Quantifying impacts of altered river flow on catches of giant mud crabs

Abstract

Natural variation in river flow can impact fisheries production in downstream ecosystems, including for the estuarine-dwelling Giant mud crab Scylla serrata. This portunid crab is widely distributed across the Indo-Pacific and makes important contributions to commercial, recreational and Indigenous fisheries.

Methods
To quantify the impact of altered flow on mud crab abundance and catch in the Gulf of Carpentaria, we first developed an age-structured, spatially-structured population model ensemble to accurately capture their population dynamics. We then explored if environmental drivers, primarily river flow, could be used to improve model estimates and help explain the observed variability in historical catches. Once validated using observed catches, this model was then applied to predict expected changes in mud crab abundance and fishery catch given a variety of flow regimes resulting from different water resource development scenarios.

Results
River flow significantly improved the ability to explain historical catches for some regions but not all. Moreover, the strength of the relationship between flow and abundance varied across catchments. Reduced flows had negligible effects for perennial rivers but for ephemeral and temporally variable rivers, a reduced flow resulted in substantial decreases in abundance and catch (range: 36 - 46% on average). The magnitude of these decreases was dependent on the type and amount of water extraction.

Conclusion
Our dynamic modelling approach provides a rigorous means of quantifying the ecological impacts of altered river flow on high-value fishery species. It has potential to help inform natural resource management, including policy decisions on the timing, quantity and method of water removed from rivers.