The usage of visual indicators in regulatory monitoring at hard-bottom finfish aquaculture sites in Newfoundland (Canada)

Finfish aquaculture can be installed over hard and patchy substrates where grab sampling is challenging and use of video can be an appropriate tool to document benthic changes. Video monitoring can show visual indicators of enrichment, namely flocculent matter, Beggiatoa-like mats, and opportunistic polychaete complexes (OPC). We examined factors influencing presence of indicators using 52 video monitoring reports collected in Newfoundland, Canada. The main driving factor was distance to cage, with indicators showing a higher probability of occurrence within 10 m from cages due to low current velocities. Indicators were less prevalent on sites dominated by hard substrates while OPC in particular were restricted to depths > 35 m. Beggiatoa-like bacteria covered a larger surface than the two other indicators; however, our results suggest the necessity of amalgamating information related to all the indicators (including bare stations that could indicate anoxia) to establish a more accurate evaluation of aquaculture impact.