Population estimation methods to quantify temporal variation in fish communities downstream of a weir
Fifty-five freshwater fish species inhabit rivers in NSW, Australia. At least 36 of these have a requirement to migrate at some stage of their life. Adult native fish have been previously observed undertaking large-scale upstream migrations of up to 1,400km whilst juveniles also perform mass upstream migrations. Dams and weirs often prevent successful migrations by blocking pathways to spawning habitats and feeding areas. These structures have been widely implicated in the declines of many freshwater fish species in NSW rivers.
Many species of fish are known to accumulate in high numbers and densities at sites where migration is obstructed. Although this phenomenon is widely documented, there are few methods currently available to measure the size and seasonality of these occurrences. Such information is of importance because it would allow fisheries managers to develop measures to ameliorate the effects of weirs that are specifically tailored to local fish assemblages.
This paper reports on the accuracy of Chapman-Petersen and Jackknife removal techniques to determine the total number of fish accumulating downstream of a low level weir. The Chapman-Petersen technique uses a mathematical formula to estimate population size based on tag and recapture information. Jacknife Removal methods are based on the idea that repeatedly removing fish from an area (termed ‘depletion sampling’) will result in fewer catches each time. Population size can then be calculated after evaluating the number of fish caught in each depletion sample.
In this study, temporal changes in the population sizes of seven migratory species were assessed over a two year period prior to the construction of a fishway on the Murrumbidgee River in southwestern NSW. No differences in population size estimates were detected between methods. However, the Jackknife removal technique generally provided more accurate estimates for a greater number of species. Total fish numbers were greatest during summer and autumn when bony herring and common carp dominated the migratory community. Both estimators proved useful and their application will help to further the development of appropriate fish passage facilities in the Murrumbidgee River. The techniques could be easily applied to other sites where the size of migratory fish populations is of interest.