Sizing up marine ecosystems

Former Rhodes Scholar Dr Rowan Trebilco has been awarded the second RJL Hawke Postdoctoral Fellowship in Antarctic Environmental Science. During his three-year project he will develop models focused on mesopelagic fish (see photo) and squid, to better understand how Southern Ocean ecosystems are structured and how they respond to fishing pressures and climate change.

Rowan Trebilco received his open water SCUBA diving ticket at just 14 — the minimum age for aspiring divers. Since then his love of the sea has seen him acquire a range of specialty SCUBA skills and inspired an academic path through the subantarctic and Southern Ocean environments.

Along the way Rowan spent almost two years on Heard and Macquarie islands, working with macaroni penguins, elephant seals, albatross and giant petrels, and led research investigating how to mitigate interactions between seabirds and Australian longline fisheries.

‘My fieldwork in the subantarctic primarily involved satellite tracking and population monitoring, and my work in bycatch mitigation sparked an interest in conservation planning,’ he said.

‘When I finished my undergraduate degree I found myself in a job where my role was to provide scientific advice to guide conservation and fisheries management, but I didn’t feel my quantitative [measurement] skills and policy knowledge were as good as they could be.’

To enhance his knowledge of biogeography and conservation policy Rowan successfully applied for a Rhodes Scholarship to study a Masters in Biodiversity Conservation and Management at Oxford University. To improve his quantitative skills, he subsequently enrolled in a PhD at Simon Fraser University in Canada.

‘I’d always wanted to do research in subtidal [reef] areas so that I could spend more time SCUBA diving,’ he said.

‘So I developed a research project focused on reef fish community ecology in the Haida Gwaii archipelago in northern British Columbia.’

His PhD research investigated the body size and number of fish in reef communities and the energy flow of the reef — the transfer of biomass through predator-prey interactions.

The work segued neatly into his new role as a Hawke post-doctoral fellow (named in honour of former Australian Prime Minister Bob Hawke) which began in July. During the two year project Rowan will develop size-based models to understand the role of mesopelagic fish and squid in Southern Ocean ecosystems, and the impact of fishing and climate change on them.

‘Size-based ecological models recognise that individual body size is often more important than species identity in determining an individual’s role in the food web,’ Rowan said.

‘Individual metabolic rates and energetic requirements, as well as life history traits like growth rate, reproductive age and lifespan, are closely tied to body size. Body sizes also play a central role in determining interactions between predators and prey — as a predator, you can only eat something if it’s in the size-range you can handle, and swallow.

‘So size-based models provide a powerful way to understand how marine communities are organised and how they respond to fishing and climate change, even if you don’t have detailed species and life-stage-specific dietary information.’

Mesopelagic fish and squid are important prey species for penguins, seals and other large marine predators, so they play a key role in transferring energy from zooplankton (which they eat) to higher predators. However the sensitivity of mesopelagic fish and squid to the impacts of fishing and climate change is poorly understood and they are generally poorly represented in current models.

To build his size-based models Rowan will focus on fish and squid in the Kerguelen Plateau region of the Southern Ocean — an area of high productivity and commercially important toothfish and icefish fisheries.

He will consolidate existing data from the region on biomass and size distributions, species abundance, life history parameters and other variables, and collect new data during the K-Axis voyage (see Spotlight on the K-Axis).

The new collection will involve trawling for fish and squid at different depths up to 1000m. The marine creatures will then be sorted into size classes within species groups. Stable isotope analysis (using two forms of nitrogen — the regular 14N and the heavier 15N) will be used to determine how trophic position changes with body size.

‘The higher up the food chain you go the more 15N is enriched in the body of the organism,’ Rowan explained.

Using this data Rowan will develop size-based models for mesopelagic organisms in the region, including representations of the effects of fishing pressure. He will also travel to France next year on a Scientific Committee on Antarctic Research Fellowship, to work with international experts on mesopelagic fish to develop and refine the models.

‘The model results will allow us to infer how sensitive mesopelagic fish and squid are to potential fishing and climate-related environmental changes in the region,’ he said.

‘They will also help us identify potential indicators of change, and determine how changes in fish and squid might affect higher predators.’

The models will contribute to larger ecosystem modelling efforts being developed by the Australian Antarctic Division and the Antarctic Climate and Ecosystems Cooperative Research Centre.

Wendy Pyper
Australian Antarctic Division