Lepeophtheirus salmonis and Caligus elongatus are the two common species of sealice responsible for serious disease problems in salmonid aquaculture. L. salmonis in particular is the most serious parasitic infection on Atlantic salmon farms in the Northern Hemisphere and is the best-known species. This review examines the voluminous literature on the biology and control of sealice and brings together ideas for developing our knowledge of these organisms. Research on the distribution, host range, structure, life cycle, epidemiology, laboratory maintenance, reproductive biology, physiology and pathogenesis is reviewed in depth. The control strategy and economic cost to the industry is discussed. The interactions between wild and cultured salmonids are examined.

Sealice on Salmonids: Their Biology and Control

Stewart C. Johnson, Jim W. Treasurer, Sandra Bravo, Kazuya Nagasawa, and Zbigniew Parasitic copepods are common on cultured and wild marine finfish, and there is a substantive literature describing their taxonomy, life cycles, and host ranges. Although many species have long been recognized to have the potential to affect the growth, fecundity, and survival of their hosts, it has only been with the development of semi-intensive and intensive aquaculture that their importance as disease-causing agents has become evident. Members of the family Caligidae are the most commonly reported species on fish reared in brackish and marine waters. These species, often referred to as sea lice, are responsible for most disease outbreaks. The impacts of sea lice on marine salmonid aquaculture are well documented, with catastrophic losses reported for disease outbreaks that have resulted in high levels of mortality. With the development of a variety of treatments and management strategies to reduce infection levels, mortality caused by sea lice has been greatly reduced. At present, economic losses due to sea lice are primarily from the costs of treatments, the costs of the management strategies, the costs associated with reduced growth rates that are a direct result of infection and/or treatment, and the costs of carcass downgrading at harvest. Indirect and direct losses due to sea lice in salmonid aquaculture globally are estimated to be greater than US$100 million annually. In other areas of marine aquaculture, the impact of parasitic copepods is not well documented. This is especially true for species such as Atlantic halibut, Atlantic cod, turbot, and haddock that have only recently entered commercialscale production. This review discusses the global importance of parasitic copepods as disease-causing agents in marine aquaculture. We also provide a brief review of the environmental and husbandry factors that may affect parasitic copepod abundance and the potential roles that parasitic copepods play as vectors for other disease agents.

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A Review of the Impact of Parasitic Copepods on Marine Aquaculture

Salmon lice, Lepeophtheirus salmonis, are naturally occurring parasites of salmon in sea water. Intensive salmon farming provides better conditions for parasite growth and transmission compared with natural conditions, creating problems for both the salmon farming industry and, under certain conditions, wild salmonids. Salmon lice originating from farms negatively impact wild stocks of salmonids, although the extent of the impact is a matter of debate. Estimates from Ireland and Norway indicate an odds ratio of 1.1:1-1.2:1 for sea lice treated Atlantic salmon smolt to survive sea migration compared to untreated smolts. This is considered to have a moderate population regulatory effect. The development of resistance against drugs most commonly used to treat salmon lice is a serious concern for both wild and farmed fish. Several large initiatives have been taken to encourage the development of new strategies, such as vaccines and novel drugs, for the treatment or removal of salmon lice from farmed fish. The newly sequenced salmon louse genome will be an important tool in this work. The use of cleaner fish has emerged as a robust method for controlling salmon lice, and aquaculture production of wrasse is important towards this aim. Salmon lice have large economic consequences for the salmon industry, both as direct costs for the prevention and treatment, but also indirectly through negative public opinion.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/jfd.12061

Salmon lice--impact on wild salmonids and salmon aquaculture.

Physiological, immunological and biochemical parameters of blood and mucus, as well as skin histology, were compared in 3 salmonid species (rainbow trout Oncorhynchus mykiss, Atlantic salmon Salmo salar and coho salmon O. kisutch) following experimental infection with sea lice Lepeophtheirus salmonis. The 3 salmonid species were cohabited in order to standardize initial infection conditions. Lice density was significantly reduced on coho salmon within 7 to 14 d, while lice persisted in higher numbers on rainbow trout and Atlantic salmon. Lice matured more slowly on coho salmon than on the other 2 species, and maturation was slightly slower on rainbow trout than on Atlantic salmon. Head kidney macrophages from infected Atlantic salmon had diminished respira- tory burst and phagocytic capacity at 14 and 21 d post-infection (dpi), while infected rainbow trout macrophages had reduced respiratory burst and phagocytic capacities at 21 dpi, compared to con- trols. The slower development of lice, coupled with delayed suppression of immune parameters, sug- gests that rainbow trout are slightly more resistant to lice than Atlantic salmon. Infected rainbow trout and Atlantic salmon showed increases in mucus lysozyme activities at 1 dpi, which decreased over the rest of the study. Mucus lysozyme activities of infected rainbow trout, however, remained higher than controls over the entire period. Coho salmon lysozyme activities did not increase in infected fish until 21 dpi. Mucus alkaline phosphatase levels were also higher in infected Atlantic salmon compared to controls at 3 and 21 dpi. Low molecular weight (LMW) proteases increased in infected rainbow trout and Atlantic salmon between 14 and 21 dpi. Histological analysis of the outer epithelium revealed mucus cell hypertrophy in rainbow trout and Atlantic salmon following infec- tion. Plasma cortisol, glucose, electrolyte and protein concentrations and hematocrit all remained within physiological limits for each species, with no differences occurring between infected and con- trol fish. Our results demonstrate that significant differences in mucus biochemistry and numbers of L. salmonis occur between these species.

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Susceptibility of rainbow trout Oncorhynchus mykiss, Atlantic salmon Salmo salar and coho salmon Oncorhynchus kisutch to experimental infection with sea lice Lepeophtheirus salmonis.

Bjorn, P. A., and Finstad, B. 2002. Salmon lice, Lepeophtheirus salmonis (Kroyer), infestation in sympatric populations of Arctic char, Salvelinus alpinus (L.), and sea trout, Salmo trutta (L.), in areas near and distant from salmon farms. – ICES Journal of Marine Science, 59: 131–139. The abundance of salmon lice was examined in two stocks of sympatric anadromous Arctic char and sea trout in sub-Arctic regions in northern Norway in June, July, and August 1992 and 1993. One stock feeds in a coastal area exposed to moderate salmon farming activity (exposed area), while the other feed in a region without salmon farms (unexposed area). The salmon lice infestation on both species differed significantly between the exposed and unexposed area as well as between years and also between weeks within the same year. We did not detect, however, any clear significant differences in salmon lice abundance between sympatric populations of Arctic char and sea trout, or between different size groups of the species. The 1992 and 1993 infestation pattern in the exposed area showed an epidemic tendency in both Arctic char and sea trout, characterised by a sudden increase in both prevalence and abundance of lice larvae in July 1992 (23.625.7 lice/fish) and August 1993 (19.920.8 lice/fish). We therefore suggest that salmon lice epidemics, previously only observed on sea trout, may also occur in populations of Arctic char, and that fish farming contributes to the elevated lice level in wild fish. The fish in the unexposed area were also infested, although at significantly lower levels than fish from the exposed area. The infestation peaked in August 1992 at 13.018.1 lice/fish and August 1993 at 3.94.5 lice/fish, suggested that lice originating on ascending wild Atlantic salmon, or lice larvae drifting from farming areas, may infest Arctic char and sea trout also in unexposed localities in Subarctic areas. 2002 International Council for the Exploration of the Sea

Salmon lice, Lepeophtheirus salmonis (Krøyer), infestation in sympatric populations of Arctic char, Salvelinus alpinus (L.), and sea trout, Salmo trutta (L.), in areas near and distant from salmon farms

In order to evaluate the performance of various sea lice management strategies, data sets from a number of salmon farm sites on the west coast of Ireland were examined. These data sets were collected as part of the ongoingNational Sea Lice Monitoring Programme in Ireland. Temporal and geographic variations in infestations of Lepeophtheirus salmonis and Caligus elongatus are described from two species of farmedsalmonids, Salmo salar L. and Oncorhynchus mykiss Walbaum. Patterns of lice transmission, both within and between fish populations, are discussed and environmental and biotic influences considered. Clear differences in infestation parameters, which persist over a number of seasons, are identified between sites. Where infestation parameters are observed to change over successive seasons, these are linked to changing husbandry practices.

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D. Hassett

Papers

Variations in parasitic caligid infestations on farmed salmonids and implications for their management