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.

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Gyrodactylus salaris Malmberg, 1957 is an important pathogen in Norwegian populations of Atlantic salmon Salmo salar. It can infect a wide range of salmonid host species, but on most the infections are probably ultimately lim~ted by a host response. Generally, on Norwegian salmon stocks, infections grow unchecked until the host dies. On a Baltic salmon stock, originally from the Neva River, a host reaction is mounted, limltlng parasite population growth on those fishes initially susceptible. Among rainbow trouts Oncorhynchus mykiss from the sam.e stock and among full sib anadromous arctic char Salvelinus alpjnus, both naturally resistant and susceptible individuals later mounting a host response can be observed. This is in contrast to an anadromous stock of brown trout Salmo trutta where only innately resistant individuals were found. A general feature of salmonid infections is the considerable variation of susceptibility between individual fish of the same stock, which appears genetic in origin. The parasite seems to be generally unable to reproduce on non-salmonids, and on cyprinids, individual behavioural mechanisms of the parasite may prevent infection. Transmission occurs directly through host contact, and by detached gyrodactylids and also from dead fishes. Relative importance of these routes and of different host species in the epidemiology of the disease is discussed with reference to laboratory experiments and existing knowledge concerning the host-parasite ecology.

The susceptibility and resistance of hatchery‐reared salmon parr, native to the rivers Neva (U.S.S.R. Baltic Sea), Alta (northern Norway) and Lone (western Norway) (both eastern Atlantic Ocean), to Gyrodactylus salaris from Norway, was examined. The level of resistance to the parasite was assessed from counts, made on anaesthetized salmon, ofthe numbers of G. salaris after an initial experimental exposure (2 weeks) to G. salaris‐infected salmon. Three experiments, all in water at c. 12° C, were carried out: (1) 50 Alta and 50 Neva salmon, initial mean parasite intensity c. 12; (2) 50 Lone and 50 Neva salmon, initial mean parasite intensity c. 60; (3) 10 Lone and 10 Neva salmon individually isolated, initial intensity one gravid G. salaris. In both the Norwegian salmon stocks, the G. salaris infrapopulations steadily increased during the experimental period of 5 weeks, in contrast to a prominent decline in the Neva salmon stock, after, respectively: (Exp. 1) week 3, average peak intensity 32.6; (Exp. 2) week 2, average peak intensity 58.7; and (Exp. 3) week 3, average peak intensity 6.3. The hatchery‐reared Baltic Neva stock demonstrated both an innate and an acquired resistance towards G. salaris, in contrast to the highly susceptible, Norwegian Alta and Lone salmon stocks.