WAMSI Node 4.2.2a - Establishment of indicators for ecosystem based fisheries management - Benthic assemblages

Brief description

There were four key objectives for WAMSI Node 4.2.2a: (1) determination of indicator regions; (2) development of monitoring strategies; (3) Implement a long term monitoring program (LTMP); and (4) provision of an assessment of expanding the LTMP to other areas. Indicator regions were confirmed as the Abrolhos Islands, Rottnest Island, and the Cape Naturaliste region. A synthesis of historical data sets for primary producers, sessile and mobile invertebrates and finfish were collected from the indicators region and an extensive review of potential indicators for finfish was also conducted (available for download from this metadata record). A program to monitor the health and biodiversity of subtidal reefs was established in April 2010 and April 2011. An Automated Underwater Vehicle (AUV) was deployed at the Abrolhos Islands, Jurien Bay and Rottnest Island to capture valuable information on the cover and abundance of benthic invertebrates and macroalgae. Replicate grids (25 x 25 m area of seabed) were surveyed. Exact co-ordinates are given below.

Lineage

Statement: *AUV details* Surveys were conducted with a state-of-the-art Autonomous Underwater Vehicle (AUV), which is operated by the Australian Centre for Field Robotics (University of Sydney) and part-funded by IMOS. The submersible is equipped with a full suite of oceanographic instruments, including a high resolution stereo camera pair and strobes, multibeam sonar, depth, conductivity and temperature sensors, Doppler Velocity Log (DVL) including a compass with integrated roll and pitch sensors, Ultra Short Baseline Acoustic Positioning System (USBL) and forward looking obstacle avoidance sonar. For biological survey work, the vehicle is also fitted with a Wetlabs Eco Puck, measuring chlorophyll-a, CDOM and scattering (red), and an Aanderaa Optode, measuring dissolved oxygen concentrations. The ‘flight path’ for each AUV dive can be precisely pre-programmed with a range of sampling designs, depending on the ecological question at hand (see below). The AUV was capable of conducting dives of up to 3 hours (limited by battery-life) and 3 or 4 dives per day. The vehicle was operated and deployed by a team of 4 scientists/technicians from ACFR; surveys were conducted from the CSIRO-operated research vessel ‘Linnaeus’ in April 2010. *Survey locations and design* Long-term monitoring was established at 3 key locations within the WCB; Rottnest Island (‘Rottnest’), Jurien Bay (‘Jurien’) and The Houtman Abrolhos Islands (‘Abrolhos’) (Fig. 2). The key objective of the sampling was to survey predominantly rocky reef/coral habitat that could be established as LTM sites and revisited as part of monitoring efforts. As such, sites were selected, based on bathymetry maps and existing knowledge (i.e. Marine Futures project), to target moderate to high relief reef. At each location, replicate sites were chosen along a depth gradient, with 2 sites at 15, 25 and 40 m depth. At Rottnest, one of the sites at both 15 and 25 m was positioned within the Kingston Reef marine sanctuary, to enable cost effective monitoring of this ecologically and socioeconomically important zone. Within each site, 3 replicate ‘grids’ were surveyed by the AUV. Each grid represented 25 x 25 m of seabed and grids were ~50 to 200 m apart. The AUV performed parallel transects at each grid, to obtain 100% coverage of the seabed. During the dive, the AUV captured overlapping geo-referenced stereo images of the benthos, as well as bathymetric data at 2 resolutions and physiochemical data (temperature, salinity, light, chlorophyll). *Post-processing and analysis* The AUV captured over 130,000 benthic images during the survey. Each grid comprised of ~1000 stereo image pairs, which were used to generate ‘meshes’ – composite images of the entire 25 x 25 m grid - of the seabed. These meshes represent a powerful tool for monitoring, amongst other things, kelp patch dynamics and coral cover at relevant spatial scales. However, due to constraints associated with swell and the physical properties of kelp, the construction of meshes was not possible for all grids; work on meshes is ongoing. For more detailed analysis, individual images, each capturing approximately 1.5 x 1.0 m of seabed, were subsampled from each grid to quantify assemblage structure using image analysis techniques. Subsamples were selected at 20 second intervals from the AUV dive to generate a sample set of 101-129 non-overlapping images that maximised spatial coverage of each grid. When analysing images 50 random points were digitally overlaid onto each sample, and the number of points covering each benthic grouping was counted. This value was then doubled to give a proxy of percent cover. Benthic groupings included dominant flora, fauna and substratum characteristics and were largely determined a priori based on previous observations and research. Care was taken to include conspicuous species of considerable ecological importance, such as the canopy-forming brown algae Ecklonia radiata and Scytothalia dorycarpa, while maintaining a general, holistic approach to describing the benthos. The benthic groupings were designed so that minimal training and experience would be required to analyse the images, while aiming to retain ecological pattern by including the complete range of biota likely to be sampled. Encrusting organisms were assumed to be living unless obvious discolouration or structural damage was observed. It was evident from the images that the methods were inappropriate for sampling mobile fauna (i.e. echinoids, gastropod molluscs), which generally have low abundances, highly variable distributions and utilise cryptic habitats in Western Australia (Vanderklift & Kendrick 2004). Therefore, mobile invertebrates were excluded from analysis and assemblage composition was derived from dominant macroalgae and sessile invertebrates. Subsamples were pooled for each grid and analysis was conducted using the three grids per site as true, independent replicates. Assemblage structure at each grid was ‘benchmarked’ by calculating the percent cover of dominant benthic groupings. Statistical differences in multivariate assemblage structure between depth increments and sites (nested within depths) at each location were tested with PERMANOVA using PRIMER 6 software with the PERMANOVA+ add-on.