Brad Pusey

A life-history account of Macrobrachium spinipes (Schenkel, 1902) (Cherabin) in a large tropical Australian River

Rivers in tropical Australia are largely pristine, and ecosystem connectivity is intact, but water resources in northern Australia are under increasing development pressure. A better understanding of the role and life histories of key migratory species is urgently needed to manage the risks posed by development. We investigated the life history of Macrobrachium spinipes (Schenkel, 1902) in the Daly River of the Northern Territory, Australia. We carried out a 13-mo trapping program (2011–2012) at 7 sites along the river between 92 and 405 km from the estuary. We provide the first detailed account of spatial and temporal changes in relative abundance and reproduction, and present information on length at first maturity and Fulton’s body condition factor. Reproduction was strongly seasonal and was restricted to the wet-season months of November to April, followed by a recruitment pulse 3 to 4 mo later during the early dry season. Length at first maturity declined as the reproductive season progressed but did not vary significantly with distance from the estuary. Condition was temporally variable and peaked for both sexes immediately after the wet season before declining throughout the dry season. Relative abundance patterns followed a similar pattern. Contrary to results for other Macrobrachium species in rivers of comparable length, we found an increase in the relative abundance of females throughout the river’s length during the reproductive season and reproductive effort occurring far upstream, over a restricted time period. This result highlights the importance of maintaining connectivity for reproductive success of this species and further recruitment into these upstream reaches.

Ontogenetic shifts in habitat use during the dry season by an amphidromous shrimp in a tropical lowland river

Caridean shrimp have considerable effects on ecosystem processes and, thus, understanding their use of key habitats is important for determining their potential ecological effect. The present study examined the meso-habitat use of Macrobrachium spinipes, a large-bodied and important amphidromous species, in the Daly River, northern Australia. We examined shrimp abundance at four common meso-habitat types; sand bank, rock bars, undercut and structurally complex banks and mid-channel areas at five sites on three occasions during the dry season (May to October). We found that habitat use changed considerably first, with ontogeny, and, second, with the colonisation of habitats with algae and macrophytes as the dry season progressed. As juveniles, their habitat use was strongly associated with well structured bank environments early in the dry season. By the mid-dry season, juveniles were more abundant within sand habitats recently colonised with macrophytes and filamentous algae. Females showed little change in habitat use, whereas large-bodied dominant males generally favoured rock bars and heavily structured bank environments. The present study has provided significant insights into the changes in use of key riverine habitats throughout the dry season by an ecologically important species. This information will be of considerable value to the determination of environmental flow requirements and food-web investigations.