Letting Closed Coastal lakes and Estuaries Open to the Sea Naturally

Thursday, February 16, 2017

Wayne Thorpe uses 'Indigenous storytelling' to tell a story of how Bung Yarnda (aka Lake Tyres) and other coastal tidal estuaries, work. It is a very important story because it applies to all estuaries and the Indigenous style of telling a story of Bung Yarnda brings the environment to life for adults and children.  Like all Indigenous stories it has layers, other stories within it reflecting the layers of history and the national history of Bung Yarnda. This estuarine lake sits in the deeply gorged limestone valley made by a large river from a time when Bass Strait was a land bridge it once flowed across. Fishing Bung Yarnda has long supported the community that lives on Lake Tyers, locals and regular visitors.

An open estuary has daily tides, and with persistent stream flow it remains tidal, restocking with marine life on each high tide. However, smaller estuaries, especially those that have lost catchment vegetation cover, often only open to the sea and become tidal again after rain.

Estuaries like Bung Yarnda, that open and close, need enough rainfall in the catchment to flood the lake and breech the sandbar that blocks tidal flow from the sea.  The higher the estuary level before it opens to the sea and the deeper the channel, the longer it stays open, the more fish and other marine life can get into the lake.

That 'flooding', just prior to opening, brings freshwater high onto banks covering bushland, farmland edges, parks etc. until the estuary opens, usually for a few days but sometimes longer. People affected naturally often seek to open the estuary manually with bulldozers, tractors or even by hand, digging trenches to the sea.  Though this seems like an entirely sensible approach to reduce the temporary freshwater flooding, people too often do not realise how it compromises the fisheries in these estuaries.

More than 50 % of the world’s commercial and recreational fish species breed in estuaries.  Either the eggs hatch in them or in nearby coastal waters with tiny larval fish, like sea mullet, coming in with the rising tide.  These estuaries and estuarine lakes contain vegetation such as salt marshes and seagrasses and logs that have washed down from upstream. This living and dead vegetation provides shelter for juvenile fish from predators – bigger fish, birds etc.

The fish that live in estuaries also live in coastal waters and bays. When an estuary that has been closed for months even years, opens, it washes the dead and living small (and not so small) animals, insects, crabs, prawns and shrimps and tiny fish into the sea. This flow forms a 'plume' often of discoloured or muddy water that sweeps along the coast with the falling tide. Coastal fish like bream and sea mullet are attracted to the 'food' in these plumes and swim into them. When the tide turns, much of the plume and the 'fish food' it contains flow back into the estuary from where fish often swim far up into the estuary and its streams in search of more 'food'.

Where estuaries are allowed to flood naturally before opening, their waters flow from what was dry ground behind the entire estuarine shoreline, washing insects, worms etc out to sea into that 'plume' as Bung Yarnda opens. This attracts even more fish into the plume and the now tidal estuary itself. Combined with the deeper channels, these naturally opened estuaries remain open longer supporting more fish and other wildlife like dolphins and seals.

Bob McDonald, Naturalist, February 2017

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