Assessing the effect of water resource development on fisheries in the wet-dry tropics

Abstract

The growing demand for freshwater resources has led to dam construction and water diversions in a majority of the world’s large rivers. The regulation of river flows, coupled with changes to land use in and around coastal areas, has far-reaching consequences not only for the ecology of rivers but also for fisheries in marine ecosystems. The challenges of quantifying the influence of regulated flows on marine fish stocks is complicated by our poor understanding of how flow induced processes interact over spatial and temporal scales throughout a species life history. The life history of the banana prawn, Penaeus merguiensis, is an ideal template to assess the potential impact of river regulation on the catch of a commercial fishery. The banana prawn is an estuarine-dependent marine species that spawn offshore and use estuaries as nursery areas. This research will ask how water resource development in three northern Australian rivers, the Mitchell, Gilbert and Flinders Rivers, affects population dynamics and catch of banana prawns in the south-eastern Gulf of Carpentaria. The extreme climatic variability in northern Australia raises concerns about how river regulation will impact marine fisheries. In Chapter 2, a spatio-temporal Bayesian modelwas developed to predict the impact of river regulation on the catch of banana prawns. The analyses of three river regulation scenarios found that catch was most impacted by river regulation during low flows. The impact of river regulation was greatest for a scenario with dams on the Mitchell River, where the model predicted catch would decline by 53% during a year with low flow. These results imply that maintenance of low-level flows is a crucial requirement for sustained fishery yields. Further, these findings suggest that water managers must balance the upstream demand for water during drier years against the impact on downstream prawn fisheries. Banana prawns have complex life histories where survival in the early life stages is largely dependent on prevailing oceanic conditions. Chapter 3 evaluated the effects of river flows, wind, tides and currents on larval connectivity from spawning areas to estuarine nurseries. To identify patterns in larval connectivity the output from a high resolution three-dimensional Regional Oceanic Modelling System (ROMS) was coupled to Ichthyop, a biophysical Lagrangian particle tracking model. The probability of larvae transported from release zones to nursery areas was computed under different river flow and coastal current conditions. This research found that the importance of each river’s role as a nursery area changed throughout the season and depended on the timing of river flows and direction of coastal currents. Further, high river flow events, >5,500 m3 s-1, blocked post-larval ingress to estuarine nurseries altogether. A key outcome of this research was the development of an index of larval connectivity for each river to be used in a population model. Alterations to the magnitude, timing and duration of flows can affect different life stages, either directly from an individual prawn’s biological response to changes in salinity or temperature, or indirectly by modifying habitat, movement patterns and the availability of food. In Chapter 4, a population dynamics model was developed to simulate the effect of different river flow regulation scenarios on larval, juvenile and adult life stages. Sensitivity analysis and Bayesian calibration were used to determine how uncertainty in key parameters were driving the variation in modelled results. The analyses of low and high flow scenarios found the contribution of juveniles from each nursery to the adult population was lower with increasing levels of water extracted. This key finding suggests maintaining historic mean flow levels during periods of low flow or drought will be critical to sustain juvenile production and fishery yield. Overall, this research has demonstrated the dynamic and complex interaction between river flows and the life of banana prawns. The models developed provide an insight into how water resource development and the fishery would benefit from an integrated planning and management approach.