Reproductive ecology of Australia’s wet-dry tropical freshwater fish

: insights into hydrological and other environmental determinants of fish recruitment

Abstract

The rivers of northern Australia are among a shrinking group of rivers which have so far largely escaped the seemingly ubiquitous impacts of human development worldwide. To date, these rivers have seen only low levels of human development relative to comparatively sized rivers elsewhere in Australia, and climatologically similar regions throughout the world. However, this may change, with governments now increasingly supporting further development of northern Australia through industry, agriculture and mining activities; all of which put pressure on the values and uses of the region’s water resources, particularly river flows.

Natural variability in a riverine flow regime is now widely recognised as being crucial for the reproductive success of fish species. However, the flow-reproductive ecology of Australian freshwater fishes has been patchily described, particularly for those species in the far northern wet-dry tropics: the region that houses a large proportion of the continents freshwater fish fauna. Even the most fundamental information that is critical for understanding environmental influences on recruitment, such as at what time of year and where does a fish species breed, is lacking for many species. Knowledge on the flow-reproductive ecology relationships of this fish fauna is essential for management of water resources in the region and for ongoing maintenance of biodiversity values.

The research described in this thesis investigated the hydrological and other environmental determinants of freshwater fish reproduction in the Australian wet-dry tropics. Using a model river catchment (Daly River, Northern Territory, Australia), the project aimed to: (1) identify temporal and spatial patterns in fish reproduction; (2) describe the abiotic and
biotic variables most strongly associated with variations in fish larval abundance and diversity; and (3) determine whether spatial and temporal variation in the contribution of groundwater out-welling to the rivers discharge affects the growth and abundance of fish larvae. Information was collected and interpreted with the objective of increasing our understanding of the drivers of fish recruitment in the tropics, in order to enhance the sustainability of water resource management in the region.

Results showed that fish reproduce in all hydrological seasons, but discrete taxa assemblages spawn in high-flow and low-flow periods, and in perennially and intermittently flowing reaches. Fish larvae were most abundant, and larval assemblages most diverse, where food was abundant and in warm and structurally complex habitats. These results were largely independent of the hydrological regime or season, thus indicating that similar biotic (i.e., high food availability) and environmental mechanisms (i.e., warm and hydraulically benign conditions) may be generally important for fish reproduction within this catchment. Furthermore, the fastest growth rates of larval fish of one species (Craterocephalus stramineus) in the low flows of late-dry season were found to occur in areas of, or downstream of, warm groundwater out-welling. In addition, groundwater out-welling zones subsequently supported increased juvenile C. stramineus recruitment in the wet season, presumably through the provision of suitable rearing conditions (i.e., warm, productive and hydraulically benign habitats) to support abundant larvae during the late-dry season.

A main result of this thesis is that both the dry and wet seasons, and perennially and intermittently flowing reaches, are important for the reproduction of species with diverse habitat requirements. This suggests that natural seasonal and spatial hydrological variation needs to be protected to maintain sufficient habitat heterogeneity and productivity to support high levels of both diversity and abundance of fish in the catchment. This thesis contributes to our understanding of the fundamentals of, and mechanisms behind, the reproductive ecology of tropical freshwater fish and the food and habitat resources that young fish require to survive and recruit into the population. Importantly, these findings highlight the spatial and temporal scales at which threats to fish reproduction, such as the impacts of water extraction, need to be managed.

Date of Award	2 Dec 2021
Original language	English
Supervisor	Sam Banks (Supervisor) & Alison King (Supervisor)