Diel shifts and habitat associations of fish assemblages on a subsea pipeline

Nocturnal studies of fish assemblages are relatively rare, particularly at depths greater than 100 m, despite the relevance of diel shifts in habitat usage to fisheries management. This study assesses fish diversity and abundance from remotely operated vehicle (ROV) video recordings that were collected by industry during the day and at night in the course of a subsea pipeline survey, at 130 m depth. A total of 34,862 fish from 41 species and 25 families were recorded along the 23 km of pipeline. The pipeline was characterised by a high abundance of commercially important snapper (Lutjanidae) and grouper (Epinephelidae) species. The fish assemblage sampled along the pipeline during the day differed markedly to that sampled at night time. Several ubiquitous predatory species, e.g. Epinephelus areolatus, Lutjanus quinquelineatus, Lutjanus russellii, where present during the day but not at night, likely moving off the pipeline to feed in nearby habitats. Structurally complex mesophotic epibenthic habitat forming invertebrates were observed on the pipeline including; mesophotic corals, crinoids (featherstars), gorgonocephalids (basket stars), hydroids, true anemones and sponges, but elsewhere in the region, historical trawling effort is thought to have removed such organisms and extensively modified the original habitat. These complex epibenthic habitats were considered to be important to commercial target species and the modification or loss of these habitats is thought to have negatively impacted the valuable commercial fisheries in the region. This study suggests pipelines can offer a significant epibenthic habitat and refuge for fish, potentially comparable to the historical habitats lost to trawling. Fish diversity and abundance was observed to be consistently greater where a gap/crevice existed beneath the pipeline and many species were frequently observed in conjunction with the complex invertebrate matrix above the pipeline, under spanning sections beneath the pipeline and at the pipeline-sediment interface, regardless of time of day. Further dietary analysis, spatially explicit fisheries modelling and off-pipeline surveys on the natural seafloor are required to further investigate the ecological value of pipelines and its influence in fish behaviour. The study builds knowledge of mesophotic coastal fish ecology and will help to inform discussions regarding the ecological and fisheries implications of decommissioning and the removal of subsea infrastructure.