The impact of variable hatching rates on parasite control: a model of an aquatic ectoparasite in a Finnish fish farm.
Many parasites of medical and veterinary importance infect via free-living stages that undergo a period of development in the environment before seeking a host. Frequently, considerable variation is observed in the development rates of these stages, such that some individuals may be considered to occupy distinct subpopulations, similar to a plant's seed bank. Under certain conditions, these subpopulations may act as a reservoir for the parasite, buffering it from the impact of management strategies and reducing control success. We assessed the impact of intraspecific variation in development rates on parasite control by developing a population dynamic model of the ectoparasite Argulus coregoni, a major parasite of Finnish fish farms. We divided argulid eggs into two subpopulations: an exposed class with fast hatching rates and an 'egg bank' with slower hatching rates. We then assessed the success of a number of potential control strategies under a range of egg bank hatching regimes. The predicted seasonal dynamics of the various life-cycle stages matched well those observed in Finnish fish farms. Simulations showed that the long-term growth rate of the argulid population tends to be reduced by the presence of a slow-hatching egg bank, unless the egg bank affords some protection in the form of reduced egg mortality. Sensitivity analysis showed that the key stages of the parasite's life cycle are those that relate to the on-fish juvenile mortality and development rates. The next most sensitive parameters relate to the number of viable eggs at the start of the year, such as female fecundity and overwinter egg survival. We showed that current control approaches that target these stages could prove highly effective at suppressing the argulid population. However, long-term control was greatly reduced in both cases as the proportion of eggs entering the egg bank increased. This was especially true if mortality of the egg bank was lower than that of the exposed eggs. Synthesis and applications. Understanding the factors affecting parasite persistence is vital for the development of effective long-term control. We have shown that inherent heterogeneity within a parasite population may have a substantial impact on its long-term growth rate, and can be an important mechanism reducing the effectiveness of targeted control strategies in the field.