This is the first study to describe benthic and fish communities associated with two oil and gas platforms within (GSP) and immediately adjacent to (HA) the BIMMA in an area of conservation significance. The distinct benthic and fish communities observed on the two structures likely relates to their size, configuration, age, the depth of water in which they are located and their proximity to natural habitat features.
Relative to the size of the two platform structures, high fish abundance and diversity was observed. The platforms were characterised by schools of small caesionids (fusiliers) and carangids (trevallies) that swam within and around the support beams. Attraction of pelagic species such as these to physical structures that span the water column is a well-known phenomenon where such structures are often referred to as fish aggregating devices (FADs; Castro et al. 2002). These behaviours are linked to species obtaining benefits in terms of finding food, predation success, avoiding predation and increasing mating opportunities (Deudero et al. 1999; Sancho 2000). Additional behaviours were noted that suggest that platforms provide a variety of specific niche habitats that benefit different species depending upon their ecology. For example, L. carponotatus (stripey snapper) would ‘line up’ along vertical beams (Fig. 5G) while G. magnificum (threadfin pearl perch) would exhibit a similar behaviour but usually in the shadows beneath horizontal beams (Fig. 5E). L. carponotatus may form this vertical configuration to remain as close to the structure as possible, likely for shelter while the behaviour of the Australian endemic G. magnificum suggests it has an affinity for caves/overhang environments.
Herbivorous fish were observed in low numbers (e.g., parrotfish, surgeonfish), perhaps due to a limitation in preferred algal species present on these structures. We did not separately include ‘corallivores’ as a feeding guild (butterflyfishes), for we saw very few, likely due to the low amount of hard coral cover on these platforms. Conversely, invertebrate and generalist carnivores were prevalent (A-Table 2), suggesting the presence of abundant invertebrate communities within and around the platforms. A high abundance of baitfish, particularly near to the seafloor likely provides a food source supporting the higher abundance of many other predatory demersal species observed in this part of the platform. An affinity of these larger, typically predatory fish species (groupers, snappers) with the seafloor-structure interface is expected as they are predominantly reef-associated species. Although the structures observed here remain relatively consistent in their structural complexity throughout the water column, species preferring to remain near the bottom may suggest defensive behaviour against open eater predation (Rilov and Benayahu 1998). An ability to utilise the structure for shelter and to ambush prey would benefit these species. The dominance of carnivorous fish species and relatively low abundance of herbivores/corallivores is likely linked to an absence of suitable benthic biota to support high abundances of these feeding guilds on platforms.
Commercial fisheries that operate in this region include the Pilbara Trap Managed Fishery, North Coast Shark Fishery, Onslow Prawn Management Fishery, wetline, beche de mer and tropical rock lobster fisheries and shell, coral and aquarium fish collecting (DBCA et al. in prep). However, most do not operate in the BIMMA or rarely fish within it (Newman et al. 2019; DBCA et al. in prep). Most recreational fishing occurs from charter vessels, although there is some recreational fishing from private vessels (Ryan et al. 2019). In regard to finfish, the platforms are located within the north coast bioregion where the principal fisheries (commercial trap and recreational) focus on high-value emperors, snappers and cods (Newman et al. 2019). The total estimated mass of fish across all depths at Harriet was 1247 kg (685 kg for fishery target species). McLean et al. (2019) reported a total mass of 2779 kg for the first 25 m depth bracket of a larger (125 m depth) jacket on the north-west shelf. The much higher mass reported by McLean et al. (2019) than here was due to large schools of big trevallies (Caranx spp) that were present at this remote offshore jacket compared to the small-bodied scad species observed at these shallower platforms. Of the top 14 species obtained in the commercial fisheries (proportion of catch), seven were observed here (L. sebae, L. erythropterus, E. multinotatus, L. vitta, L. nebulosus, L. russellii, Plectropomus spp.). The remaining six species not observed here all occur in deeper water or generally associate with different seabed morphologies. While it is unlikely that fishery target populations on these platforms have a significant contribution to fisheries, they do provide sanctuary for these species through an enforced exclusion zone. Research that seeks to monitor fishery target species inside and outside no-take sanctuary zones around the Montebello and Barrow Islands should seek to include oil and gas platforms. In this way, their role as de facto marine reserves may be assessed in addition to providing an understanding of how fish communities on these structures compare to those in surrounding natural ecosystems.
The cover of benthic biota on both platforms was high, yet diversity is unknown given that it is very difficult to identify many corals and sponges to genus or species level without physical examination of specimens. Thus, while a high diversity of biota is known for this region (Richards and Rosser 2012; Fromont et al. 2016), few distinct groups could be identified on platforms. This highlights the challenges of utilising imagery, no matter the quality, to assess the diversity of benthic communities where specimen collection or physical assessment is required. However, imagery is particularly useful for the development of reference libraries that become especially valuable when paired with specimen identifications. On GSP, some of the beams nearest the surface were dominated by barnacles (Fig. 3G). A high diversity of barnacle species exist in shallow waters of north-west Australia (Jones 2012) and their growth and settlement is positively associated with productivity (Menge et al. 2003). Presence of particular biota on various sections of platforms can be related to shading or sheering effects of currents (Love et al. 2019b). Here, high cover on GSP platform beams near the surface likely provides them with a vantage point to utilise their cirri (feathery feeding appendages) to filter feed and trap food such as plankton that may pass them by. Barnacle production on platforms has been shown to serve important ecological functions in the Gulf of Mexico where they are prey for many important commercially fished species (Reeves et al. 2018). While imagery suggests large monospecific mats of barnacles on the structures, some species of barnacle only associate with particular organisms (Jeffries and Voris 1996), with the latter not observable from imagery. Instead of barnacles, HA had a higher prevalence of macroalgae cover (up to 62% of quadrats possessed an undifferentiated form). Macroalgae genera could not be distinguished from imagery but was of low complexity/height. The presence of macroalgae on HA likely limits space for barnacles to colonise. The ubiquity of two different colonising groups of benthic biota on two relatively close platforms may reflect varying processes of succession (linked to structure age), the presence of different predatory fish communities or even cleaning regimes.
Sun corals (Tubastraea spp) were reasonably ubiquitous (5–38% of quadrats) but present in low cover (< 6%) on HA and were not observed at all on GSP. Unlike other jackets that have been surveyed in Australia (> 50% cover of quadrats in McLean et al. 2019) and the Gulf of Mexico (Kolian et al. 2017; Sammarco et al. 2013), their cover was not extensive here where it was intermixed with soft corals and sponges. The two species observed are most likely to be T. faulkneri (orange sun coral) and T. micranthus (black sun coral), although formal identification is required. T. faulkneri is known from the Indo-Pacific Ocean and for producing a toxic compound that inhibits the settlement and growth of competitor corals (Koh and Sweatman 2000). T. micranthus naturally occurs in the Indian (Comores and Madagascar to the Fiji Islands) and Pacific Oceans. T. micranthus are now, however, widespread across the globe and significant invasive species in the Atlantic (Sammarco et al. 2013; Capel et al. 2019) where they appear particularly dominant on artificial structures while tending not to exhibit this same dominance on natural reefs (Sammarco et al. 2014). This is the third study on offshore platforms of north-west Australia that has documented the presence and abundance of Tubastraea spp. While they are present within their natural range here, further research is required to understand the distribution and extent of sun coral species on artificial and natural reefs in Australia and the ecological implications of anthropogenic impacts (climate change, artificial structures) on its distribution and abundance.
Sea fans (Melithaeidae sp1) were observed only on GSP but identification to genus level of this family is not possible without physical examination of the sclerites. Observed only in two quadrats, the ‘tree-like’ soft coral was light pink in appearance (Fig. 3E). The deep blue coloured soft coral, Dendronephthya sp1 (Nephtheidae), was only observed on HA while Carijoa sp1 (Alcyonacae) was observed on both platforms. Each of the soft coral taxa observed occur in moderate to high current flow environments where they can filter feed on zooplankton and other food particles (e.g., phytoplankton) that come within reach. Research off the coast of Eilat, Israel, has shown that soft corals of the family Nephtheidae can be dominant on artificial reefs but not natural reefs, likely due to the vertical nature of the artificial structures exposing them to currents that can enhance their development (Perkol-Finkel and Benayahu 2004). A high diversity of benthic biota is known to exist in this offshore region (Richards and Rosser 2012; Fromont et al. 2016) and further targeted research is required to assess diversity on infrastructure present nearby. The benthic communities almost completely covering each of the platform structures would provide important shelter and food opportunities for a range of fauna species (e.g., van Lier et al. 2017).
In summary, the two relatively small platform structures surveyed here are within the BIMMA and possess marine communities that share species with those known to exist in surrounding natural ecosystems. The diversity of fish communities was high, although likely higher than that we were able to quantitatively assess using ROV video imagery. The platforms provide a unique habitat that a subset of fish species utilise to serve their specific ecological niche. While all species present on platforms occur in natural surrounding ecosystems, the platform structures facilitate their co-occurrence in a single place with habitat spanning the water column from the surface to the seafloor. The significance of marine communities present on these, and other platform structures in this conservation significant region, requires investigation. This can occur by comparing communities present on platforms to those in natural ecosystems, both within and outside sanctuary zones (to better understand their role as de facto marine reserves). Further, and perhaps most importantly, research should examine the processes that facilitate species establishment and dispersal in addition to the movement of organisms between these structures and surrounding ecosystems – to understand the ecological role these structures play in the marine ecosystem. Lastly, the research demonstrates the value of science-industry partnerships where simple modifications to enhance industry ROVs can yield important scientific information of broad relevance to management and industry (Macreadie et al. 2018; McLean et al. 2020).