In my part of the river, over recent years, why have yabbies taken up residence in their millions in areas where sea grasses once were?
When you have the very good fortune most days to be able to sit on the end of your jetty with rod in hand, you soon learn a few fundamentals. When it rains in the catchment it takes a few days for the water to come downstream. Saltwater fish and crabs get ‘flushed’ out to sea and you start catching catfish. The estuary cod, bream and flatties that we regularly catch temporarily disappear and freshwater fish take their place. The inter-tidal zone is covered in a dark, muddy sedimentary layer.
It is sediment that I suspect has choked the sea grasses and given oysters on sacks a very hard time. Happily. though, it has created a new opportunity for life such as yabbies and other crustaceans. These days we still see a lot of juvenile fish species in and around our jetties, but what the change to yabby patches has seen is an influx of many stingrays, shovel nosed sharks and flatheads now grazing the river bottom for yabbies – something rarely seen in the past.
Having owned (and lived on) property on the Noosa River in three very different locations, over time I’ve built an understanding of both the good and the bad things that can happen. Only a small number of residents permanently live on the river proper and are able to witness each and every day the subtle changes that occur very, very slowly – but constantly evolving. This by no means qualifies me or anyone else as a marine biology expert, but what we see firsthand is long-term, real and valuable – not just a study at a point in time.
I vividly recall more than 20 years ago buying the property where I live today. The river bed – or, as scientists call it, the benthic layer – was very different then. Changes to the benthic layer since prompts me to ask – is all change bad? Is some change necessarily Darwinian in the sense that environments change and life adapts?
I came across a very simple definition from Berkeley University of California’s Museum of Paleontology so I could understand better what I was seeing.
“The benthic zone….. The bottom of the zone of water that consists of sand, silt, and/or dead organisms. Here, temperature decreases as depth increases toward the abyssal zone, since light cannot penetrate through the deeper water. Flora are represented primarily by seaweed while the fauna, since it is very nutrient-rich, include all sorts of bacteria, fungi, sponges, sea anemones, worms, sea stars, and fishes”.
Other data makes it clear that a benthic layer changes dramatically over the length of a river as it transitions from catchment to sea with different salinities, different substrates, different temperatures, different everything…. Accordingly the benthic layer also ranges from excellent to maybe less than ideal depending upon where and how we humans have intervened in the environment.
Simply put, you cannot simplify or make a statement in general terms about the condition of the entire river because, along its length, it is variable.
A good example of a current issue is sedimentation. Intuitively our reaction is that siltation is a bad thing and we must make greater efforts to stop solids entering the catchment. The recent Oyster Bed Trial reported that siltation was a major threat to the viability of oysters in the river.
Seeing firsthand Oyster Bed Trial coir sacks covered in silt suggests there is far more sediment coming down the river than we think and considerably more that one would naturally expect. The immediate reaction of most people is that we need to stop all sediment entering the river from the catchment because it will kill off everything. Good science supports this and every effort is being made to correct the situation in our river system.
Going further, if we have much higher levels of siltation this probably explains why sea grasses have died off (in combination with boat wash/propeller churn) in some areas of the lower river. But what is interesting is that for every action there is a reaction which at times is not disastrous.
I am not for one moment suggesting that do nothing about sediment run-off but what I have observed is that nature has a wonderful way of adapting and, from my perspective at least, this is not always a disaster. To some degree, despite all the human influences on the river over the past 150 years, we have not seen invasive species take over the river and we do not see desolation – we see change and adaptation to our interventions.
Yes, there are some parts of the Noosa River that have changed because of human influence but, unless you change human intervention, it is highly unlikely that any benthic plants and animals will respond by re-colonising the substrate of the river.
We have known for a millennium that most forms of submerged structure provide substrate for the colonisation of species and today these are plentiful in the river, so why haven’t we seen a massive return of things like native species oysters? if you look anywhere in the inter-tidal zone, e.g., in Noosa Sound where everywhere there are rock walls, mangrove roots, jetty posts and so on, why aren’t there more oysters settling?
There are many places along the river that have structures but no oysters. In my view there is more at play in the busy lower sections of the river that, if we do not address them today, will see efforts such as Oyster Reefs become just a nice idea without too much long-term impact.
We know that pollution, sedimentation, chemical run-off and other influences affect our river, but unless we control their impact upon the substrate where needed (which I suspect is likely to be between Lake Cooroibah and the river mouth), in my view no amount of proactive projects will have a real and positive long-term impact.
It is known that dredging or netting that drags along a bottom will disturb the benthic layer. In Noosa these disruptors are pretty much a non-issue these days. But what causes ongoing and irreparable damage, and goes unnoticed, is the impact of vessels anchored in the river.
By MSQ law, vessels have to be on a swing anchor. This means they are constantly dragging heavy anchor chains in a circle of up to 30 metres along the bottom of the river – ripping up the substrate and not allowing anything to grow or re-colonise it.
When you consider that from Woods Bay to Tewantin there are some 130 vessels at anchor which 24/7 are dragging the bottom, what hope is there for rejuvenation?
This is why, as a priority step in the overall management of the river, remedying this particular impact, above all other projects, must be the priority.
Then, over time, the substrate will rebuild so that oysters and everything else can grow in an environment that compares to how it was before.
