Mesophotic fish communities of the ancient coastline in Western Australia

Abstract

Marine diversity across the Australian continental shelf is shaped by characteristic benthic habitats which are determined by geomorphic features such as paleoshorelines. In north-western Australia there has been little attention on the fish communities that inhabit an ancient coastline at ~125 m depth (the designated AC125), which is specified as a key ecological feature (KEF) of the region and is thought to comprise hard substrate and support enhanced diversity. We investigated drivers of fish species richness and assemblage composition spanning six degrees of latitude along sections of the ancient coastline, categorised as ‘on’ and ‘off’ the AC125 based on depth, across a range of habitats and seafloor complexity (~60–180 m depth). While some surveyed sections of the AC125 had hard bottom substrate and supported enhanced fish diversity, including over half of the total species observed, species richness and abundance overall were not greater on the AC125 than immediately adjacent to the AC125. Instead, depth, seafloor complexity and habitat type explained patterns in richness and abundance, and structured fish assemblages at both local and broad spatial scales. Fewer fishes were associated with deep sites characterized by negligible complexity and soft-bottom habitats, in contrast to shallower depths that featured benthic biota and pockets of complex substrate. Drivers of abundance of common species were species-specific and primarily related to sampling Areas, depth and substrate. Fishes of the ancient coastline and adjacent habitats are representative of mesophotic fish communities of the region, included species important to fisheries and conservation, and several species were observed deeper than their currently known distribution. This study provides the first assessment of fish biodiversity associated with an ancient coastline feature, improving our understanding of the function it plays in regional spatial patterns in abundance of mesophotic fishes. Management decisions that incorporate the broader variety of depths and habitats surrounding the designated AC125 could enhance the ecological role of this KEF, contributing to effective conservation of fish biodiversity on Australia’s north west shelf.

Regional patterns in demersal fish assemblages among subsea pipelines and natural habitats across north-west Australia

“…Proximity to these features was the primary split in the MRT analysis, followed by depth, with strong alignment in patterns observed by the MRT with DISTLM analyses. Depth is often among the most common predictors of fish distributions (Moore et al, 2009;Monk et al, 2010;Galaiduk et al, 2017b), assemblage composition (Cappo et al, 2007;Moore et al, 2010;Harvey et al, 2013) and fish species richness (Young & Carr, 2015;McLean et al, 2021;Currey-Randall et al, 2021). Proximity to infrastructure, such as platform jackets and/or pipelines, featured in both statistical analyses but were ranked low as predictors each explaining <2% of variation in the fish abundance data in both instances.…”

“…These techniques quantify the relationships among depth values in small neighbourhoods to reveal textural differences. Calculations are run on a small number of neighbourhood cells surrounding each pixel and the value assigned to the central cell in the output, thus creating a derivative dataset (see Cure et al, 2021;Currey-Randall et al, 2021;McLean et al, 2021) for additional information on secondary rasters). This produced a set of 23 secondary rasters that describe the structure and complexity of the seafloor (Supplementary Data Table S1).…”

“…The fish assemblages of natural shallow marine ecosystems across north-west (NW) Australia have been relatively well studied (Wilson et al, 2012;Evans et al, 2014;McLean et al, 2016), particularly in comparison to those present in mesophotic depths and beyond (Currey-Randall et al, 2021;Saunders et al, 2021;Wellington et al, 2021). Unique and diverse fish communities exist across the inshore shallow coral reefs, offshore emergent oceanic reefs, oceanic banks and shoals of the region (Heyward et al, 2012;Moore et al, 2014;Wilson et al, 2014;Moore et al, 2017) with lower fish diversity in sanddominated regions offshore (Currey-Randall et al, 2021;McLean et al, 2021). The NW supports a number of commercial finfish fishery operations such as Northern Demersal Scalefish Managed Fishery, Pilbara Demersal Scalefish Fisheries and Northern Territory Demersal fishery as well as several others throughout this region (Newman et al, 2020;Gaughan & Santoro, 2021).…”

Habitat value of subsea wells and pipelines for fishery target species in Australia

“…This variable was a strong predictor of abundance of target species (including lutjanids and serranids) with numbers declining as depth increased. Depth has also been shown to be a strong predictor of fish communities in natural ecosystems of this region (Abdul Wahab et al, 2018;Currey-Randall et al, 2021). In this case, shorter wells were found in the shallowest depths, which may have confounded the relationship between surface area and abundance.…”

“…), yet assemblages were similarly dominated by lutjanids, serranids, and carangids (total 46% species in these families). These families are known to be ubiquitous in natural reef ecosystems across the NW (McLean et al, 2016;Abdul Wahab et al, 2018;Currey-Randall et al, 2021) and, excluding tropical snappers, also in the SE marine region (Williams and Bax, 2001;Butler et al, 2002). As richness and abundance counts are intrinsically linked to sampling effort, and with disproportionate sampling effort across pipelines (Table S2), it is not possible to compare these measures across pipelines.…”