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Big Floods = Big Barras

barramundi

Study co-author Ian Halliday catches a barramundi, a popular species for anglers in rivers such as the Daly River in the Northern Territory.

By Tim Jardine, Brad Pusey and Ian Halliday

More barramundi survive to adulthood during big flood years due to increased feeding opportunities.

Floods can be devastating natural forces that wreak havoc on just about anyone and anything living in low-lying areas. We often see images of homes, other property and human lives tragically destroyed by floods.

Yet for all this devastation and destruction, surely there must be some benefit? After all, floods have come and gone on this continent for millions of years, and the plants and animals that we see today are the product of this flood–drought cycle.

This is especially true in the wet–dry tropics, an area stretching from Broome in Western Australia across to the tip of Cape York in Queensland, which contains many vast floodplain rivers. Do these river systems benefit from annual floods that turn a parched landscape into an aquatic utopia just about every Christmas?

One animal that does very well in rivers that flood is the barramundi (Lates calcarifer), an Australian icon that has great commercial, recreational and cultural value. Keen anglers flock from southern locations for a chance to hook this fish, and it features prominently on the plate of fish-and-chip lovers. There is no better place to find it than the lower reaches of a large floodplain river like the Mitchell River in north Queensland or the Daly River in the Northern Territory.

For a long time it has been known that floods and barramundi ecology and production are intimately linked, but the nature of those linkages was largely unknown. How many more barramundi do you get with a bigger flood? Conversely, if you take the flood away (as has been done in many large rivers worldwide), how many barramundi might you lose?

It was also unknown why floods are important. What is the ecological mechanism? Such important questions have been left unanswered.

New research is shedding light on the links between barramundi and floods. The Tropical Rivers and Coastal Knowledge (TRaCK) research program uses cutting-edge technology to better understand the growth and survival of fish in relation to flow, and the food web pathway leading from food sources on flooded land to top predators such as barramundi.

The first novel method used by the TRaCK team is to look at the ear bones, or “otoliths” (Fig. 1), of barramundi caught by commercial and recreational fishers. Information about the age and growth of each fish is recorded in these ear bones, allowing researchers to determine how many small fish each year survive to be big fish (and end up on a dinner plate). Each otolith has a series of rings, much like a tree, that show annual growth and the age of each individual, allowing scientists to determine the best years for growth and survival.

It turns out that survival and growth of barramundi are intimately linked to the size of flooding in the river system. In big flood years, juvenile barramundi grow at about twice the rate experienced in years with small floods. This increases their survival, meaning that about twice the number reach adulthood in years of big floods.

These measurements established the link between flood size and the number of barramundi surviving to adulthood, but a further question remained. What is it that allows these fish to grow faster and survive better during big flood years? The answer appears to lie in tiny, humble algae that grow out on the floodplain when the land is under water; algae that look like nothing more than slime on the surface of submerged plants.

To uncover the source of food for barramundi, the TRaCK team used a second technique called stable isotope analysis. Natural chemical fingerprints based on the ratios of different naturally occurring isotopes of carbon, nitrogen and sulfur allow us to identify where and on what an animal has been feeding. Importantly, algae growing out on floodplains have a very different isotopic signature (ratios of different isotopes for each element) than other sources of food such as terrestrial plants, aquatic macrophytes, algae growing in the river itself, and algae growing in saltwater.

An animal at the top of the food chain can be linked back to the food eaten by the organisms it has consumed. In this case, barramundi feed on small herring-like fish (bony bream) and catfish that feed on the algae growing on flooded plants. In this way, the algal energy is transferred up through the food chain to barramundi. The logical conclusion from all this is that a bigger flood means more area underwater, and more time and space for algae to grow, fuelling the food chain and providing prey for barramundi. The end result: more barra in big flood years.

Barramundi return to waterholes, river channels and estuaries as the flood waters recede during the dry season, making them able to be caught by commercial fishermen and keen anglers. Analysis of samples collected from around the Mitchell River delta shows that algae growing on the floodplain accounts for about half of the barramundi’s biomass, with most of the remainder coming from prey living in saltwater (estuaries and coasts). This large contribution from the floodplain is despite the fact that it is normally underwater for only about 2 months of the year.

Barramundi grow best at high water temperatures, and the highest temperatures occur in mid-summer when the river is flooding. All of these factors form the recipe for a fish that is strongly connected to the floodplain.

But barramundi are not alone in their affinity for floodplains. What other creatures might benefit from the annual floods?

Research and anecdotal information constantly illustrates the importance of flood size and duration in determining catches of other commercially important species, such as prawns in Australia and Mozambique, and fish in the Mekong River of Cambodia, Laos and Vietnam. Floods can therefore help to generate a major source of protein in developing countries, as well as providing livelihoods for families who live in flood-prone areas.

There are other less commercially valuable but no less important species that use the floodplain for feeding, growth and reproduction. Several species of fork-tailed catfish are found in great quantities in flooded areas of northern Australia, and the bony bream that are eaten by barramundi are normally found in saltwater but breed on inundated floodplains. These are just a few examples of the many species that have come to rely on the floods brought by the predictable monsoonal rains in the wet–dry tropics. The net benefit to the ecosystem of a flood in any given year is therefore poorly understood but is likely to be enormous.

Together these measurements illustrate the importance of the floodplain, and why scientists and managers often talk about the “natural flow regime”. In many developed river systems, humans have greatly modified the riverine landscape, often eliminating natural floods altogether. These modifications include the construction of dams and weirs, channelling of water to increase the efficiency of run-off, and extraction of water for irrigation and other purposes. While these actions do wonderful things for humans, they also effectively disconnect the river from its floodplain.

In river systems that historically have received a large flood, native plants and animals have adapted to take advantage of the flood and do as much growing as possible. Taking away that flood, either through water extraction, diversion or impoundment, could therefore have serious consequences for these ecosystems and the fisheries they support.

Finding the right balance between water resource use and the ecological needs of aquatic ecosystems is a key challenge for both scientists and managers in the future.

Tim Jardine and Brad Pusey are Research Fellows at the Australian Rivers Institute, Griffith University. Ian Halliday is a biologist with the Queensland Department of Employment, Economic Development and Innovation.