Journal of Aquaculture Research & Development

Abstract

Effect of Different Filter Methods on Seawater Quality at a Marine Scallop Hatchery

Thorolf Magnesen, Anita Jacobsen and Malebo Hellen Moepi

The effect of using two different filtering methods in the main seawater inlet to a scallop (Pecten maximus) hatchery in Norway was tested. Seawater was filtered through active filter media (AFM) and a protein skimmer, or through a drum filter and a protein skimmer. Seawater quality was characterized and tested on algal growth rate, egg development and larval activity. Tests were performed under winter and spring conditions (March, April and May 2009). Both seawater treatments reduced the dissolved organic carbon concentrations in the inlet seawater. The total bacterial number was stable in both seawater treatments, except for an increase in the drum filter in March. The bacterial community showed seasonal development: Actinobacteria and Alphaproteobacteria dominated in March, while Gammaproteobacteria dominated in April and May. In a cluster analysis, samples from both seawater treatments showed high similarity on similar sampling dates. Vibrio spp. occurred, but was never observed in seawater coming from the skimmer after the drum filter. This sampling point was often clustered as most similar to the incoming seawater. The fraction of scallop eggs that developed into veliger larvae increased from 10% to 50% during the sampling period, and no significant differences were found between the two seawater treatments. The fraction of 8 day old active larvae was lowest in March, in experiments with both undiluted and diluted (1:10, 1:100) seawater from both treatments. No significant difference in activity was found between the treatments, except for undiluted (April) and 100-fold dilution (April and May) from the drum filter, when the larval activity was significantly higher. The effect of both seawater treatments was tested by growing the diatom Chaetoceros muelleri in small volumes for 4-5 days. Daily growth rates (μ) varied between 0.75 and 1.15, and were highest in May. No significant difference in cell concentration was found between the treatments. The results showed that the skimmer attached to the drum filter had the best performance overall in reducing dissolved organic carbon and potentially lethal bacteria. These findings have important implications for hatchery seawater management protocols.