For several years starting in 1974, I was lucky enough to go out on a prawn trawler out of Port Douglas on many occasions. As a budding marine biologist I was like Alice in Wonderland - the “critters’ that came up from the bottom in nets to land in the sorting tray along with the prawns were animals I’d only seen in esoteric invertebrate taxonomy books.
There were ”Superman Balls” Sea Cucumbers, Basket Stars that looked like a Gorgon’s head, Brittle Stars and Starfish, tiny crabs and big ones, Sea Pens, bivalves, sponges of all shapes and sizes. Of course there were prawns, as well as fish of various shapes, sizes and species, but mostly with trawling at night, it was the invertebrates that dominated.
It became clear to me that this living carpet were mostly filter feeders - filtering the seawater for organic matter, or detritus feeders - scavenging on the bottom and through complex tiny mouths or digestive systems, removing the goodness from the inorganic waste.
Over the next 10 years the quantity of this underwater wonderland coming up on the sorting trays seemed to me to decrease. With sometimes a dozen or more trawlers going over these fairly shallow (10-20m depth) soft bottoms, you would have to wonder how much damage we would be doing to the physical structure and the complex life systems that remained unseen in the usually turbid but fecund waters below us.
It was interesting over the same period to watch the change in coral species structure and increasing turbidity at nearby Low Isles. In the early 70’s there were large areas of plate Acropora. These disappeared - partly due to storms - but also to be replaced by more turbid water corals over the years.
It is just a personal thought, but this living carpet, when it is intact, must contribute to trapping sediment and preventing it from being resuspended during strong trade winds close to the coast. Trapping nutrients and organic matter attached to sediment is almost like a giant pool filter - converting sediment and matter into living organisms in a giant flat trophic system coating the muddy bottom. Delicate and easily disturbed by the physical action of tickler chains and nets, I hope our original bottom fauna is not irretrievably damaged.
We all love to eat prawns, but if we are concerned with building resilience in coral reefs, and we are doing all we can to minimise sediment and nutrient load coming into our Great Barrier Reef waters from land via rivers, then surely we must also look at restoring our living carpet of soft bottom benthos in that very unsexy, dirty water that lies in the no-mans land between the coast and our mid shelf reefs. I don’t know how that might happen but perhaps its a question we should be thinking about.
Prawn trawling effort has been dramatically reduced, which is great, but are the important areas - such as off our river deltas sufficiently protected? The recent discovery of the deep reef in dirty water off the Amazon poses some interesting questions for us here on the GBR…
So much we don’t know because we only see what we see…. the workings of nature are so much more than we mere humans comprehend.
https://news.nationalgeographic.com/2016/04/220416-Amazon-coral-reef-Brazil-ocean-river-fish/.