I’ve written before about how important great habitat is if we want great fisheries. I strongly believe that as fishers, being custodians for the environments we spend so much time enjoying is vital. One area that is sometimes overlooked is the importance of freshwater inflows for the health and vitality of our estuary fisheries. This is especially important on regulated rivers like the Murray, Shoalhaven and many many more, where natural flow regimes no longer occur. This can mean that high flow events cease to occur, occur at times of year they wouldn’t normally or occur less regularly than they would under natural conditions.
This matters for a number of reasons. Estuaries are the places where our freshwater and marine environments mix. Habitats that aren’t fully marine or freshwater. This mixing means that the health of these habitats are affected the freshwater flows into them, both its quality and flow. While it is well known that flow events are important drivers of both spawning and recruitment success in freshwater species such as Australian Grayling, Silver Perch and Golden perch (and for the growth of non-flood spawners such as Murray and Trout cod), fewer people realise the importance of freshwater inflows for the spawning and recruitment success of a number of highly prized estuary species such as Mulloway, Estuary perch, Dusky flathead and others.
Recent work has highlighted just how important freshwater inflows might be when it comes to the successful spawning and recruitment in a number of these species. For example, work done on Mulloway in the Coorong, the mouth of the Mighty Murray river, one of the most heavily regulated in Australia, has demonstrated that the year class strength of Mulloway is strongly related to the magnitude of freshwater inflows. Recent strong age classes (2010/11, 2011/12) correspond with the breaking of the millenium drought and high fresh water outflows in the Coorong. Similar results have been demonstrated in the Shoalhaven river and NSW more generally.
The same is true for Estuary perch, with strong year classes closely linked to year of high freshwater inflows during the breeding season. In most populations, strong year classes of fish coinciding with years of high freshwater inflows during the breeding season dominate populations.
Similar links between flows and recruitment have been demonstrated in dusky flathead, luderick, sea mullet sand whiting and black bream, the bread and butter species of our south coast estuaries (as well as Barramundi and King Threadfin up north). It is clear that natural flow regimes and freshwater inflows aren’t just important to freshwater species, they are also an important driver of productivity in our estuary environments. In regulated rivers, we may be able to boost our fisheries by the judicious use of environmental flows at the right time of year. Doing so may be important if we want to rebuild stocks of highly prized fish such as Mulloway, or to simply make sure that our estuary fishers are performing at their best.