triploid fish have three sets of chromosomes instead of two, which leads to larger, but fewer cells in all tissues and organs. Behavioural differences between fish of different ploidies have been described, with reduced aggressiveness, inferior overall performance in sub-optimal conditions, feeding at deeper water depths and lower responsiveness to environmental stimuli reported for triploid Atlantic salmon. Studies have also suggested that triploids could be more susceptible to pathogen or parasite infection due to reduced immune activity as compared to diploids.
For example, triploid Atlantic salmon have been found to be more susceptible than their diploid counterparts to infection by Gyrodactylus salaris, a monogenean ectoparasite. The authors suggested this might be due to compromised complement-dependent immune pathways in triploid salmon. It is uncertain, however, as to what extent the observations in these various studies relate to ploidy per se rather than to the interaction of ploidy with particular genotypes. In many of the previously published trials, fish size was unaccounted for. Results from tank and cage trials have shown that large fish tend to be more heavily infected.
Given that triploid smolts show higher growth potential than diploid salmon smolts, the sizeof the fish can be a potential confounding factor with respect to comparison of sea lice infection levels. Salmonid fish are capable of generating an immune response to salmon lice, however, no acquired protection against re-infection has been observed. Various studies tested individually tagged diploid salmon in separate challenges with sea lice and demonstrated that the infection level for a single salmon in one challenge is a poor predictor of its infection level in a subsequent challenge.
Persistent infection may lead to compromised host immunity and tissue damage followed by a period of hyporesponsiveness and delayed healing. It has been suggested that weakening of the animal could be expected to be more pronounced in triploid fish compared to diploid fish due to reduced immune activity. The aim of the present study was to compare the susceptibility of triploid and diploid Atlantic salmon to infection by the salmon louse Lepeophtheirus salmonis in several experimental and commercial settings in Scotland and Norway. In addition, a re-infection trial was undertaken to determine if a correlation existed between the outcomes of infection events for individual fish.
April 2015
