Abstract
Gyrodactylus salaris (Monogenea, Platyhelminthes) is a notifiable freshwater pathogen responsible for causing catastrophic damage to wild Atlantic salmon stocks, most notably in Norway. In some strains of Baltic salmon (e.g., from the river Neva) however, the impact is greatly reduced due to some form of innate resistance that regulates parasite numbers, resulting in fewer host mortalities. Gyrodactylus salaris is known from 17 European states; its status in a further 35 states remains unknown; the UK, the Republic of Ireland and certain watersheds in Finland are free of the parasite. Thus, the parasite poses a serious threat if it emerges in Atlantic salmon rearing regions throughout Europe. At present, infections are generally controlled via extreme measures such as the treatment of entire river catchments with the biocide rotenone, in order to remove all hosts, before restocking with the original genetic stock. The use of rotenone in this way in EU countries is unlikely as it would be in contravention of the Water Framework Directive. Not only are such treatments economically and environmentally costly, they also eradicate the potential for any host/parasite evolutionary process to occur. Based on previous studies, UK salmon stocks have been shown to be highly susceptible to infection, analogous to Norwegian stocks. The present study investigates the impact of a G. salaris outbreak within a naïve salmon population in order to determine long-term consequences of infection and the likelihood of coexistence. Simulation of the salmon/ G. salaris system was carried out via a deterministic mathematical modelling approach to examine the dynamics of host-pathogen interactions. Results indicated that in order for highly susceptible Atlantic strains to evolve a resistance, both a moderate-strong deceleratingly costly trade-off on birth rate and a lower overall cost of the immune response are required. The present study provides insights into the potential long term impact of G. salaris if introduced into G. salaris-free territories and suggests that in the absence of external controls salmon populations are likely to recover to high densities nearing 90% of that observed pre-infection.
Highlights
Gyrodactylus salaris Malmberg, 1957 is a viviparous freshwater ecto-parasite that infects both wild and farmed populations of Atlantic salmon (Salmo salar L.), potentially resulting in juvenile host mortality. It is an Office International des Epizooties (OIE) listed pathogen that was first described from the fins and skin of a Baltic Atlantic salmon strain from a hatchery in Sweden located near the Indalsalv river [1]
Using mathematical modelling approaches based on the existing knowledge of G. salaris, the present study aims to simulate salmon/G. salaris interaction dynamics in order to investigate the potential for natural recovery of susceptible salmon populations post introduction of G. salaris infection
Using the single host model, Eq (1), two different cases were considered (Fig 1): firstly, a highly susceptible Atlantic salmon strain with no immunity, m % 0; secondly, a resistant salmon strain, m > 0 (m = 0.0175)
Summary
Gyrodactylus salaris Malmberg, 1957 is a viviparous (i.e., live-bearing) freshwater ecto-parasite that infects both wild and farmed populations of Atlantic salmon (Salmo salar L.), potentially resulting in juvenile host mortality. Within 5 years of initial introduction to a susceptible host population reductions in outbound smolts can be as high as 98% [10,12,17] This has caused severe damage to the Norwegian economy and to wild salmon fisheries. The last time loss figures were estimated annual loss of wild juvenile salmon was suggested to be in the region of 250–500 metric tonnes as a consequence of parasitic infection reducing the average density of salmon parr in infected rivers [19] Such annual loss costs the Norwegian economy over US$ 55m per annum through surveillance and eradication (circa US$ 23m per annum) along with losses incurred by fisheries, associated industries and tourism (circa US$ 34m per annum) [14]. G. salaris poses a serious threat if it establishes in territories that are currently G. salaris free [9]
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