Host density thresholds and disease control for fisheries and aquaculture

Abstract

The outbreak, persistence, and eradication of infectious diseases often depend on the density of hosts. In coastal seas, many fisheries are fully or over-exploited; meanwhile, farmed popu- lations are increasing rapidly with aquaculture growth. Marine aquaculture facilities are typically open to the surrounding ecosystem and, therefore, wild and farmed populations are connected by their shared parasites. At the core of epidemiological theory are host density thresholds, above which diseases can persist or invade and below which diseases can be eradicated. Host density thresholds in aquaculture-fishery interactions likely function at regional scales that encompass multiple farms, which are connected by pathogen dispersal and the movement of wild hosts. Sudden outbreaks of parasitic copepods in wild-farmed salmon systems may be linked to aquaculture growth exceeding host density thresholds. Abiotic (e.g. temperature and salinity), management (e.g. husbandry and farm siting), and biotic factors (e.g. migrations of wild hosts) likely affect threshold values. A con- nected wild-farmed host population can exceed a host density threshold due to an influx of wild hosts via migration, increases in aquaculture production, or environmental change such as climate warm- ing. Coastal management and policy should heed the disease implications of climate warming, aqua- culture growth, and fisheries restoration that suggest increasing host densities and decreasing threshold values.