Seasonality in live fish movements and its effects on epidemic dynamics

Live fish movements between salmon farms risk spreading pathogens, and movements between freshwater farms (FW-FW) or from freshwater to seawater (FW-SW) show clear seasonality. In this study, we quantify the effects of seasonality of live fish movements on epidemic dynamics, using a network model populated with data from live fish movements between Scottish salmon farms from 2002 to 2004. We used three types of networks: A) timing and pair-wise movements between farms were as observed; B) as network A, but with a random reordering of FW-FW and FW-SW movements; and C) number of movements were kept the same as in the data, but connection between nodes was random. We compared the time-course of simulated epidemics in a stochastic model for all three networks. We showed that seasonality had the strongest effect in networks B and C, especially when local transmission was high, and this effect was stronger in SW farms compared with FW farms.