The susceptibility of Atlantic salmon (Salmo salar L.) x brown trout (Salmo trutta L.) hybrids to Gyrodactylus salaris Malmberg and Gyrodactylus derjavini Mikailov.

Salmo salar and Salmo trutta co-exist in coastal river systems in Europe and produce hybrids with little loss of viability or growth. This report describes the susceptibility of pure full-sibs of S. salar and S. trutta and their reciprocal half-sib hybrids to their respective gyrodactylids, Gyrodactylus salaris and Gyrodactylus derjavini. The pure-bred salmon and trout, and half-sib hybrids, were produced using eggs and sperm from wild anadromous S. salar (River Alta stock, North Norway) and wild anadromous S. trutta (River Fossbekk stock, Southwest Norway). Infections were initiated by exposing experimental fishes (0+) to S. salar naturally infected with G. salaris (River Lierelva strain) or S. trutta naturally infected with G. derjavini (River Sandvikselva strain). Fishes were then kept individually isolated under standardized conditions at 12 degrees C. Pure-bred S. salar were susceptible but frequently mounted a response to G. salaris without eliminating the infection, whereas pure-bred S. trutta were innately resistant to this species. Pure-bred S. trutta ranged from innately resistant to susceptible to G. derjavini but later most of the susceptible trout mounted a host response to G. derjavini. Pure-bred S. salar were also susceptible to this species, although parasite population growth rates were reduced and a host response frequently appeared eliminating G. derjavini. The abundance of both gyrodactylids was lower on the hybrids than on their respective pure-bred natural hosts, and a parental sire- and dam-influence on the resistance of hybrids was observed. When the sire was S. salar, the susceptibility of hybrids to G. salaris was similar to that of pure S. trutta; when the dam was S. salar both innately resistant, intermediately susceptible and responding individuals were present. In the case of G. derjavini, when the sire was S. trutta, infections on hybrids were similar to those on pure S. salar; when the dam was S. trutta, an increased level of susceptibility was observed. The present results provide evidence that: (1) Norwegian salmon stocks are variable in their susceptibility/resistance, with some fish able to control S. salaris infections; (2) trout stocks are innately resistant to G. salaris; (3) individual trout show a spectrum in susceptibility/resistance to G. derjavini, ranging from innate resistance through slightly susceptible to highly susceptible but with acquired resistance controlling infection; (4) although G. derjavini infections grow poorly on salmon, this host stock is susceptible to the parasite, but can limit infection by a host reaction; (5) susceptibility/resistance traits to gyrodactylids are genetically controlled and resistance can be transferred as a dominant trait through interspecific crosses between different salmonids; (6) interspecific hybrids between susceptible and resistant salmonids have a pattern of susceptibility to gyrodactylids intermediate to that of the parents; (7) resistance to gyrodactylids may be controlled by relatively few genes in salmonids; (8) epidemiologically, hybrids may act as a reservoir for gyrodactylids, may support a wider diversity of species than either parent and may disseminate gyrodactylids of both host species.