Publication Abstract

The Influence of Myxobolus pseudodispar (Myxozoa) on the Dynamics of Roach (Rutilus Rutilus) Populations

N.G.H. Taylor*, S.W. Feist*, and M. Longshaw*

Myxobolus pseudodispar infections are common in UK roach (Rutilus rutilus) populations. Juvenile fish are usually infected in the first few months of life via a triactinomyxon actinospore released from tubficid worms. Following successful infection, parasite cysts containing myxospores develop in individual muscle fibres. The parasite plasmodia increase in size and can cause pressure on neighbouring muscle fibres. Destruction of individual muscle fibres as a result of parasite growth and subsequent host responses can lead to reductions in muscle function. As a consequence, it has been hypothesised that infected fish are more prone to death or more subtle sublethal effects that may ultimately lead to death. In order to establish the significance of such infections at the host population level, a two-pronged approach was taken. Initially a 13-year longitudinal dataset was mined to establish trends and correlations between host-parasite dynamics in five English river sites. Subsequently a simple mechanistic model was developed in order to determine impact at the host population level. 

The primary model aimed to describe the population dynamics of roach by using estimates of recruitment and mortality obtained through a review of available literature. The resulting model was deterministic, age-structured and incorporated a density-dependent effect. By using parameter estimates gained independently from those obtained through statistical analysis of the long-term datasets, the theoretical dynamics could be compared to the dynamics observed in the field, thus providing a method of model validation.

Preliminary results from the model predict a chaotic dynamic if it is assumed that the parasite is absent from the system. By introducing varying levels of parasite induced mortality into the model, the system appears to stabilise, becoming more predictable and leading to periodic cycles. The level of parasite-induced mortality required to induce this stabilizing effect is currently being compared to estimates gained through the statistical analysis of the long-term datasets, in order to determine whether this is biologically plausible. If confirmed by further study, there maybe important implications for fisheries management, as the findings suggest that parasites play an important role in determining the persistence of wild fish populations.