Quantity and Quality of Aquaculture Enrichments Influence Disease Epidemics and Provide Ecological Alternatives to Antibiotics

Abstract

Environmental heterogeneity is a central component influencing the virulence and epidemiology of infectious diseases. The number and distribution of susceptible hosts determines disease transmission opportunities, shifting the epidemiological threshold between the spread and fadeout of a disease. Similarly, the presence and diversity of other hosts, pathogens and environmental microbes, may inhibit or accelerate an epidemic. This has important applied implications in farming environments, where high numbers of susceptible hosts are maintained in conditions of minimal environmental heterogeneity. We investigated how the quantity and quality of aquaculture enrichments (few vs. many stones; clean stones vs. stones conditioned in lake water) influenced the severity of infection of a pathogenic bacterium, Flavobacterium columnare, in salmonid fishes. We found that the conditioning of the stones significantly increased host survival in rearing tanks with few stones. A similar effect of increased host survival was also observed with a higher number of unconditioned stones. These results suggest that a simple increase in the heterogeneity of aquaculture environment can significantly reduce the impact of diseases, most likely operating through a reduction in pathogen transmission (stone quantity) and the formation of beneficial microbial communities (stone quality). This supports enriched rearing as an ecological and economic way to prevent bacterial infections with the minimal use of antimicrobials.

Keywords: Salmo salar; Salmo trutta; aquaculture; biofilm; disease epidemiology; enriched rearing; environmental microbes; microbial community.

Figure 1

Survival of trout and salmon in experimental bacterial exposure. Cumulative survival curves of a) sea-migrating brown trout (Salmo trutta) and b) landlocked Atlantic salmon (Salmo salar m. sebago) exposed to the pathogenic bacterium Flavobacterium columnare. Exposures of groups of 100 fish took place in 50 replicated tanks (0.4 m²) with different combinations of quantity (no/few (6 stones)/many (60 stones)) and quality (unconditioned/conditioned) of natural stones. Conditioning was done by keeping clean stones in the lake water before the experiment. Survival was followed every 4–6 h until 112 h post-exposure. In trout, survival was lowest in tanks without stones, but increased in the presence of conditioned stones as well as with an increasing number of unconditioned stones (a). Similar trends of lowest survival in tanks without stones and with few unconditioned stones were evident also in salmon at the end of the experiment (b). Note differences in Y axis scales.

Figure 2

Standard and enriched tanks used in the experimental bacterial exposures. Replicated tanks with no stones (left panel), few stones (6 stones) or many stones (60 stones, right panel) were used. In addition, stones were either clean, unconditioned or conditioned in lake water before the experiment. Each tank (0.4 m²) had 100 fish (either trout or salmon) and 60 l of water (16.5–18.9 °C).