Micro-particles in recirculating aquaculture systems: microscopic examination of particles

In order to determine the composition, shape and structure of micro-particles in a cold water recirculating plant raising juvenile Atlantic salmon to the smolt stage, samples of suspended solids, collected on classification meshes, and samples of particles in the culture water were examined microscopically. Concurrent gravimetric tests were attempted which method proved unusable due to clogging of the meshes by a viscid material, especially for particles under 40 μm. It was determined that this viscid material, also evident on the classification screens, constituted a major portion of the less dense particles (1050 kg/m³) by floccing fine particles. Staining indicated that the viscid material was primarily mucus with some gelatinized starches from the feed. Undigested feed stirred in distilled, filtered water were also examined microscopically. Again a viscid coating was found on the filter media, but in this case the primary source appeared to be gelatinized starches. The lighter fraction was essentially absent. In both the culture water and the feed samples, the heavier fraction, 1150 kg/m³, appears to originate primarily from the heavy, brown, indigestible cellulose fraction of the ground wheat in the feed.