AEI prepress abstract   -  DOI: https://doi.org/10.3354/aei00487

ABSTRACT: Norway employs an environmental management system to assess benthic organic loading from aquaculture activities. Monitoring surveys, mostly performed through grab sampling, scrutinise responses to enrichment within soft-sediment macrofaunal communities. However, new methods and indicators of organic enrichment (IOE) need to be identified for mixed- and hard-bottom substrates. In this study, we used image characterisation to examine temporal changes in the abundance and coverage of benthic IOE on mixed-bottom substrates (organic pellets, sulphur-oxidising bacterial-mats, opportunistic polychaete complexes [OPC], polychaete tube aggregations [PTA]) in relation to changes in organic deposition pressure through 1 yr of production at a rainbow trout farm on the western coast of Norway. Rates of organic deposition on the seafloor around the farm showed an increase towards the end of the survey period as fish biomass increased. PTA were significantly associated with low levels of organic deposition and their abundance declined in 2 of the 3 study cages as the production cycle progressed. The coverage of OPC significantly increased with organic deposition on the seafloor and showed the greatest coverage in June 2021, 4 mo before the end of the production cycle. The coverage of organic pellets closely followed the patterns of organic deposition on the seafloor, whereas coverage of bacterial mats showed no relationship with deposition. Our findings provide new knowledge on the annual impact of organic enrichment on IOE beneath fish farms over mixed-bottom substrates. This study highlights the application of image characterisation methods to improve benthic monitoring on mixed-bottom substrates. The outcome of the study can contribute to the development of an environmental proxy to assess the enrichment stage around aquaculture farms placed over mixed- and hard-bottom areas.