The role of flow regime and movement in stream shrimp assemblages

Researchers have long sought to understand what governs the distribution of organisms. For riverine animals and plants, there is often a strong relationship between hydrology (flow) and distribution and assemblage patterns. The dispersal behaviour and “inherent mobility” of organisms is thought to play role in this relationship. This study examined the distribution patterns and movement behaviour of three Caridean shrimp species in the south-eastern region of the Murray-Darling Basin in the southeast of Australia, and related those patterns to flow. To do this, I applied multiple methods, including: basin scale occupancy modelling; monitoring of spatiotemporal distribution and abundance patterns; directional trapping to detect dispersal events; and laboratory experiments that compared the ability of the three species to maintain position and resist displacement.

The results demonstrated that flow regime seems to play a substantial role in the distribution and composition of shrimp communities throughout the Murray-Darling Basin, and dispersal and “inherent mobility” is likely to play strong roles in this relationship. Caridina mccullochi was found to be restricted to lowland rivers that experience relatively few high flow events in summer, which is typical of the natural flow regime in the southern Murray-Darling Basin. Macrobrachium australiense and Paratya australiensis, on the other hand, seem to be more tolerant of the more frequent summer high flow events in many of the more heavily regulated rivers.

Caridina mccullochi seems to be restricted to lowland river habitats for its entire life history, whereas Paratya australiensis can complete their life history in both lowland and upland river reaches. Macrobrachium australiense occurs further upstream than Caridina mccullochi, but it appears that this species cannot complete its life cycle in upland reaches, and it is likely that individuals that occur in the upper reaches were spawned further downstream. Macrobrachium australiense was found to undertake upstream movements during early-to-mid summer. In contrast, there was no evidence to suggest that Paratya australiensis and Caridina mccullochi undertake significant movements, and are likely to be restricted to the relatively small area for their entire life history. Key to these findings was the discovery that Caridina mccullochi has a more limited ability to maintain position in flow compared to Paratya australiensis and Macrobrachium australiense, and also lacks a strong rheotactic response to flow.

These results have highlighted the importance of recognising both dispersal behaviour and “inherent mobility” of shrimp to understanding distribution and assemblage patterns. In the past, it has been hypothesised that differences in both habitat preferences and spawning and recruitment strategies could also result in different distribution and assemblage patterns under different flow regime conditions. Consequently, any useful framework for understanding freshwater shrimp populations should incorporate their movement and life histories, as well as their large-scale habitat relationships. This study has also highlighted the potential importance that flow regime management plays in the future conservation of not just shrimp but also the broader ecological community in the Murray-Darling Basin.