Production of lumpfish (Cyclopterus lumpus L.) in RAS with distinct water treatments: Effects on fish survival, growth, gill health and microbial communities in rearing water and biofilm

Lumpfish (Cyclopterus lumpus L.) in Norway is currently produced in traditional flow-through systems (FTS). Hatcheries frequently show signs of bacterial infections, unstable microbial communities in the rearing water and varying mortality. Recirculating aquaculture systems (RAS) is proposed to create stable and healthy microbial environments, with less probabilities for blooming of opportunistic microbes. Studies have also shown that RAS increases the survival of marine fish. The aim with this study was to investigate the effect of various RAS water treatment designs on water and biofilm microbiota, survival, growth and gill health of lumpfish. An experiment with lumpfish was conducted, from 2 months post hatch to the transfer into sea cages. Five different water treatment regimens were compared: 1. RAS with no additional water treatment, 2. RAS with a filtration unit for removal of small particles, 3. RAS with filtration and disinfection with UV-irradiation, 4. RAS with filtration and disinfection with UV-irradiation and ozone and 5. FTS as a reference. The microbiota of the rearing water and tank wall biofilm were sampled and characterized by Illumina sequencing of 16S rDNA amplicons. Lumpfish juveniles reared in the RAS treatments were exposed to a more stable and diverse rearing water microbiota, with a lower share of opportunistic bacteria, a probable reason for the higher survival and better gill health of the fish compared to siblings reared in the FTS. Lumpfish reared in RAS without disinfection were exposed to a more diverse and stable water microbiota, with a lower share of opportunistic and potential harmful bacteria, compared to the lumpfish reared in RAS with disinfection and FTS. This resulted in better gill health. Fish in RAS with filtration, but no disinfection, had a better gill health than the fish in the RAS without filtration, possibly due to the reduction of small particles. The lumpfish were exposed to different microbial communities of both water and biofilm, due to the different treatments of the incoming tank water. In conclusion, our results indicate that implementation of RAS in the production of lumpfish has a potential to increase both survival, growth and gill health of the fish and that RAS with filtration of small particles, but without disinfection, result in the best fish health and performance among the investigated treatments.