Autonomous Underwater Vehicle-based benthic fauna surveys and monitoring of Huon Marine Park and Freycinet Marine Park shelf habitats 2022/2023

Autonomous Underwater Vehicle (AUV) image-based surveys of benthic habitats and the biological communities that inhabit them were conducted in the Huon Marine Park (HMP) Multiple Use Zone and the Freycinet Marine Park (FMP) Multiple Use Zone within the South-east (SE) Marine Park (MP) Network in 2022 and 2023 respectively. These surveys represent (1) a continuation of the time-series of repeated cross-shelf surveys in Australian Marine Parks (AMPs) conducted as part of the Integrated Marine Observing System (IMOS) AUV facility in Tasmania (see Perkins et al. 2021), and (2) an extension of the program to survey and represent newly mapped habitats not represented by prior surveys. For the HMP, two long-term survey sites have now been surveyed four times: in 2009, 2010, 2014 and 2022. In the FMP surveys began in 2009, with revisits conducted in 2010, 2011, 2012, 2014, 2016 and 2023. However, in the FMP not all sites were surveyed in each time step. Joe’s Reef, the dominant granite reef feature within the FMP has now been surveyed four times: in 2011, 2014, 2016 and 2023.

The long-term AUV monitoring sites were initially established under the IMOS AUV program on representative seabed features in shelf waters of each MP based on multibeam mapping undertaken by Geoscience Australia (Nichol et al. 2009b) of portions of the shelf in each MP prior to the baseline surveys. The AUV sites were primarily targeted on reef-like features identified from the mapping in both Huon and Freycinet MPs. Additional multibeam mapping on the shelf conducted in 2019 in each MP considerably expanded the knowledge of the benthic topography and reef features in each of these parks, allowing an opportunity to undertake further AUV surveys for both describing the biodiversity of these habitats and underpinning potential future monitoring. Subsequently, additional AUV survey sites were identified for the 2022/23 campaign, with four additional AUV surveys being undertaken in both MPs. The four new sites in HMP were primarily targeted on reef features identified further offshore from the long-term monitoring sites, extending the maximum depth of coverage from around 75 m to over 100 m, the deepest extent of mapped reefs in shelf waters in this park. The four new sites in FMP were primarily focussed on shelf break reef features in depths of 100-140 m (i.e. in the rariphotic zone), offshore from sites previously surveyed and representing deeper reef habitats not previously examined in the park, allowing the full depth range of reefs in the park to be represented by AUV surveys.

This report therefore has two major points of focus: (I) to describe the habitats and biological communities present in the previously unsurveyed sites in both HMP and FMP, and (ii) to explore the time series at the repeat survey sites in both HMP and FMP to describe temporal change.

In the HMP the four new survey sites revealed the reef habitat and associated biota on the deeper mesophotic to rariphotic reefs were generally similar to that found on the prior sites, which were further inshore, with the new sites providing spatial replication and greater understanding of the depth relationships of key species. In the FMP the four new survey sites at the shelf-break in the rariphotic zone provided a first habitat characterisation of this region and the associated biota. Much of this area was characterised by extensive beds of bryozoan rubble and shell rubble amongst soft sediments, with relatively rare outcropping reef. Typically, the cover of emergent biota was very sparse regardless of the habitat type, even on outcropping reef, particularly where this was composed of mudstone. The two new surveys conducted on mid-shelf locations in the rariphotic zone over the common dune-like features showed them to be similar to existing mid shelf survey locations, with a sand-inundated seabed with litle to no emergent bedrock or reef-like features and very litle emergent fauna other than the fine biological matrix (presumably predominantly bryozoan in composition) that was also commonly encountered during most of the deeper shelf surveys in the FMP.

The time-series in the FMP had two key components; (1) repeat surveys of the complex reef associated fauna on Joe’s Reef, the only major shelf reef identified in this park spanning lower mesophotic to rariphotic zones, and (2) repeat surveys of the mid-shelf dune-like features in the rariphotic zone. The fauna on Joe’s Reef was relatively stable over the decadal period of the study, although several key morphospecies such as gorgonian red Pteronisis-like octocorals and repent yellow sponges did vary in order of magnitude over this period, though they were typically less than 2% individual cover, and as there were no consistent paterns of variation shared across these, presumably they are each responding to different (yet unknown) environmental drivers. On the dune-like mid shelf seabed, the cover of readily identifiable morphospecies was extremely sparse and remained that way over the period of the study at these sites (2009-2023). At one survey loca􀆟on, MPA site 2, the overall cover of fine biological matrix did increase notably over this period, although further time-series data will be required to determine whether this is an ongoing response to protection or a result of a lengthy period without storm-related seabed disturbance.
The sites associated with the time-series in the HMP were established on complex reef systems primarily in the mesophotic zone. These showed a somewhat similar patern to that observed in the FMP on rocky reef. Most morphospecies remained at less than 2% average cover, and although some varied at up to an order of magnitude over the period of the study (2009-2022), as found at FMP, there was no consistent trend and trends were often in opposite directions to that seen in the same morphospecies in the FMP. For both the FMP and HMP a longer-term time series coupled with the recording of oceanographic and climatic variables is required to determine the likely key drivers of observed variation, as this can be derived from a wide range of physical drivers, including severe storms through to changing ocean current systems with corresponding influences on food availability and water temperature.