Effects of Reef Proximity on the Structure of Fish Assemblages of Unconsolidated Substrata

Smith SDA (2012) Effects of Reef Proximity on the Structure of Fish Assemblages of Unconsolidated Substrata. PLoS ONE 7(11): e49437. doi:10.1371/journal.pone.0049437 Effects of Reef Proximity on the Structure of Fish Assemblages of Unconsolidated Substrata Arthur L. Schultz 0 Hamish A. Malcolm 0 Daniel J. Bucher 0 Stephen D. A. Smith 0 Simon Thrush, National Institute of Water & Atmospheric Research, New Zealand 0 1 National Marine Science Centre, Southern Cross University , Charlesworth Bay, Coffs Harbour, New South Wales , Australia , 2 Aquatic Ecosystem Research, NSW Department of Primary Industries , Coffs Harbour, New South Wales , Australia , 3 Marine Ecology Research Centre, School of Environment, Science and Engineering, Southern Cross University , Lismore, New South Wales , Australia Fish assemblages of unconsolidated sedimentary habitats on continental shelves are poorly described when compared to those of hard substrata. This lack of data restricts the objective management of these extensive benthic habitats. In the context of protecting representative areas of all community types, one important question is the nature of the transition from reefal to sedimentary fish assemblages. We addressed this question using Baited Remote Underwater Videos (BRUVs) to assess fish assemblages of sedimentary habitats at six distances from rocky reefs (0, 25, 50, 100, 200, and 400 m) at four sites in subtropical eastern Australia. Distance from reef was important in determining fish assemblage structure, and there was no overlap between reef sites and sedimentary sites 400 m from reef. While there was a gradient in assemblage structure at intermediate distances, this was not consistent across sites. All sites, however, supported a mixed 'halo' assemblage comprising both reef and sediment species at sampling stations close to reef. BRUVs used in conjunction with high-resolution bathymetric and backscatter spatial data can resolve differences in assemblage structure at small spatial scales (10s to 100s of metres), and has further application in unconsolidated habitats. Unless a 'reef halo' assemblage is being examined, a minimum of 200 m but preferably 400 m distance from any hard substrate is recommended when designing broader-scale assessments of fish assemblages of sedimentary habitats. - Funding: All funding for this project was supplied by a postgraduate (PhD) research budget from Southern Cross University. The funder had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. Despite the fact that sedimentary environments are the most extensive subtidal habitat of the worlds oceans [1], research on fish assemblages in shallow coastal waters has primarily been conducted on reefal habitats, and understanding of fish assemblages of unconsolidated habitats is relatively poor [23]. Sedimentary habitats have often been characterised as less topographically complex and more physically stressed than reef environments [4], providing fewer niches and lower primary production than reef habitats [5]. However, many species utilise this habitat, and species richness may be comparable to that found on adjacent reefs [6]. The lack of available data on unconsolidated habitats and associated biotic patterns restricts the objective management of these habitat types. Marine Protected Areas have been developed worldwide with conservation of biodiversity as a primary objective [7], and a representative sample of all habitats and biota is required if conservation outcomes are to be met [89]. The use of habitat as a surrogate for biodiversity is often unavoidable, as detailed species and assemblage inventories are rare [1012]. However, this will only be effective if biotic patterns consistently and predictably match physical attributes, which requires testing with biological surveys [13]. In New South Wales (NSW) marine parks, a Habitat Classification Scheme (HCS) is employed as a surrogate for biodiversity, and uses depth and habitat type (such as rocky reef and unconsolidated sediment) as primary categories. While the HCS has been tested for effectiveness against patterns of fish assemblage structure on rocky reefs [1416] this has not been assessed for fish assemblages of unconsolidated habitats. Many species found on unconsolidated habitats are morphologically and behaviourally adapted to this environment, posing problems for visual surveys. For example, fish such as those from the family Platycephalidae bury themselves in the sediment to ambush prey [17], and sillaginids are highly mobile, and very sensitive to sound and vibration [18]. These traits, combined with the practical depth limitations for SCUBA-based visual census (see [19] for a review), creates logistical difficulties in accurately and non-destructively sampling entire fish assemblages in these environments. For this reason, destructive trawl sampling has often been employed [2021], but this method is not compatible with marine park objectives. Baited Remote Underwater Video (BRUV) has been developed to reduce constraints involved in fish assemblage research by SCUBA methods [2223]. BRUVs have been successfully used in many hard substrate environments to assess fish assemblages [24 25] but have only recently been applied to shallow unconsolidated habitats [2629]. The effectiveness of BRUVs for assessing abundance of carnivorous fish otherwise shy of divers is well documented [22,30], and [26] suggest that BRUVs may have Figure 1. The location of the 72 individual BRUV deployments at Forty Acres Reef and Split Bommie Reef in the Solitary Islands Marine Park (SIMP), New South Wales, Australia. doi:10.1371/journal.pone.0049437.g001 a particular role in studies of larger elasmobranchs or teleost species of special conservation interest that other sampling methods do not reliably record. There is also great application for BRUVs in habitats below depths accessible by SCUBA [31], and they have been widely employed in deep-sea research since the 1970s [3233]. Not only are data lacking for fish assemblages of unconsolidated habitats in general, but also relatively little is known about linkages between unconsolidated and reefal habitats (but see [34]). Top down influences by consumers foraging across multiple habitats can be strong. Predators [34] and herbivores [35] will often most intensively forage in areas closest to their primary shelter habitat. Predatory species associated with reef habitats may effectively forage across adjacent sedimentary environments, while cryptic species and ambush predators may concentrate predation effort close to hard substrata (e.g. [36]). Gradients in assemblage structure have been inferred by studies demonstrating a reduction in predators [34,3738], and an increase in prey densities [3942] with increasing distance from reef. Decreases in the abundanc (...truncated)