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Impact of fishery harvesting regimes on the evolution of parasitic virulence

Final Activity Report Summary - VIRUFISH (Impact of fishery harvesting regimes on the evolution of parasitic virulence)

Important not only for sustainable resource management, but also for disease management in general, is the understanding of the forces that drive pathogens towards higher virulence. Today, fishing is the dominant source of mortality in most commercially exploited fish stocks, leading to changes in the life-history patterns of fish and their parasites. Fishery thus acts as an external constraint influencing the evolution of virulence, i.e. the ability of a pathogen to kill its host. This may pose serious threats to exploited stocks and to their value as resources for humankind. So far, however, there has been no systematic attempt to investigate the effect of fisheries on parasite virulence evolution.

It is very difficult to ad hoc assess the implications that fishing activity may have on the evolution of virulence. Thus, modelling presents the adequate means of exploring this complex problem. In general, models of virulence evolution focus on the co-evolution of host and parasitoid under non-changing environment, using mathematical equations or standard epidemiological models. But dealing with fishery-induced evolutionary change requires a new type of models be developed. These models have to do justice both to the ecological and the genetic intricacies involved in the dynamics of a particular stock with its pathogens and predators. Thus, an individual-based host and pathogen model coupled with genetic algorithms is used for generating and testing hypotheses of parasitic virulence and its consequences under selective harvesting regimes.

We focussed on herring as the host and ichthyophonosis as the 'reference disease'. The pathogen Ichthyophonus hoferi in herring Clupea harengus L. is an example of a disease in an economically important, migrating fish stock that has been known since 1893 to cause recurrent epizootics with significant losses. It is contracted via the digestive tract by oral ingestion of spores from the water, by feeding on infected tissue or due to direct exposure to spores and causes poor motor coordination in swimming until starvation results.

The host model considers recruitment, growth, migration as well as length and age-dependent mortality, and the pathogen model is based on interventions in the mortality process. Pathogen traits are subject to adaptive changes and inherited from the pathogens that managed to infect susceptible hosts.

We analysed a unique long-term dataset to gain insight into the factors and processes related to the disease to be included as ecological and behavioural realism in the simulation model. We conclude that a variety of environmental and host-pathogen ecological mechanisms were involved in the perpetuation of the disease. We used these data to calibrate the disease part of the model as well as the relevant processes. To this end, we impose a 'virtual fishery' that samples according to the real-world fishery data. Then, these virtual fishery data were analysed in the same way as the field data and act as 'filters' to select plausible processes and parameters. The calibrated model was used to test how ecological and behavioural aspects of the herring as well as harvesting regimes affect the disease evolution, because harvesting regimes are altering the internal structure of the host population.

We conclude from our simulation experiments that ecological processes, like seasonality in migration, significantly alter the direction and strength of evolution, which makes general statements about harvesting and virulence evolution strongly dependent on the specific host-pathogen system. Still, the drivers of marine epizootics, i.e. disease outbreaks in animal populations, are poorly known. Merging epidemiology with evolutionary ecology has widespread potential to help us to further marine disease ecology.