Measuring the effect of river rehabilitation for fish: logistical constraints on experimental design
Dead trees and branches occur naturally in many undisturbed streams or rivers. Accumulations of wood in rivers are often referred to as snags (or more technically as ‘Structural Woody Habitat’). Snags benefit fish populations by increasing channel complexity and by providing many different sorts of micro-habitats. The loss of habitat complexity that results from the removal of snags can limit the productivity and diversity of aquatic ecosystems. The previous removal of snags and the clearing of native vegetation along river banks can also allow alien fish such as carp to replace native fish in Australia’s rivers and streams. Consequently, the reinstatement of snags is seen as an important technique to improve native fish survival and reduce the dominance of alien fish. A current research project in a 10 km segment of the Upper Hunter River below Muswellbrook in NSW is investigating resnagging as a means of conserving native fish and improving fishing opportunities for recreational anglers.
The response of the fish community in this stretch of the Hunter River will be assessed following the introduction of new snags. Control segments of the river (with no new snags) and treatment segments (with new snags) will be sampled for 1 year before, and for at least 2 years after, the snags are placed in the river. Large, pre-constructed snags (affectionately known as ‘fish cabins’) will be placed in pools and smaller ones (called ‘log jams’) will be built in the shallower areas between pools to represent a range of different fish habitats.
This paper describes the key elements involved in assessing the performance of these new snags: predictive statements about how each fish species is expected to respond to snags, the arrangement of the new snags in the study area, electrofishing surveys before and after the introduction of the cabins, use of directional nets, and tagging/radiotracking of individual fish.
Additional sampling will monitor the fish community throughout the Upper Hunter catchment and will examine in detail how native and alien fish use snags and other types of habitats. Eventually, the project hopes to provide crucial information on issues of experimental design, field sampling techniques, snag design and placement and fish response to snags, all of which will help guide future river restoration efforts.