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 an d produce hybrids with little loss of viability or growth. This report desc ribes the susceptibility of pure full-sibs of S. salar and S. trutta and th eir reciprocal half-sib hybrids to their respective gyrodactylids, Gyrodact ylus salaris and Gyrodactylus derjavini. The pure-bred salmon and trout, an d half-sib hybrids, were produced using eggs and sperm from wild anadromous S. salar (River Alta stock, North Norway) and wild anadromous S. trutta (R iver Fossbekk stock, Southwest Norway). Infections were initiated by exposi ng 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 un der standardized conditions at 12 degrees C. Pure-bred S. salar were suscep tible but frequently mounted a response to G. salaris without eliminating t he infection, whereas pure-bred S. trutta mere innately resistant to this s pecies. Pure-bred S. trutta ranged from innately resistant to susceptible t o G. derjavini but later most of the susceptible trout mounted a host respo nse to G. derjavini. Purebred S. salar were also susceptible to this specie s, although parasite population growth rates were reduced and a host respon se frequently appeared eliminating G. derjavini. The abundance of both gyro dactylids was lower on the hybrids than on their respective pure-bred natur al hosts, and a parental sire- and dam-influence on the resistance of hybri ds 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. sa lar both innately resistant, intermediately susceptible and responding indi viduals were present. In the case of G. derjavini, when the sire was S. tru tta, 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 vari able 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. der javini, ranging from innate resistance through slightly susceptible to high ly susceptible but with acquired resistance controlling infection; (4) alth ough G. derjavini infections grow poorly on salmon, this host stock is susc eptible to the parasite, but can limit infection by a host reaction; (5) su sceptibility/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 sus ceptible and resistant salmonids have a pattern of susceptibility to gyroda ctylids intermediate to that of the parents; (7) resistance to gyrodactylid s may be controlled by relatively few genes in salmonids; (8) epidemiologic ally, 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.