Fish use of deep-sea sponge habitats revealed by long-term, high-resolution monitoring

In this study, we used long-term high-temporal resolution data to gain insights into the functional use of the Sambro Bank Conservation Sponge Grounds by fish. In particular, we aimed at capturing fish behaviour and complex benthopelagic interactions over spatial and extended temporal scales (i.e. 30-minute intervals for 2-8 months, in two separate time periods through 2021-2023). To achieve this, an integrated ecosystem-based monitoring approach was used, involving data collected on the biology (time-lapse image cameras, telemetry, Chl a, video and trawl surveys), food supply (sediment traps), and oceanography (temperature, salinity, current speed and direction). Random forest models, along with time-series analytical approaches, were used to determine what drives the observed spatial and temporal differences in fish occurrences. A total of 21 different planktivorous and benthivorous fish taxa were found utilising the seafloor. We provide the first evidence that sponge grounds are utilised as nurseries by Redfish, urophycid hakes, Silver Hake, and American Plaice. Distinct diel and seasonal patterns were found. Our results also indicated that food availability, sponge density and current speeds are associated with the presence and behaviour of some juvenile and adult fish. We demonstrated that a high temporal resolution ecosystem monitoring approach, in combination with other data types, is essential for understanding how benthic habitats and environmental drivers impact valued natural resources. Such information is crucial for developing and implementing robust, evidence-based policy and management decisions.