Abstract
New Zealand’s diadromous fish populations face vast networks of road culverts that often limit upstream dispersal as a function of efficient hydrological design. Migration barriers, such as perched pipe culverts, fragment and isolate viable stream habitat from recruitment by overcoming migratory adaptations of fish, inevitably eroding populations through genetic loss. This study addressed several concerns of conservation managers in the eastern Otago region through examination of impassable perched pipe culvert impacts on local fish distributions, as well as trialling perched migration barriers in the laboratory and field in order to protect a threatened non-migratory galaxiid population from an invasive species. The latter trial was found to have great promise as a freshwater conservation management tool.
The interaction between poor road culvert design and fish dispersal has been examined across New Zealand’s differing landscapes. In eastern Otago, commercial forestry plantations provided homogenous land use type and pipe culvert design to examine perched pipe culvert prevalence and fish distribution in relation to pipe culverts as seen in Chapter Two. By using a unique approach identifying pipe culverts as passable or impassable a priori, based on previous fish passage understanding and research, the relationship between fish distribution and culverts could be examined despite inherently dynamic interactions between fish migratory adaptations and culvert characteristics. Surveys found just over fifty percent of pipe culverts were perched to some degree, and upon analysis that a negative relationship existed between species richness and fish abundance above versus below pipe culverts. The findings galvanise current understanding of the limiting effects of pipe culverts on upstream fish migration and identify the scale of perched culvert prevalence in commercial forestry plantations of the eastern Otago region.
Abundance trends, although statistically inconclusive but scientifically supported, affirm that impassable pipe culverts likely have a limiting/barrier effect on the dispersal and migration of kōaro (Galaxias brevipinnis). The climbing migratory adaptations which kōaro rely on to navigate in-stream obstacles were examined in Chapter Three. The relationship between climbing success and juvenile kōaro size was trialled in a controlled environment and revealed a significant trend in the size of juvenile kōaro climbers that were successful at navigating a simulated migration barrier. Disrupting the wetted margin, and thus hydrological connectivity, through the use of a perched barrier proved totally effective at halting juvenile kōaro climbing.
Kōaro are considered an invasive threat to populations of the rare non-migratory dusky galaxias (Galaxias pullus) in several tributaries of the upper Waipori River in eastern Otago. Attempts by the Department of Conservation to install migration barriers on a research weir to limit kōaro recruitment into a dusky galaxias stronghold population had proven unsuccessful. Chapter Four documents the development and deployment of a perched aluminium migration barrier which continued to be extremely successful over a period of several years at halting juvenile kōaro recruitment as confirmed by electrofishing and kōaro relocation data pre and post installation. This system provides conservation managers with a tool to protect non-migratory galaxiids across New Zealand that are at risk of juvenile kōaro invasion, and for the protection of historic habitat for non-migratory galaxiid translocation.
Investigation into kōaro migratory capabilities also raised new questions about the interactions between form and function of climbing adaptations and how these may have influenced the evolutionary ecology of ancestral climbing galaxiids in response to geological processes. Discussions regarding the complexity of kōaro migration as a dynamic process of timing, growth and distance are also presented.
Understanding the mechanisms by which perched pipe culverts limit upstream fish migration, and on which migratory fish species rely, allows for conservation managers to improve fish passage or control invasive fish species encroachment, both with the purpose of conserving endangered species and in-stream habitats. Overall, critical thinking and understanding of fish migratory adaptations, both in a natural setting and a controlled environment, has proven a robust method in developing a useful conservation tool for endangered species isolation management while working closely alongside front line conservation managers.