Macro-parasites pose a major threat to fish populations, especially those that have had little contact with the parasites previously. A prime example of this is the situation in Norway, where outbreaks of Gyrodactylus salaris can quickly devastate populations of Atlantic Salmon by up to 98%. However in other Baltic countries, where the parasites have been present for much longer, it appears Salmon have evolved resistance mechanisms to the parasite to a point where they stably co-exist. This talk will introduce the models we used to study this host-parasite interaction and subsequently extend them to carry out evolutionary analysis of how resistance may evolve. We aim to predict the amount of time between the introduction of the parasite to a fully susceptible strain of Atlantic salmon, like that in the UK, and the time when resistance is sufficient such that fish stocks may naturally recover to a reasonable level.