Scott Denholm's Home Page

MATHEMATICAL MODELS FOR ESTIMATING THE LONG-TERM IMPACT OF GYRODACTYLUS SALARIS INFECTIONS IN THE UK

Scott John Denholm, Rachel Norman, Andy Hoyle, Andy Shinn, Nick Taylor

Gyrodactylus salaris Malmberg, 1957, is a viviparous freshwater ecto-parasite that infects both wild and farmed populations of Atlantic salmon (Salmo salar, L.) and can result in the death of the host. It gained notoriety when its introduction into Norway caused a catastrophic decline in salmon numbers, reducing salmon populations in some rivers by 98% over a period of five years. In order to treat infected rivers the Norwegian authorities adopted the use of the biocide rotenone which kills all life in the river, fish, parasite and plant alike. Currently the UK is recognised as being free from G. salaris and hence has G. salaris free status under EU law. However, it is believed that if the parasite found its way to the UK, the impact would be similar to that witnessed in Norway. If Gyrodactylus salaris was introduced to a UK river, it is believed that the main priority of authorities will be the prevention of further spread. The use of rotenone in the UK is highly unlikely due to environmental and legislative constraints. In order to plan for a G. salaris outbreak a greater knowledge of the possible long-term consequences is required. Mathematical models, of some form, have been used to understand disease and host-parasite interactions for many years. The aim of this project is to use mathematical modelling techniques to gain a greater understanding of the long-term impact of a G. salaris infection. This will be achieved by studying the dynamics and evolution of single and multiple strains of both parasite and host and answering questions of the form - ``under what circumstances and in what timeframe would the parasite population become extinct? the host population become extinct? Low level host/parasite coexistence occur? Host/parasite coexistence with host population recovery occur?

The final outcome will be a mathematical model that can be used to describe interactions between Atlantic salmon and Gyrodactylus salaris infections as well as evolution under a range of different scenarios. Funding for this PhD project is provided by the Department for Environment, Food and Rural affairs (Defra) via the Centre for Environment, Fisheries and Aquaculture Science (Cefas).

Impact Statement: The results gained from the different models developed throughout this PhD project will be used to predict whether or not Atlantic salmon populations would recover from Gyrodactylus salaris infections in the long term and how long such a recovery might take. The models will also illuminate what additional information is needed to improve the predictions. Our predictions will ultimately be used to improve/assist in the future contingency planning against possible Gyrodactylus salaris outbreaks in the UK. The models may also be used as a basis for future models to describe other fish/ecto-parasite systems.