Standard bivalve hatchery practices rely on good ambient water quality for reliable production, however ambient water quality can change and have caused issues for hatcheries in years past. A collaborative effort is currently underway between Virginia Institute of Marine Science- Eastern Shore Lab (VIMS-ESL) and Virginia Tech Seafood Agriculture Research and Extension Center (VSAREC) to develop a recirculating aquaculture system (RAS) to allow bivalve hatcheries to reuse production water and minimize impact from fluctuating ambient water quality. Two pilot scale systems were developed for testing. The system design consisted of a 600 L traditional moving bed bioreactor with K1 Kaldnes carrier elements (AnoxKaldnes, Lund, Sweden) for ammonia reduction with the addition of a cultured heterotrophic microbial community to reduce organic waste and remove bacteria from larval production waters. A conventional foam fractionator was also used to assist with removal of dissolved organics. Initial efforts have developed and standardized a heterotrophic conditioning protocol.
Three trials were conducted in 600 L bioreactors to test the preliminary biofilm development procedure and survival of bivalve larvae in system water. In trial 1, multiple 2-day cycle spikes of organics, at a level expected as worst-case scenario for larval culture failure, were absorbed and treated by the system. Very little perturbation of the biomass in the filter was noted. Plate counts of heterotrophic bacteria showed slow growth of colonies indicating planktonic phase bacteria were semi-dormant. Trial was conducted at both VIMS-ESL and VSAREC sites.
A replicated experiment was conducted for trial 2 at VIMS-ESL with 60 L batch cultures of C. virginica larvae (n=4). Larvae were reared in either water treated by the experimental 600 L RAS (treatment) or standard hatchery water (control). Larval survival was slightly lower (d2-5 post hatch =35.5%, ± SD 3.9%; d5-7 post hatch = 49.3%, ± SD 6.5%) in the RAS treatment when compared to the standard treatments (d2-5 post hatch = 48.3%, ± SD 1.0%; d5-7 post hatch = 57.0%, ± SD 5.5%). Initial qualitative assessment of larval growth was promising, but indicated a potential negative impact from the RAS rearing water that needs to be addressed.