Abstract
The spatial distribution of a species assemblage is often determined by habitat and climate. In the marine environment, depth can become an important factor as declining light and water temperature leads to changes in the biological habitat structure. To date, much of the focus of ecological fish research has been based on reefs in less than 40 m with little research on the ecological role of mesophotic reefs. We deployed baited remote underwater stereo video systems (stereo-BRUVS) on temperate reefs in two depth categories: shallow (20–40 m) and mesophotic (80–120 m), off Port Stephens, Australia. Sites were selected using data collected by swath acoustic sounder to ensure stereo-BRUVS were deployed on reef. The sounder also provided rugosity, slope and relief data for each stereo-BRUVS deployment. Multivariate analysis indicates that there are significant differences in the fish assemblages between shallow and mesophotic reefs, primarily driven by Ophthalmolepis lineolatus and Notolabrus gymnogenis only occurring on shallow reefs and schooling species of fish that were unique to each depth category: Atypichthys strigatus on shallow reefs and Centroberyx affinis on mesophotic reefs. While shallow reefs had a greater species richness and abundance of fish when compared to mesophotic reefs, mesophotic reefs hosted the same species richness of fishery-targeted species. Chrysophrys auratus and Nemodactylus douglassii are two highly targeted species in this region. While C. auratus was numerically more abundant on shallow reefs, mesophotic reefs provide habitat for larger fish. In comparison, N. douglassii were evenly distributed across all sites sampled. Generalized linear models revealed that depth and habitat type provided the most parsimonious model for predicting the distribution of C. auratus, while habitat type alone best predicted the distribution of N. douglassii. These results demonstrate the importance of mesophotic reefs to fishery-targeted species and therefore have implications for informing the management of these fishery resources on shelf rocky reefs.
Highlights
The spatial distribution of a species assemblage is strongly determined by habitat and physical conditions [1,2], and in the marine environment depth is an important factor [3,4,5]
This study took place along a ~40 km length of coastline between Port Stephens and Seal Rocks within the waters of the Port Stephens-Great Lakes Marine Park (PSGLMP) and Hunter Marine Park (HMP) in New South Wales, Australia (-32 ̊S; Fig 1). This region is dominated by temperate species, some tropical vagrants often arrive in summer, reflecting the strong presence of the East Australian Current (EAC) that originates in the tropics [57,58,59,60]
The fish assemblages on rocky reefs at lower-mesophotic depths (80–110 m) were found to be distinct from those associated with adjacent shallow rocky reefs (20–40 m)
Summary
The spatial distribution of a species assemblage is strongly determined by habitat and physical conditions [1,2], and in the marine environment depth is an important factor [3,4,5]. On the inner continental shelf the decreased light conditions with increasing water depth results in a change from macroalgal to sessile invertebrate dominated habitat composition [6,7]. There are few standardised tools to quantitatively survey fish at greater depths This is despite the significant range of pressures on deeper rocky reefs across the continental shelf, such as commercial and recreational fishing that target reef-associated species [11,12,13]. In recent decades there has been anecdotal evidence that recreational fishers have an increased technical capacity such as side-scan or multibeam sonar and electric reels and are able to target deeper reefs These reefs may have provided refuge for older, mature individuals [14,15]. With increasing recreational fishing activity at these depths it is important that we gain a better understand on the abundance and diversity of fishes at depths >30m
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