Modelling the impact of disease-induced mortality on the population size of wild salm
By Sophie des Clers
Fisheries Research, Volume 17, Issues 1-2, Pathological Conditions of Wild Salmonids, June 1993, Pages 237-248, ISSN 0165-7836, DOI: 10.1016/0165-7836(93)90023-Z.
A simple age-structured population model is used to discuss the impact of disease-induced mortality at different stages of the life-cycle of salmonids. A population of wild Atlantic salmon (Salmo salar) in Scotland is taken as an example. In a situation of demographic equilibrium, the numbers of fish are estimated from natural and fishing mortalities and stage durations reported in the literature. The population is assumed to be controlled through a regulation of the density of fry-0+ parr on the nursery grounds. It appears that diseases which kill the foundation layers of the population, fry-0+ parr, parr and post-smolt, reduce numbers to a greater extent than diseases killing older fish at sea or in the river. The parr stage which incurs high annual natural mortality over several years, proves to be the most vulnerable of all life-history stages in the life-cycle. Parasites which attack fish at sea (e.g. sea lice) have a smaller impact on population numbers than parasites such as, Gyrodactylus salaris which kill parr in the river.