Sustainable finfish aquaculture is dependent on a benthic environment that can assimilate and process farm particulate wastes. As outlined above, in MH, bottom and mid water DO levels have reached very low levels and we have observed an associated increase in the presence of bacterial mats and a significant decline in the abundance and diversity of benthic fauna. As a consequence, multiple cage sites across the harbour are now subject to mandatory fallowing. The challenge facing farmers and regulators is understanding and predicting the length of fallowing required for benthic recovery; this has major implications for future stocking plans in the harbour. FRDC project 2014/038 and 2015/024 provide the benthic baseline over the past 2 years of farming; repeating these surveys following the observed decline in oxygen levels and fauna is needed to understand benthic recovery following fallowing. It is clear that DO levels have been, and will be, a major determinant of the benthic response over the coming months and years. Thus, it is imperative that real time observations of DO levels are coupled with the benthic observations. However, it is also extremely important to improve our understanding of the drivers of oxygen drawdown and recharge, including identifying the flushing rates of the various bays and basins in the harbour as it is not just how low the oxygen levels get, but also how long they stay low that will influence ecological outcomes. The existing CSIRO Hydrodynamic and Oxygen Transport Model can help to address these critical questions.
The future of salmon farming in Macquarie Harbour depends on its long term environmental sustainability. This project will help inform the likely effectiveness and duration of any given fallowing or remediation strategy, and as such is essential for both operational management of farming activities and the long-term management of the harbour.
This report provides an update on the status of dissolved oxygen (DO) and benthic conditions in Macquarie Harbour. It follows on from the results outlined in the IMAS reports released in January, May, September 2017 and January 2018. These reports described a deterioration of benthic and water column conditions in Macquarie Harbour in spring 2016, early signs of faunal recovery observed in May 2017 and a subsequent decline in benthic conditions in spring 2017 when oxygen concentrations in middle and bottom waters returned to very low levels. Oxygen concentrations in the middle and bottom waters have since improved through the summer of 2017/18 due to recharge events that commenced in late spring 2017. This report presents the results and preliminary interpretation of a repeat survey of benthic communities in January 2018 and DO monitoring data up until the beginning of May 2018. This work is part of the research project (FRDC Project 2016-067: Understanding oxygen dynamics and the importance for benthic recovery in Macquarie Harbour to address these needs) funded by the Fisheries Research Development Corporation with the support of both industry and government (EPA and DPIPWE); the scope and funding for the project was recently extended for a further two years (until April 2020).
The bottom water recharge that commenced in early November again appeared strongly tied to salinity at the surface, and hence river flow. Through winter and into early spring river flow was high and surface salinities low and this coincided with the decline in DO levels at depth. From October river flow decreased, surface salinity increased and in early November DO levels at depth increased markedly. These are the same combination of conditions that have coincided with previous oxygen recharge events. An important feature from a farming and management perspective are the concomitant changes observed in DO levels towards the surface. In the weeks prior to the onset of the oxygen recharge (from late October through November), DO levels at shallower depths (5-15m) declined significantly, most notably at the 5 and 7.5m sensor depths, before increasing again in early December. This would appear to be strongly related to the reduction in river flow, as evidenced by increased surface salinities, and likely reflects the lower DO, more saline water that had previously been pushed deeper in the system now moving closer to the surface. During a recharge event this situation may be further exacerbated, as the denser more saline oceanic water entering the bottom waters will effectively enhance the displacement of this water upwards from below.
In the October 2017 survey we reported that the early signs of recovery in benthic fauna across lease sites observed in May 2017 had declined, concomitant with the return of very low DO concentrations. Faunal abundances had returned to levels similar to those observed in the October 2016 and January 2017 surveys. The number of species recorded also decreased but not to the same extent. In the January 2018 survey there was little change from October 2017 in faunal abundance and number of species recorded at most of the leases, except at the more southern leases where there had been a further decline. At the majority of external sites (>1km from the leases), the improvements seen in May 2017 remained in October 2017; there has been little further change in January 2018. At the deeper external sites to the south, faunal abundances continue to remain low relative to observations prior to the noted decline in spring 2016 – early 2017, although there has been some improvement at the shallowest of these sites.
The May and October 2017 video surveys indicated a clear reduction in the presence of Beggiatoa at both lease and external sites compared to the October 2016 and January 2017 surveys. In the January 2018 video survey there was an increase in the number of dives that Beggiatoa was observed on, but this reflected more observations of patches of Beggiatoa at lease sites and fewer observations of Beggiatoa forming extensive mats relative to the previous two surveys. At the external sites, Beggiatoa was observed at the same number (2) of the 28 sites in the January 2018 survey as it was in the September 2017 survey, which is significantly fewer than reported in January 2017 and October 2016. The January 2018 ROV has also seen a reduction in the number of sites where Dorvilleid polychaetes were observed; there also appears to have been a reduction in the 4
number of sites with very high Dorvilleid scores (>300 abundance categories).
- the maintenance of the CSIRO profiler and ongoing delivery of near real time and short term forecast model results
- updates to the visualisation dashboard
- a description of the simulated harbour water quality, evaluation of oxygen and nitrogen budgets, scenario results and analysis
- analysis of observational process studies for phosphate addition and oxygen drawdown