Linking the presence of visual indicators of aquaculture deposition to changes in epibenthic richness at finfish sites installed over hard bottom substrates

Effects of finfish aquaculture on benthic communities at hard bottom sites can be assessed using visual indicators of organic enrichment, namely bacterial mats, opportunistic polychaetes, and/or barren substrates (i.e., with no visible epifauna). Under some regulatory frameworks, the presence of visual indicators beyond a certain threshold proportion (e.g., >70%) of mandatory sampling stations at a site signals an unacceptable degree of benthic organic loading. However, relationships between visual indicator presence and other biological characters such as epibenthic taxon richness are unclear, and should be examined to advise on the validity of existing legislative frameworks. Here, we used video data collected for regulatory purposes before and after aquaculture production to document (1) change in epibenthic taxon richness (TR) and its discriminatory power in determining aquaculture impact and (2) the association between TR change and the presence of visual indicators. Despite low richness values overall, our results show a significant post-production decrease in TR in the near-cage area, which was predicted to be affected by aquaculture. Decreases in TR were associated with visual indicator presence, validating the use of a suite of visual indicators to detect organic deposition. Importantly, visual indicators should be considered together in the context of regulation, given that relationships between TR and indicators were not linear when the latter are considered individually.