Ecological changes affect pathogen epidemiology and evolution and may trigger the emergence of novel diseases. Aquaculture radically alters the ecology of fish and their pathogens. Here we show an increase in the occurrence of the bacterial fish disease Flavobacterium columnare in salmon fingerlings at a fish farm in northern Finland over 23 years. We hypothesize that this emergence was owing to evolutionary changes in bacterial virulence. We base this argument on several observations. First, the emergence was associated with increased severity of symptoms. Second, F. columnare strains vary in virulence, with more lethal strains inducing more severe symptoms prior to death. Third, more virulent strains have greater infectivity, higher tissue-degrading capacity and higher growth rates. Fourth, pathogen strains co-occur, so that strains compete. Fifth, F. columnare can transmit efficiently from dead fish, and maintain infectivity in sterilized water for months, strongly reducing the fitness cost of host death likely experienced by the pathogen in nature. Moreover, this saprophytic infectiousness means that chemotherapy strongly select for strains that rapidly kill their hosts: dead fish remain infectious; treated fish do not. Finally, high stocking densities of homogeneous subsets of fish greatly enhance transmission opportunities. We suggest that fish farms provide an environment that promotes the circulation of more virulent strains of F. columnare. This effect is intensified by the recent increases in summer water temperature. More generally, we predict that intensive fish farming will lead to the evolution of more virulent pathogens.

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We describe an unusually high infection rate of Gyrodactylus salaris Malmberg in juvenile Atlantic salmon Salmo salar L. of Baltic Sea origin, which are generally believed to be more resistant to G. salaris than East Atlantic salmon populations. Based on analyses of mitochondrial (complete cytochrome oxidase 1 [CO1] gene, 1548 bp) and nuclear (ADNAM1, 435 bp; internal transcribed spacer [ITS] rDNA region, 1232 bp) DNA fragments, the closest relatives of the characterized Estonian G. salaris strain were parasites found off the Swedish west coast and in Raasakka hatchery, Iijoki (Baltic Sea, Finland). Analyses of 14 microsatellite loci of the host S. salar revealed that approximately 40% of studied fish were triploids. We subsequently identified triploid Atlantic salmon of Baltic origin as more susceptible to G. salaris infection than their diploid counterparts, possibly due to compromised complement-dependent immune pathways in triploid salmon. This is in accordance with earlier studies that have shown elevated susceptibility of triploids to various viral or bacterial pathogens, and represents one of the first reports of increased susceptibility of triploid salmonid fish to an ectoparasite. However, further experimental work is needed to determine whether triploid Atlantic salmon is generally more susceptible to G. salaris compared to their diploid counterparts, irrespective of the particular triploidization method and population of origin. KEY WORDS: Atlantic salmon · Gyrodactylus salaris · Pathogen susceptibility · Triploid · Microsatellites · Baltic Sea Resale or republication not permitted without written consent of the publisherDis Aquat Org 91: [129][130][131][132][133][134][135][136] 2010 tect significant differences in susceptibility between diploid and triploid fish. For example, both diploid and triploid rainbow trout exposed to Vibrio ordalii, Aeromonas salmonicida, or infectious hematopoietic necrosis virus demonstrated similar mortality rates (Yamamoto & Iida 1995). Similar mortality rates between diploid and triploid Atlantic salmon were also observed after experimental infection with Renibacterium salmoninarum (Bruno & Johnstone 1990).Gyrodactylosis is a parasitic disease of salmonid fishes caused by the viviparous ectoparasite monogenean flatworm Gyrodactylus salaris, which belongs to the G. wageneri species-group, of the subgenus Limnonephrotus (family Gyrodactylidae, Malmberg, 1957). Due to 'hyperviviparity' (also known as a 'Russian doll' style of reproduction), combined with a rapid generation time, it reproduces fast, and in a matter of weeks a single worm can produce thousands of progeny , Buchmann 2008. G. salaris damages fish not only by consuming mucus and epithelial cells of the host but also by piercing the fish epithelium with its hooklets, compromising its osmoregulatory function, and leaving it vulnerable to fungal and bacterial infections. Importantly, Atlantic salmon populations exhibit marked differences in susceptibility to G. salaris infection, with populations fr...

Aeromonas salmonicida was studied at fish farms producing salmonid smolts in northern and central Finland from 1982 onwards. Aeromonas salmonicida subsp. salmonicida has been causing epizootics in salmon, Salmo salar L., and trout, S. trutta L., at two coastal farms in northern Finland sinee 1986, involving 1-29% mortality in the fish-rearing units affceted. The disease causes more serious losses of sea trout yearlings and brood fish than of salmon. The achromogenic atypical A. salmonicida proved to be the most common bacterial disease in brown and sea trout at one farm in northern Finland throughout the period, causing constant heavy losses, mainly of fingerlings, especially in 1982 -1986. It was found only occasionally in central Finland. Pigment-producing atypical A-salmonicida caused mortality among brown trout at one farm in northern Finland and one in centra! Finland.