To focus expertise, wisdom and enthusiasm for the benefit of Moreton Bay
While we’re staying at home, their autumn migrations begin.
Autumn sees the migration of mullet out to Moreton Bay, to have their yearly spawning. This is, of course, a boon for local fisheries, since almost 90% of these fish are mature adults; however not all of the mature adults will breed every year. Unlike many other fish species, mullet undergo a partial migration, where only part of the population spawns each year. Mullet are incredibly cosmopolitan – they can live in salty, brackish or fresh water – but they only spawn out at sea[1]. This gives them a great deal of flexibility in their choice of habitat and has allowed them to colonise a larger range than if they were simply fresh or saltwater fish.
Once a popular food source in Minjerribah, or North Stradbroke Island, the local Noonuccal and Goenpul people would work alongside dolphins to herd and catch mullet[2]. The dolphins would herd the mullet into dug-out areas in the beach, where the fishermen would use nets to catch the fish. After catching their fill, the people would share their catch with the dolphins, fostering this special bond[2]. Aside from their role in the pre-colonial history of Moreton Bay, however, these fish have interesting lives of their own.
Mullet are among the few large fish who routinely dine on the organisms which live at the bottom of oceans and rivers. Since they are found in fresh, brackish and salty water, mullet spread nutrients between marine environments, helping to maintain a healthy marine ecosystem[3]. A French study found that the newly spawned fish would feed non-stop in saltmarshes and brackish creeks, mostly on small crustaceans and plankton[3]. Some of Moreton Bay’s most important habitat for species is found in saltmarshes, so this is just one more reason to give them greater protection – if we do, we can benefit the surrounding ecosystems as well!
According to research conducted in Moreton Bay, mullet occupy one of three migratory patterns. They can be cyclical migrators, following a more regular spawning pattern from estuary to sea; they can be irregular migrators, who move between fresh and estuarine waters before occasionally making their own spawning run to the ocean; or they can be residents, staying in a single marine environment until it’s time to breed[1]. All three of these types of mullet will coexist in various water bodies, but when autumn comes around and the seasonal migration approaches, the migrators will usually move ahead, while residents lag behind[1]. Once again we can see the mullets’ unique strategies for survival; by leaving a portion of their mature adults behind, there will be a continuous and robust population of mullet who are ready to breed!
A study conducted on the west coast of Greece found results similar to those seen in the Bay. Mullet will spawn in the open sea, and quickly migrate toward fresh and estuarine waters, where they will inhabit for much of their life. When the time comes, the mullet make their return to the ocean for spawning, or to take refuge during the colder months[4][5]. It seems that in the northern hemisphere, both temperature and wind currents are important factors in determining mullet migrations – both factors which could be potentially affected by the looming threat of climate change[5].
While we remain locked up indoors to stop the spread of COVID-19, it’s always good to be reminded that the natural world will continue on without us. What better example of this than the mullet? Right now, we’re stuck inside like the residential mullets, but hopefully soon enough we can continue our migrations, be they cyclical or irregular.
by Stephen Molan, Intern