In Victoria’s Port Phillip Bay, commercial and environmental interests can live in harmony, just like the vast variety of organisms that inhabit the bay’s spectacular rocky reefs.
University of Melbourne researcher Dr Eric Treml says reefs are rich in both biodiversity and potential profits, and understanding ‘population connectivity’ can help make the most of both offerings.
Port Phillip Bay, on which Melbourne is located, has many habitats, including sub-tidal rocky reefs and vast sea grass meadows. And it is home to many species, from the beautiful weedy sea dragons, to commercially important fish species like snapper and whiting, plus fur seals, dolphins, and whales. With 264km of shoreline, it’s an environmental and economic playground offering boating, tourism, shipping, fishing and more.
Dr Treml, a lecturer in marine biology from the School of Biosciences, says getting the balance of human activity and conservation right is important to maintain a sustainable bay, but we don’t need to necessarily reduce our activities in the bay to achieve this – we just need to understand how the bay operates and which areas need special conservation measures.
If you think of Port Phillip Bay as a country, the reefs it contains are the towns and cities. Like major cities, there are reefs that are huge and contain big populations, which produce large numbers of offspring that can spread to surrounding reefs.
And it is those offspring that are the key to reefs repopulating and becoming healthier. When individuals from key populations – like the big city ‘source reefs’ – can easily move to other, more discrete populations to repopulate damaged reefs the whole ecosystem is more resilient.
“There are different grades of coral grades; it’s a real patchwork,” Dr Treml says. “Different areas of the bay will have different characteristics, based on things like fishing pressure, winds, and proximity to Werribee treatment plants.”
Population connectivity, as it’s called, has significant implications for reef resilience –the ability of reefs to withstand and bounce back from disturbances.
Dr Treml has previously studied this mechanism in other reefs, and is now applying his models to the reefs in Port Phillip Bay.
Population connectivity focuses on the movement of individuals from one distinct population to another. Like people moving from one town to another for better job opportunities, young sea creatures move between reefs to find a place where they will thrive.
Source reefs are key to repopulating smaller, more isolated reefs, but intermediate, or ‘stepping-stone’ reefs are also crucial for connecting different populations that would otherwise be unreachable in one trip, or even in one generation.
This makes stepping-stone reefs critical for repopulating more isolated reefs.
Dr Treml says reef degradation is increasing worldwide.
“Unfortunately, I think with the current state of things, with population and climate impacts, the stresses that we see on the natural systems, that’s kind of locked in for the next few decades,” he says.
However, that does not mean it’s all over for the reefs. Connectivity means populations can be restored from outside sources. For example, Dr Treml says areas of the Great Barrier Reef that died during this year’s bleaching event could be recolonised by larval rains (the offspring of spawning corals, drawn in by currents) from the Coral Sea.
Dr Treml says that knowing which source reefs are critical for repopulating others means that we can determine which reefs need to be most vigorously protected.
“We are still researching, as it’s very context specific; there might be one rocky reef that is important for urchins or for one for important fish of the bay,” he says.
Using field-based research and computer modelling of Port Phillip Bay, Dr Treml has developed predictions about where a reef’s offspring will travel to. He says that thanks to the mountain of previous research, much of it funded by the state government’s ‘state of the bay studies’, we know the tidal flows, currents, and the physics of the bay.
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But these theories and models are being tested using an unusual method – tracking baby fish.
Known as larvae tagging, Dr Treml and his team analyse the unique chemical signatures of otoliths, which are the tiny structures within a fish’s ear bone and multiply as the fish grows.
By analysing the chemistry in the ear bone, Dr Treml can link that chemical signature back to the chemistry of the bay; the otolith can identify where fish have spawned and where they end up. He can then use this data to estimate the connectivity between reefs.
Given the important role that stepping-stone reefs play, Dr Treml believes that many of them should be protected, while leaving other parts of the bay free for recreational and commercial pursuits.
Dr Treml says that redistributing fishing pressures to make sure that we have healthy fisheries, happy people, full bellies, plus a sustainable system, is possible, with the right knowledge in place.
Banner Image: Melbourne’s Port Phillip Bay. Picture: Flickr/Wilson Afonso