Role of the physical landscape in determining patterns in a critically endangered stream fish – Galaxias 'species D'

The importance of understanding a species' intrinsic traits and relationship with the environment is key to implementing successful evidence-driven conservation management and protection. But for many species, especially small, rare, cryptic freshwater fish, significant knowledge gaps exist that limit the effectiveness of management action. This thesis examines and discusses key biological traits of a critically endangered stream fish, the Clutha flathead galaxiid (Galaxias 'Species D'), and the relationship between patterns in the species and the physical environment. This is in the hope of filling key knowledge gaps associated with the protection of this critically endangered species.

The Clutha flathead individuals examined from the upper Clutha Mata-Au possessed relatively large eggs, and both low absolute and size relative fecundity. Therefore, it has been labelled as an intermediate/slow life history galaxiid species. I have presented the first age data for the species using sagittal otolith analysis and gonad histology, which showed that the species is relatively late to reproductive maturity (approximately three years old) and possesses a long lifespan (ten years). Apart from somatic weight, there was no observed significant change in life history traits across environmental gradients, likely as a result of similar habitat use across the surveyed distribution.

Analysis of observed patterns of abundance in multiple age classes of the species revealed the effect of the physical environment across multiple scales. On a catchment scale, I examined a range of environmental variables and observed that the best predictor of adult Clutha flathead abundance was the amount of low stream order habitat within the stream network. On reach and microhabitat scales, it was observed that discharge and low order network area significantly increased larval habitat occupancy (reach scale). Backwater size was associated with both the presence and abundance of larvae, whereas conspecific abundance was only a significant driver of presence (microhabitat). This is the first research to examine the relationship between stream order extent and river resident galaxiid species.

The results of this thesis have important implications for the management of this species, and potentially also for the conservation of other river resident galaxiid species. Filling knowledge gaps surrounding intrinsic traits of a species can help inform what management decisions a species would benefit most from. The identification and protection of elements of the physical environment that are associated with increases in the abundance of a river resident galaxiid will help management action prioritisation. This will ensure that conservation efforts are as effective as possible in restoring galaxiid abundance.