Abstract
The adoption of new investigative strategies based on environmental DNA (eDNA) can be used to monitor parasites, associated bacterial microbiomes, and physical-chemical parameters in fish farms. In this study, we used the economically important and globally distributed fish ciliate parasite Cryptocaryon irritans as a model to understand the parasite abundance and potential drivers of its presence in marine fish farms. Environmental (rainfall) and physical-chemical (temperature, oxygen, salinity, pH) data collected from a marine fish farm in Hong Kong were analyzed together with the eDNA approach targeting C. irritans abundance based on digital droplet PCR and 16S metagenomics to determine associations and triggers between parasites and specific bacterial groups. Rainfall and temperature demonstrated positive associations with high abundance of C. irritans (eDNA) at the studied marine fish cage farm. However, rainfall was the only parameter tested that demonstrated a significant association with parasite eDNA, indicating that the raining season is a risky period for fish farmers in Hong Kong. Coraliomargarita was the bacterial genus with the most significant relationship with low abundance of C. irritans in water. Understanding the environmental triggers of ciliate parasites propagation and associated bacterial microbiome could elucidate new insights into environmental control, microbial management, and promote the reduction of chemical use in marine fish farms.
Highlights
The aquatic environment in marine fish farms is complex and interactive
A marine fish farm located at Three Fathoms Cove, Hong Kong reporting yearly mortalities with C. irritans was adopted as the location for this study
Two main components of a principal component analysis (PCA) explained 71% of the variances observed among physical-chemical and environmental parameters monitored on farm (Figure 2)
Summary
The aquatic environment in marine fish farms is complex and interactive. These farms are exposed to multiple aquatic microbes which propagate and co-exist with fish [1,2,3].Recent studies have started to unveil close relationships between the bacterial microbiomes and the proliferation of parasitic ciliates within aquatic environments [3,4]. The aquatic environment in marine fish farms is complex and interactive. These farms are exposed to multiple aquatic microbes which propagate and co-exist with fish [1,2,3]. Limited knowledge exists on the interaction between ciliate protozoan and bacterial communities in marine fish farms . Some of these interactions can increase stress, compromise immune capacity, and reduce fish’s ability to fight infections, while the exact triggers behind such interactions are usually difficult to determine as a series of abiotic (e.g., water parameters, water current, weather) and biotic (e.g., bacteria responsible for nitrogen cycle) factors can influence the multiplication of these microorganisms [3,5]. The processes behind such interactions are largely unknown in marine environment [5]
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