It has in recent years been a shift in the land-based aquaculture production from flow through systems (FTS) to recirculating aquaculture systems (RAS). With emerging use of RAS, it is necessary with knowledge regarding potential issues, whereas one potentially increasing problem is accumulation of microplastics (MP) in the RAS water. The objects of this study were to determine MPs concentration in RAS water, if the MPs originated from within the RAS, and provide a size distribution of the particles. RAS water were sampled in triplicates from three different RAS at three different locations in Norway. The water for each RAS were sampled effluent of the fish tank, effluent of the drum filter, and effluent of the biofilter as well as make-up water and sludge. Bio-media from the corresponding RAS were also sampled. Organic material in the water samples were digested prior to filtration, and filters were stained with Nile Red for quantification. There were found MPs in all the RAS water samples with mean concentration of 61 MP/L at Sunndalsøra RAS, 371 MP/L at the commercial site RAS and 41 MP/L at Kårvika RAS. There were higher concentrations in the RAS water than in the make-up water for all locations, indicating that the MPs originated from within the systems. Sludge samples were only possible to quantify from Kårvika RAS, while sampling were not performed at the commercials site and MPs concentration were too high to perform a count in Sunndalsøra RAS. The sludge samples support that the origin of MPs are within the system, as more MPs leave then enters the RAS. Nuclear magnetic resonance (NMR) spectroscopy analyses gave almost identical signals as their reference, which were the corresponding bio-media. The MPs also showed indistinguishable similarities with polyethylene (PE), which the bio-media were made off. There were found no significant trends regarding concentration at the different sampling sites. All RAS had similar size distribution of the MPs with approximately 75% of the particles being less than 100 μm.
In conclusion, there were high concentrations of MPs in RAS which likely were due to shattering of the bio-media. Future research is necessary to determine an accumulation rate, but the high abundance of MPs proves the emerging issue regarding MPs. And although RAS has the potential to be the environmental winner within aquaculture, there still are opportunities for improvement
