Cabled observatories as multidisciplinary platforms for marine fish community monitoring

An increasing number of submerged infrastructures are being established in the worlds oceans, including permanent cabled observatories equipped with biogeochemical and oceanographic sensors. Although often focusing on abiotic factors, technological advances have enabled the installation of instruments such as underwater High-Definition cameras and hydrophones capable of targeting biotic components, such as fish communities. Additionally, the recent emergence of novel indirect monitoring approaches such as environmental DNA (eDNA) has further enhanced the ability to study marine life. Thus, in situ marine observatories provide numerous advantages over ship-based marine studies, hence beholding great potential for the monitoring of marine biodiversity. The aim of this research was to carry out a multidisciplinary case study at the EMSO SmartBay cabled observatory located in the north east Atlantic (Galway Bay, Ireland), where we combined Underwater Video (UV) with environmental DNA (eDNA) metabarcoding data for monitoring of the local marine fish community. A total of 25 teleost fish could be identified to the species level, including organisms of ecologic and economic importance such as Atlantic cod, whiting, Atlantic mackerel and monkfish. Up to 20% of species were detected by both UV and eDNA, indicating that the two approaches complemented each other, but at the same time highlighting limitations of each method. The present study not only demonstrated that fixed cabled observatories such as EMSO SmartBay can provide reliable platforms for the implementation of multidisciplinary approaches, but also that such approaches provide the means to gain a more in-depth evaluation and interpretation of results from novel indirect methods such as eDNA metabarcoding, hence advancing significantly the monitoring capability of marine fish communities.