Population biology of infectious diseases shared by wild and farmed fish

Abstract

Global fisheries landings ceased increasing decades ago, causing an increasing shortfall in wild seafood supply and an expansion of aquaculture. The abundance of domesticated fishes now dwarfs related wild fishes in some coastal seas, changing the dynamics of their infectious diseases. Transport and trade of seafood, feed, eggs, and broodstock bring pathogens into new regions and into contact with naïve hosts. Density-dependent transmission creates threshold effects where disease can abruptly switch from endemic to epizootic dynamics. Hydrodynamics allow pathogens to disperse broadly, interconnecting farms into metapopulations of domesticated host fish in regions that also support related species of wild fish. Spillover and spillback dynamics of pathogen transmission between wild and farmed fish can create novel transmission pathways or bioamplify pathogen abundance, potentially depressing or endangering wild fish. Mortality from natural predator-prey interactions may be synergistic or compensatory with these increased infections. Domestic environments may favour the evolution of undesirable pathogen traits, such as virulence and drug resistance, leading to the emergence of strains that cause high mortality and (or) evade treatment. Overall, these changes to the dynamics of infectious disease in coastal seas impose new constraints on the sustainability of both wild and farmed fish. Résumé :Les débarquements des pêches à l'échelle planétaire ont cessé d'augmenter il y a des décennies, entraînant un manque à gagner croissant dans l'approvisionnement en fruits de mer sauvages, ainsi que la croissance de l'aquaculture. L'abondance des poissons domestiqués est maintenant beaucoup plus grande que celle de poissons sauvages reliés dans certaines mers littorales, ce qui modifie la dynamique de leurs maladies infectieuses. Le transport et le commerce de fruits de mer, d'aliments, d'oeufs et de géniteurs introduisent des pathogènes dans de nouvelles régions et les mettent en contact avec des hôtes naïfs. La transmission dépendante de la densité crée des effets de seuils qui font en sorte que la dynamique de la maladie peut passer soudainement d'endémique à épizootique. L'hydrodynamique permet aux pathogènes de se disperser largement, inter-reliant des fermes pour en faire de métapopulations de poissons hôtes domestiqués dans des régions qui supportent également des espèces reliées de poissons sauvages. Les dynamiques de débordement et de retour de la transmission de pathogènes entre les poissons sauvages et d'élevage peuvent créer de nouvelles voies de transmission ou bioamplifier l'abondance de pathogènes, réduisant ou menaçant potentiellement les populations de poissons sauvages. La mortalité découlant d'interactions prédateur-proie naturelles pourrait être en synergie avec ces infections accrues ou les compenser. Les milieux domestiques pourraient favoriser l'apparition de caractères non souhaitables des pathogènes, comme la virulence et la résistance aux drogues, menant à l'émergence de lignées qui causent u...

Introgressive hybridization between wild and domestic individuals and its relationship with parasitism in brook charrSalvelinus fontinalis

“…Prevalence and diversity of parasites were found to decrease with a greater proportion of domestic genes in lakes. This result was unexpected and somewhat contrasts with the idea that the introduction of farmed individuals into wild populations could increase the number and diversity of parasites (Krkošek, ; Naish et al ., ; van Oosterhout et al ., ; Wootten, , but see Kennedy et al ., ; Valtonen & Koskivaara, ) or increase prevalence through the introduction or attraction of new hosts (Dick et al . , McGuigan & Sommerville , van Oosterhout et al .…”

“…At the population‐level, stocking should also affect host–parasite relationships in different ways. Domestic individuals brought in the wild can become vectors for the introduction of new parasites (Naish et al ., ; Valtonen & Koskivaara, ; Wootten, ) and create favourable conditions for their establishment (Krkošek, ; Krkošek et al ., ). Additionally, since supplementing a lake implies increased density of fish ( i.e., potential hosts), the transmission of parasites can be facilitated and prevalence of infection ( i.e., the proportion of infected hosts in a population) could increase (van Oosterhout et al ., ).…”

Impacts of marine and freshwater aquaculture on wildlife: a global meta‐analysis

“…), indicating that farm environments may have a higher carrying capacity for wild fish than reference sites. However, any potential positive effects – such as higher reproductive potential – may be opposed by orders of magnitude higher infection loads near farms (especially sea lice on salmonids: Krkošek ) and possible impacts of a dietary shift from marine‐derived to terrestrially derived fatty acids in commercial aquaculture feed (Lavens et al . ; Mazorra et al .…”

“…, ; Costello ; Lafferty et al . ; Krkošek ). Wild fish populations may also act as reservoirs for parasites and diseases, and as they move between cages to take advantage of feeding opportunities, they act as potential transmission vectors that may increase reinfection rates for farms, driving positive feedbacks (Uglem et al .…”

Responses of migratory species and their pathogens to supplemental feeding

“…The loss of migration may place susceptible host life stages and infected individuals in closer proximity or more frequent contact. This has occurred in Pacific Northwest salmon, for which migration normally separates adult salmon from susceptible juvenile salmon and truncates the transmission potential of sea lice between these groups (migratory allopatry; [109]). The introduction of fish farms in outdoor semi-enclosed sea pens, however, has altered this relationship.…”