Populations and communities

Drivers of galaxiid fish distributions and population dynamics in trout-invaded riverscapes

A large rainbow trout, Oncorhynchus mykiss.

The distribution of fish populations across the landscape of a river catchment, or “riverscape”, can offer insights into the biotic and and abiotic factors controlling population dynamics. Trout appear to have extirpated non-diadromous galaxiid fishes from streams across New Zealand, but still co-occur with two species in the Canterbury high country. As part of his PhD research, Darragh Woodford investigated the spatial configuration of Galaxias vulgaris and G. paucispondylus populations within river networks partially invaded by trout in the Waimakariri basin.

Galaxias vulgaris (Fig. 2) appear to form source-sink metapopulations within trout-invaded river networks, with trout-invaded reaches becoming demographic sinks, while streams above trout barriers become demographic sources. Galaxias vulgaris are excluded from small, stable trout streams that are far from potential source populations in the riverscape. Darragh is in the process of modelling these spatial interactions in GIS (Fig. 3) to provide a management tool for G. vulgaris.

Figure 2. Galaxias vulgaris	Figure 3. A GIS-based map predicting ability of trout to eliminate Galaxias vulgaris within a river network

In contrast to G. vulgaris, G. paucispondylus appears to co-exist with trout in most situations, although it is also eliminated from reaches containing good holding water for large trout. Darragh is investigating how G. paucispondylus may utilise habitat cover to more effectively avoid trout predation than G. vulgaris.

People involved in project

University of Canterbury
Darragh Woodford (Completed PhD student)
Angus McIntosh
Marie Hale
Tom Cochrane

Department of Conservation
Dave Kelly