Soft-sediment communities key to assessing impacts of salmon farms

Studying soft-sediment communities is key to assessing the environmental impacts of salmon farms and is assisting salmon producers in monitoring and managing their operations for a sustainable industry, now and into the future, as IMAS Technical Officer Jimmy Hortle explains.

Food availability plays an important role in shaping many ecosystems, including the muddy, sandy soft-sediment environments on the sea floor where crabs, worms, shellfish and other aquatic creatures gather. These animal communities are known as benthic macrofauna assemblages.

The food source for these soft-sediment communities is organic material that comes from various sources, such as terrestrial (farm runoff, leaf litter, soil), marine primary production (macroalgae, phytoplankton settling) and, in some locations, faeces and uneaten feed from aquaculture production.

Image, above right: Polychaete worms (Ophryotroca shieldsii) are used as an indicator species in Macquarie Harbour. Here they are feeding on a salmon pellet in a lab experiment. Photo: Adam Davey.

The structure of these communities can change rapidly in response to increases in food. This is usually found immediately under salmon pens, where researchers often observe a community dominated by opportunistic animals such as Capitellid worms and Nebaliid crustaceans.

These opportunistic species play a vital role in processing the waste, much like the worms in a compost heap, so having an abundance of them breaking down the waste under a salmon pen is vital.

However, regulators do not want these communities to proliferate outside the farm lease area, as this indicates that excess waste has spread beyond the lease area. It might also indicate that macrofaunal communities inside the lease area are not effectively processing the excess food.

Most of Tasmania’s salmon farms are located over soft sediment environments, where the release of nutrient rich organic matter can impact the environment if not managed appropriately. IMAS researchers use the composition of the animal community living in the sediment as a useful indicator of food availability, and can then quantify the salmon farm’s enrichment ‘footprint’.

In Tasmania, the unique nature of the various salmon farming regions has enabled researchers to establish specific indicator species for each region. By placing the change in sediment communities at different locations and times, against the backdrop of each environment’s many unique features, we now have a better understanding of overall system health.

This has become a useful tool for farm management, with changes in the animal community giving salmon producers the ability to gauge whether current production levels are too high or on target to meet compliance obligations.

While this sounds like a simple process, it is a huge undertaking to match what lives in the sediment with all the other physical and chemical intricacies of a system and involves many committed scientists.

The Aquaculture Environmental Team at IMAS is regularly out sampling the sediments from the Huon and Channel regions, Storm Bay and Macquarie Harbour as part of a range of research projects.

This level of sampling means there is often a large stockpile of post processing to complete and, with important adjustments, the work has continued during the COVID-19 pandemic (see Research and Resilience, below).

IMAS research into soft-sediment communities under and around salmon farms continues to support monitoring requirements, while fit-for-purpose monitoring for new growing areas is also being developed. This research assists Tasmanian producers in managing their operations for a sustainable industry, today and into the future.

Also see: Latest monitoring finds improved environmental conditions in Macquarie Harbour

Research and resilience during COVID-19 pandemic

You’ll usually find Technical Officer Jimmy Hortle out in the field or in the lab, gathering and processing sediment samples and conducting other research with his fellow-scientists in the award-winning IMAS Salmon Environment Interactions Team at the University of Tasmania.

Among other projects, the team uses benthic macrofauna assemblages to assess the environmental impacts of Atlantic Salmon aquaculture (see project details).

Processing the sediment samples involves separating the animals from the sediment and identifying them – no small task considering there are about 600 species in Tasmania’s south east region alone.

This level of sampling means there is often a large stockpile of post processing to complete, so the team had to quickly adjust to the COVID-19 lockdown requirements while continuing this important work.

IMAS management and Taroona’s Laboratory Manager, Lisette Robertson, helped staff to set up and operate safely outside of the usual laboratory space. Processing continued with two co-workers social distancing in an onsite laboratory at Taroona and two staff set up with microscopes in their home labs.

These small isolated teams were rotated each fortnight so staff could get a change of scenery, while maintaining adherence to government regulations on social distancing. Working options that would once have been scoffed at became the new normal, and are a credit to the resilience and ingenuity of our IMAS people.

Image, right: Jimmy Hortle collecting sediment samples in Macquarie Harbour (pre-COVID-19)