Sea lice as a density-dependent constraint to salmonid farming

Fisheries catches worldwide have shown no increase over the last two decades, while aquaculture has been
booming. To cover the demand for fish in the growing human population, continued high growth rates in
aquaculture are needed. A potential constraint to such growth is infectious diseases, as disease transmission
rates are expected to increase with increasing densities of farmed fish. Using an extensive
dataset from all farms growing salmonids along the Norwegian coast, we document that densities of
farmed salmonids surrounding individual farms have a strong effect on farm levels of parasitic sea lice
and efforts to control sea lice infections. Furthermore, increased intervention efforts have been unsuccessful
in controlling elevated infection levels in high salmonid density areas in 2009–2010. Our
results emphasize host density effects of farmed salmonids on the population dynamics of sea lice and
suggest that parasitic sea lice represent a potent negative feedback mechanism that may limit sustainable
spatial densities of farmed salmonids.