Links between Gulf rivers and coastal productivity

The Mitchell, Gilbert and Flinders rivers flow into the southern Gulf of Carpentaria, supporting healthy ecosystems and nationally significant wetlands as well as important recreational and commercial fisheries. Banana prawns, barramundi and other important fishery species use estuarine ecosystems at different points throughout their life cycle. The Queensland State Government has also identified these river systems as catchments where irrigated agriculture is already occurring or likely to occur.

Specifically, water managers needed to know which flow characteristics of the rivers earmarked for future development are most important for the region’s plants and animals so they can make informed management decisions. This project has provided important new information about why freshwater flows in the Gulf estuaries and floodplains are essential for banana prawn and barramundi fisheries and the broader estuarine ecosystem, and modelled the potential scale of the impact of water extraction on the fisheries.

This project:

• quantified end-of-river nutrient concentrations and loads from wet season flows
• measured coastal/estuarine primary productivity in the three different Gulf estuaries
• quantified densities and size structure of juvenile banana prawns in the estuaries
• modelled how changes in flow may affect banana prawn catch
• determined how flows affect growth rates of barramundi and their use of the river systems
• quantified the economic impacts of changes to flows on the commercial barramundi fishery and the Northern Prawn Fishery in the Gulf of Carpentaria
• identified areas and periods of floodplain inundation using remote sensing and used this to estimate potential floodplain primary productivity.

Location of the three study rivers in north Queensland.

See executive summary and final reports for full findings and implications.

• Maintaining flows in low- and medium-flow years is critical for sustaining estuarine productivity. These flows deliver essential nutrients that fuel primary productivity. This has flow-on effects to the whole food web that relies on the primary producers such as algae.
• First-flush flows at the start of the wet season are also crucial for delivering nutrients which stimulate productivity, providing food for fisheries species and other species such as migratory shorebirds. The first flush is also important in reducing estuarine salinity, which is typically hypersaline in the late dry season and stressful for the plants and animals living in the estuaries.
• In low- to medium-flow years, reduction of flows from water extraction will keep salinities high, making prawns less likely to move from the estuaries into the offshore fishery and contribute to the next generation. Fisheries catch and profits will be affected in both the short and long term.
• Water extraction in a year following multiple years of low-to-medium flow will have major impacts. This scenario is not unusual, particularly for the Flinders which has the highest interannual variability in flow of the three rivers and can have multiple consecutive years of no to low flow. Climate and weather changes could also amplify these effects in coming years.
• Modelling suggests that reductions in flow will affect commercial prawn and barramundi catch. Significant infrastructure projects such as dams may have major impacts on these fisheries, including substantial reductions in short-term fisheries profits.
• Commercial fishery harvests reflect only one value delivered by water resources in these catchments. Other economic values include tourism revenue from recreational fishing and Indigenous harvesting of fish and other species. Species and habitats also have significant environmental value. Water-resource development could put this full suite of values at risk.
• Gulf rivers do not have discrete catchments. During the wet season, these rivers sometimes connect to become one large floodplain, e.g. the Norman, Gilbert and Flinders rivers. The cumulative impacts of water-resource development should be considered across multiple river systems, not by individual catchments.
• Given the variability in flow from year to year, and from river to river within a year, all Gulf rivers are important for sustaining downstream fisheries as well as estuarine ecosystems in general, as the network of rivers flowing into the Gulf of Carpentaria buffer one another in low-flow years.
• It may take some years to determine the impacts of water development on productivity in the estuaries, given the natural variability of the Gulf systems. Water allocation should occur on a precautionary basis.
• Inundated floodplains are crucial habitats for many species including barramundi, and provide a nutrient source for rivers and estuaries as floodwaters drain. This study has provided important information on the scale and duration of flooding and how it differed from year to year between the three river systems. The longer periods of inundation of the Mitchell River floodplains (see more here) mean that floodplains play a more important role for barramundi in the Mitchell than in the other two rivers. Water development needs to ensure that floodplain inundation is maintained.
• The modelling capacity developed in this project has the potential to be used by management agencies to evaluate future water-resource development scenarios and provide a platform which can be enhanced over time as new knowledge is developed.
• A key remaining challenge is linking water development in the upper reaches of the rivers with impacts hundreds of kilometres downstream at the end of the catchment. This project could not directly address the explicit connections between upstream development and downstream impacts, but this should be given consideration in terms of predicting impacts.

There were many similarities in the physical characteristics of the Mitchell, Gilbert and Flinders estuaries. However, the Mitchell River usually has more consistent flow year-to-year, while the Flinders has the most extreme conditions, often ceasing to flow in the dry season. There were also no consistent differences in the nutrient loads between the estuaries, with the scale of the freshwater flow from year to year being the major driver of variability. The Flinders estuary was generally the most productive in terms of primary productivity rates (i.e. algal growth rates), compared with the Mitchell and Gilbert estuaries.

The project tested whether the input of nutrients to the mudflats was important for stimulating primary production on the mudflats. Primary productivity rates increased in all three estuaries in response to experimental nutrient inputs in both the dry and wet seasons, indicating that that all three estuaries were chronically nutrient-limited during our study and that wet-season inputs of nutrients are critical to maintain primary productivity. This has flow-on effects to food availability for animals within the system.

Research was conducted in Gulf estuaries through fieldwork and remote sensing. Photo Stephen Faggotter.

The large-scale assessment of the productivity of the Flinders and Gilbert floodplains integrated remotely sensed biophysical indicators (vegetation and inundation) with hydrological data (rainfall and river discharge).

In the Flinders, long-term (10–30 years) modelling showed that the area of the catchment inundated in the wet season ranged from less than 50 km² up to 2,500 km², although in most wet seasons the area inundated was less than 500 km². Areas ranging from less than 50 km² up to 1,000+ km² were inundated in the Gilbert, and estimated average inundation in the Mitchell ranged from 50 to 500 km², with most years less than 200 km². Overall, the Gilbert and Mitchell catchments had similar areas of inundation in the wet season over the long term. However, the Mitchell catchment was inundated for longer compared to the other two catchments, meaning more time for aquatic vegetation to become established and increase productivity.

Spatial patterns of floodplain inundation (blue) and impact on the connectivity of freshwater habitats in downstream Flinders. Panels (a–f) show the characteristics of inundation in riverine systems and intermittent waterbodies for some wet season periods as indicated, in 2009, 2010, 2016 and 2019.

Juvenile banana prawns use Gulf estuaries each year for months as feeding and refuge areas. This project found that the densities and the total number of juvenile banana prawns in each estuary varied substantially between years, with no statistical differences between estuaries – so the relative importance of each river for this fishery varies from year to year.

The project confirmed previous studies in other Gulf rivers showing that during the wet season, most juvenile banana prawns leave the estuaries for offshore areas in response to low salinity and food availability. Three water development scenarios were tested and the effects of these reduced flows on prawn catch in the offshore fishery were examined. The model predicted a significant decline in banana prawn catch with decreasing flow levels from water extraction. Catch was most heavily impacted by water extraction during low-flow years (one year in five) for all three rivers. The greatest impact of water extraction occurred in the scenario with a dam on the Mitchell River, where a 53% reduction in catch was predicted during low-flow years. Overall, these results imply that maintenance of low-level flows is a crucial requirement for sustained fishery yields.

Building on the modelling research component, the project also assessed the impact of irrigation extractions on the financial performance of the banana prawn fishery. Results suggest that vessel-level business profit could reduce by between 7 and 12% if currently granted entitlements and planned allocations were extracted from Gulf rivers, or by around 22% if major dams are also constructed in the Mitchell River [link here]. Fishing effort could be reallocated from the highly productive and economically important south-eastern Gulf to other regions of the Northern Prawn Fishery, but it is unclear how much revenue shortfall could be recovered.

Banana prawns are a key prawn species fished in the Gulf of Carpentaria. Photo: Michele Burford.

Estimating the economic impact of potential extractions from Gulf rivers on the banana prawn fishery (component final report).

Barramundi otoliths (ear bones) from the three rivers were analysed to quantify the relationship between river discharge volumes and growth. Across all three regions, total river discharges from January to March each year (i.e. strong wet seasons) had a strong positive effect on growth rate of barramundi up to three years old. This relationship was particularly strong in the Gilbert and Flinders regions which experience intermittent, highly seasonal flows.

Juvenile barramundi growth rates were modelled under a hypothetical severe water extraction scenario for the Mitchell region. Models predicted a 19% reduction of juvenile barramundi growth rates from spawning to about two years of age. This means that fewer, smaller fish are available for the commercial and recreational fisheries, given that smaller fish are more vulnerable to predation.

Analysis of barramundi otoliths indicated variable use of freshwater habitats by fish in the Mitchell, Gilbert and Flinders rivers. Most fish in the Mitchell River stayed in the freshwater reaches, most fish in the Gilbert River stayed in the brackish areas, while most fish in the Flinders River stayed in the estuary. These results indicate that differences in the geomorphology and inter-annual river flow patterns between the Mitchell, Gilbert and Flinders rivers affect spatial and temporal connectivity of aquatic habitats (including seasonal floodplains) used by barramundi. Water-resource development should aim to minimise disruption to the frequency and duration of floodplain inundation and connectivity along the river system to maintain the natural productivity of barramundi in Gulf of Carpentaria catchments.

Currently, the long-established Gulf of Carpentaria Inshore Fin Fish Fishery harvests barramundi and other species (e.g. king threadfin and mud crabs) that rely on estuarine productivity. This is supported by the patchwork of river flows in time and space. This reiterates that the Mitchell, Gilbert and Flinders rivers all contribute to the production of estuarine fisheries, although variably between years.

The Gulf of Carpentaria and associated catchments are vulnerable to extremes in climate, as evidenced by successive poor wet seasons (e.g. 2013–2015). Our findings suggest barramundi population dynamics are most sensitive to altered patterns of river flow during and immediately after successive years of low flow. Water-resource development in the Mitchell, Gilbert and Flinders rivers should aim to avoid exacerbating or prolonging the impacts of climate variability on Gulf of Carpentaria barramundi populations.

The relationship between monthly barramundi commercial catch, effort and river flow in each catchment between 1990 and 2010 was determined to estimate the economic cost to the barramundi fishery from water extraction. The project tested the most likely water-development scenarios in each catchment. This study component showed that the net present value (NPV) of the average commercial barramundi licence under baseline conditions would range from approximately $146,000 in the Flinders River catch zone to approximately $451,000 in the Mitchell River catch zone. Water extraction reduced NPV by 6%, 9% and 6% for the Mitchell, Gilbert and Flinders catch zones respectively. For irrigated agriculture, obtaining an NPV sufficient to deliver an acceptable return on investment would only be possible under a limited set of circumstances, i.e. optimal growing conditions for agricultural crops and major local processing facilities funded by a third party.

The project assessed the relationship between monthly barramundi commercial catch, effort and river flow in each catchment between 1990 and 2010 to estimate the economic cost to the barramundi fishery from water extraction. Photo Michael Lawrence-Taylor.

You can find out more about the implications for the barramundi fishery in this paper and the trade-offs between irrigated agriculture and fisheries in this component report. This project has improved understanding of estuarine and riverine food webs in the Gulf rivers.