Reproductive Trade-Offs May Moderate the Impact of Gyrodactylus salaris in Warmer Climates

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.

Transmission dynamics of the monogeneanGyrodactylus salarisunder seminatural conditions

“…Cable, Harris & Bakke (2000) suggested that the timing of the first birth, in particular, was critical for the population dynamics. In accordance with this, Denholm et al (2013) found that delaying the first birth by 24 hours subsequently caused a major drop in the number of parasites, and an increased risk of population extinction of G.…”

“…According to the same schedule, we find that the intrinsic rate of increase in the individual infra-populations of the recipient fish was, not surprisingly, highly variable. Small populations are highly sensitive to demographic stochasticity (Lande 1993), and in G. salaris where each individual give birth to a single daughter, even small variations in individual timing can give considerable differences in life-history trajectories and fitness, as described above (Cable et al 2000, Ramirez, Harris & Bakke 2012, Denholm et al 2013.…”

“…Water temperature is known to play a pivotal role in the population dynamics and life history of gyrodactylids (Jansen & Bakke 1993a,b, Andersen & Buchmann 1998) through a strong impact on reproduction and survival (Scott & Nokes 1984, Olstad et al 2006. As a consequence of this, temperature influences the potential for spread 13 to previously unaffected populations (Denholm et al 2013). Transmission rate of G. salaris increases with increasing water temperature (Bakke, Jansen & Hansen 1991, Soleng et al 1999, and studies of free-living populations of Atlantic salmon have shown that the observed seasonality in parasite abundance is related to water temperature (Jansen & Bakke 1993a,b, Appleby & Mo 1997.…”

Predicting the Potential for Natural Recovery of Atlantic Salmon (Salmo salar L.) Populations following the Introduction of Gyrodactylus salaris Malmberg, 1957 (Monogenea)

“…More recently stochastic models have become popular in studying G . salaris infections in salmon and modelling techniques such as Leslie matrix population models and individual based models have also been employed [47–49]. Though a great deal of effort has been placed on understanding the risks and routes by which the parasite may be introduced, little has been done to predict its long-term impact.…”

Mathematical Modelling of Parasite Dynamics: A Stochastic Simulation-Based Approach and Parameter Estimation via Modified Sequential-Type Approximate Bayesian Computation

“…However, given that the initial birth occurs before the parasite reaches two days of age, the population can persist with just two births (Denholm et al. 2013 ). Additionally, Denholm et al.…”