SPECIAL SEMINAR
June 7, 2005
12:30 PM, B9242, Simon Fraser University, Burnaby
The population dynamics of sea lice on wild fish and sea-cage farmed fish
by Neil Frazer (speaker) and Martin Krkosek
In 1978 Anderson and May explained many features of host-parasite population dynamics with two
non-linear ordinary differential equations based on the principle that host mortality increases with
parasite burden. Here we first extend Anderson-May theory to the case where wild hosts are
regulated by resource-limitation as well as by parasites; then we extend it to the case of wild hosts and
farmed hosts where free-living parasite larvae can infect both classes of hosts. Spatial effects are
incorporated by an infection model based on the diel migration of sea lice and the distributions of
hosts and larvae. The main result is that, when wild fish and sea-cage fish are sympatric, wild fish
decline unless lice burdens on farm fish are kept at less than a few percent of pre-farm wild burdens;
this result holds even if wild fish are mainly regulated by resource limitation. Under the usual
scenario of fixed grow-out times and treatment rates, if the farm-wild and wild-farm cross-infection
rates are low compared to the wild re-infection rate, the decline of wild fish with increasing sea-cage
fish is initially small and then increasingly steep, and if the farm re-infection rate is non-zero, wild
fish can be extirpated. If grow-out times and treatment rates are adaptively adjusted to hold farm
burdens to very low thresholds, a high farm re-infection rate is desirable; above such thresholds,
increasing amounts of farm fish push wild fish to extinction.
Dr. Frazer is in the School of Ocean and Earth Sciences, University of Hawaii.
All welcome; bring your lunch