In the glassy, clear pools of the Sandy Bay Rivulet something stirred. The slow, languid movement of fish suddenly erupted and in a silver flash they chased each other over the sand and shingle stream bed.
The rivulet in its lower reaches is alive with three species of galaxias at this time of year, twisting and turning in the chilly waters as they prepare for their migratory journeys out of the rivulet into the wide expanse of the Derwent estuary.
The galaxias make their way to the tidal zone just upstream from the Sandy Bay shore to await a king tide under a new or full moon to carry them off into the estuary.
The galaxias form what is known among anglers as whitebait, and the whitebait runs were once so great in the Derwent they were at the centre of a canning industry in Hobart.
Those days might have gone but the common galaxias or jollytails, which can grow up to 19 centimetres in length, spotted mountain galaxias, and climbing galaxias still survive in the rivulet in good numbers and can be seen easily from the bridge which spans the watercourse on Parliament St in Dynnyrne.
The breeding success of the galaxias has been enhanced in part by the Friends of the Sandy Bay Rivulet whose members have planted reeds on which the galaxias can attach their eggs. Like the migration, the egg-laying also takes place on ultra-high tides and eggs can survive out of the water until a following king tide carries the larvae out into the Derwent and as far as Storm Bay. The young develop over six months before returning to freshwater.
The galaxias’ conservation program has also been supported by the Hobart City Council which has built a fish ladder in the rivulet to aid fish migration. This was constructed under the direction of a leading freshwater ecologist Professor Peter Davies and the unique design utilises concrete pits, usually reserved for use in a reticulated stormwater system. The pits proved to be the ideal size and shape to be negotiated by the fish and were easy to install.
A flat concrete surface under the Parliament St bridge also had corrugations cut into it to further assist fish movement upstream. This site can also be viewed from the Parliament St bridge, and there is signage there to identity all the fish species found in the rivulet.
Watching the fish from the bridge I marvelled at how the galaxias and another rivulet species, sharp-finned eels which migrate to the Coral Sea, were attuned to the seasons and knew instinctively when it was time to move.
A few days later I noticed the arrival of a white-faced heron, a familiar sight on this part of the rivulet when the whitebait are marshalling for their run. The heron also clearly knew how to read the nuances of the changing seasons, and the phases of the moon. And when the galaxy started to move, the heron was waiting for them.