Abstract
Gyrodactylus salaris is a notifiable freshwater ectoparasite of salmonids. Its primary host is Atlantic salmon (Salmo salar), upon which infections can cause death, and have led to massive declines in salmon numbers in Norway, where the parasite is widespread. Different strains of S. salar vary in their susceptibility, with Atlantic strains (such as those found in Norway) exhibiting no resistance to the parasite, and Baltic strains demonstrating an innate resistance sufficient to regulate parasite numbers on the host causing it to either die out or persist at a low level. In this study, Leslie matrix and compartmental models were used to generate data that demonstrated the population growth of G. salaris on an individual host is dependent on the total number of offspring per parasite, its longevity and the timing of its births. The data demonstrated that the key factor determining the rate of G. salaris population growth is the time at which the parasite first gives birth, with rapid birth rate giving rise to large population size. Furthermore, it was shown that though the parasite can give birth up to four times, only two births are required for the population to persist as long as the first birth occurs before a parasite is three days old. As temperature is known to influence the timing of the parasite's first birth, greater impact may be predicted if introduced to countries with warmer climates than Norway, such as the UK and Ireland which are currently recognised to be free of G. salaris. However, the outputs from the models developed in this study suggest that temperature induced trade-offs between the total number of offspring the parasite gives birth to and the first birth timing may prevent increased population growth rates over those observed in Norway.
Highlights
Wild Atlantic salmon, Salmo salar L., are in decline and their conservation is of great concern [1]
As a consequence of its impact, countries recognised by the European Commission (EC) as free of G. salaris, such as the United Kingdom (UK) and Ireland (Commission decision 2004/453/ EEC), are keen to determine whether the parasite could establish under their environmental conditions if introduced, and if so, the impact it may have
Published data [26] pertaining to the population dynamics of G. salaris on resistant Baltic strains of Atlantic salmon when infected with one parasite show an initial increase in numbers before a decline after day 15 post-infection (Figure 1a, 1b)
Summary
Wild Atlantic salmon, Salmo salar L., are in decline and their conservation is of great concern [1]. Norway has experienced massive declines in its Atlantic salmon populations, which have largely been attributed to gyrodactylosis. This disease, which predominantly affects juvenile stages of Atlantic salmon in freshwater, is caused by the viviparous ectoparasite Gyrodactylus salaris Malmberg, 1957 [2,3,4,5]. Since its first report in Norway in 1975, G. salaris has since been reported from 46 rivers [6]. It is estimated that G. salaris has reduced the average density of salmon parr in infected rivers by up to 86% [7], and costs the Norwegian economy over US $50 M p.a. through the costs of surveillance and eradication (circa US $23 M p.a.), and losses to fisheries associated industries and tourism (circa US $34 M p.a.) [6]. As a consequence of its impact, countries recognised by the European Commission (EC) as free of G. salaris, such as the United Kingdom (UK) and Ireland (Commission decision 2004/453/ EEC), are keen to determine whether the parasite could establish under their environmental conditions if introduced, and if so, the impact it may have
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