This project addresses FRDC program 1 (Environment) priority 23.
In order to double production by 2030 the Salmon industry in Tasmania must consider alternate production approaches and expansion into new areas. However, maintaining high environmental performance (a priority for both the industry and its regulators) requires an understanding of how farming in new areas might change environmental interactions. In order to ensure that management remains best practice, and farms continue to be efficient and sustainable, assessment of the local scale impact/ recovery dynamics and potential interactions with other resource users is required in newly developed farming environments and under different farming technologies. In addition understanding how farms interact with the various processes and ecosystems in the water-bodies where they occur and the various scales for those interactions (local, medium and broad-scale) will provide an important basis for establishing an effective strategy for system-wide management, including management of interactions with other users of the water-bodies.
It is clear from discussions with various resource users (i.e. fish farmers and commercial and recreational fishers) that the perception of potential risks differs between Macquarie Harbour and the southern farming regions. In Macquarie Harbour a critical issue is whether the current on-farm monitoring (and local scale impact indicators are “fit for purpose” i.e. do they support sustainable management by providing an accurate understanding of sediment conditions. Whilst in the new farming areas in the southern regions (Lower Channel/ Storm Bay), although establishing the effectiveness of the local scale monitoring is important, the key concern is whether there may be adverse effects on reef health (i.e. off-site interactions) as a result of increased aquaculture activities. Therefore a key element of this study will be to provide a better assessment of the potential risk to reef systems from sediment deposition and nutrient dispersion from fish farms directly.
Final report
Summary
The overarching aim of this research was to provide an improved understanding of the environmental interactions of Atlantic Salmon farming and to provide recommendations to both government and industry on monitoring and management strategies that are appropriate to the level of risk associated with these interactions.
The criteria for monitoring and assessment of sediment impacts and recovery associated with intensive Atlantic Salmon farming were established more than 15 years ago. However, changes in farming practices, innovations in technology, and expansion of the industry has highlighted the need to review the underlying principles to ensure for new areas management and monitoring strategies remain best practice, and that farming operations continue to be sustainable in all regions. Consequently, a program of research was established to assess current monitoring and management strategies, provide an understanding of regional and operational variability in local scale (sediment) response, and define both common and regionally specific local scale response principles. The study itself was focussed on:
- Macquarie Harbour in western Tasmania, a Harbour 5 times larger than Sydney Harbour with a restrictive entrance to the sea and significant inflow of freshwater creating a unique habitat.
- Storm Bay in south-eastern Tasmania, a significant Bay at the entrance to Hobarts Derwent River which is heavily exposed to oceanic influences as well as those from the Derwent River and to some extent the D’Entrecasteaux Channel.
- The very southern end of the D’Entrecasteaux Channel which is also influenced by oceanic conditions but also strongly from outputs of the D’Entrecasteaux Channel including the Huon River.
In addition, it was noted there was increasing concern in the community about the potential for broader scale interactions with reef systems and it was recommended this should be a feature of any resultant research plan seeking to inform monitoring and management. It was also recognised that modelling capabilities have increased markedly since the original research was conducted, and so an important component of this study was to evaluate how currently available modelling tools could support monitoring.
Key Findings
Comparison of the biotic and abiotic factors in Macquarie Harbour and the two new growing areas in southern Tasmania showed the sediments in Macquarie Harbour were inherently depauperate (low faunal abundance, species richness and diversity) whilst Storm Bay and Southern Channel sites supported diverse and species rich communities. Differences in sediment grain size, a factor that influences the macrofauna community composition, were also apparent with Macquarie Harbour sediments being much finer than those of the other two study regions. Measurements of sediment redox levels indicated Macquarie Harbour sediments were highly reducing (i.e. inherently low in oxygen/ anaerobic) for much of the system whereas the two southern study regions were generally oxidising (i.e. aerobic). This is not surprising given Macquarie Harbour’s highly stratified water column, deep central basin and shallow entrance to the ocean that has been shown to result in reduced mixing of bottom waters and naturally low dissolved oxygen conditions (Ross et al. 2021). In contrast, the sediment conditions at the other two study regions reflect the open nature of those systems and the increased levels of water exchange, which result in higher oxygen concentrations in the bottom waters and sediments. The carbon and nitrogen signatures of the sediments also highlighted the different background sources of organic material in each of the regions. In Macquarie Harbour the isotopic signature of the sediments indicated a far greater contribution of terrestrial and freshwater inputs to the organic matter pool. In addition, there was a clear change in the signal along the harbour, with the terrestrial signal increasing with distance from the harbour entrance. Isotopic signals from the two southern study regions were consistently marine.
The review highlighted there is the potential for dissolved and particulate nutrient inputs from salmon farming to interact with reef ecosystems, both directly and indirectly. Outcomes vary depending on environmental conditions; the level of wave exposure and water movement will influence not only the basic ecology but can also affect the response to particulate and dissolved inputs. Understanding how reef systems respond to inputs from Atlantic Salmon farming requires a detailed understanding of the local environment, the broader regional and global pressures, and the inherent characteristics of the reef community itself. These factors need to be considered carefully in the design and implementation of any monitoring programs and in the interpretation of the resultant data.
In terms of the modelling aspects both versions of the commercially available model DEPOMOD performed well at predicting total sediment deposition (loads) at the Storm Bay 1 lease, with NewDEPOMOD performing well at the Franklin lease in Macquarie Harbour. DEPOMOD is a model developed for modelling the deposition of solid farm wastes from fish pens at a scale of the farming lease to determine the benthic footprint of the farming activity.
NewDEPOMOD was designed with an updated sediment transport module that more realistically captures the flow of waste over dramatic shifts in bathymetry as seen for example at the Franklin lease in Macquarie Harbour. At the Lippies lease in the Southern D’Entrecasteaux Channel neither model accurately predicted total sediment deposition. We suggest this is because Lippies is a more dispersive site owing to the much higher bottom currents and deeper bathymetry. However, challenges with the sediment trap deployment may have also compromised our assessment at this site.
There were also differences between the study locations in how well the models predicted overall benthic footprint (i.e. the area affected by both deposition and re-suspension), and this too would seem to be largely due to differences in bathymetry and current speeds. At the Storm Bay 1 lease where the seabed was quite level and bottom current speeds were relatively low, the predictions from both DEPOMOD and NewDEPOMOD were similar and closely resembled the measured benthic footprint. However, at Lippies where current speeds were higher and at Table Head in Macquarie Harbour where the bathymetry is steep, DEPOMOD and NewDEPOMOD predicted quite different footprints. NewDEPOMOD provided a more accurate assessment due to its improved ability to adjust for complex bathymetry and account for sediment resuspension at dispersive sites.
