Fungal infections primarily caused by water mold (Saprolegnia sp.) and bacterial infections caused by common aquatic pathogens result in annual losses in the hatchery production of rainbow trout and other freshwater fishes. The management of fungal and bacterial infections has historically relied on the use of chemical products which may have negative effects on human and environmental health. This study aimed to test the effect of AquaVate+ (MTS Environmental, Exeter, Ontario) which is derived from oxidized lignite coal, as an alternative treatment to reduce fungal and bacterial infections during the incubation of rainbow trout eggs.
Fertilized eggs were incubated in stacked incubator trays at 9oC for 42 days and AquaVate+ was added to the influent water in the treatment incubator at a rate of 0.005 mg/L. Every 10 days from fertilization to hatch, 1L water samples were collected from the trays for a total of 5 time points (0, 10, 20, 30 and 40 days post fertilization) for extraction, amplification and next generation sequencing of 16S rDNA. Fertilized eggs were enumerated at the eyed stage and at hatch to assess survival to the eyed egg stage and survival to hatch.
At 24 days post fertilization (dpf) (eye-up) survival among treated eggs was significantly higher than the control group (P = < 0.001). At 35 dpf, the control incubator showed evidence of fungus growth whereas no signs of fungus were observed in the treatment incubator. At 42 dpf (hatch), survival among eggs treated with AquaVate+ was significantly improved compared to the control incubators (P = 0.001).
The diversity of bacteria was significantly different between treatment groups (P = 0.011) and over time (P < 0.001). Specifically, alpha diversity decreased over time between samples collected on day 0 compared to days 20, 30 and 40 (P < 0.01). The composition of bacteria (beta diversity) was different between treatments (P= <0.001), specifically the abundance of Flavobacterium and Aeromonas species, which are common bacterial pathogens.
The results of the current study have identified major findings that are relevant to the incubation of rainbow trout eggs in a hatchery environment. Specifically, the application of AquaVate+ at a rate of 0.005 mg/L improved survival, prevented the growth of fungus, reduced the abundance of pathogenic bacteria and significantly altered the microbial community of the water in which the eggs were incubated.