Comparison of fish populations in riffles of three braided rivers of Canterbury, New Zealand

Abstract

Abstract Populations of small fish were sampled in 12–20 riffles of the lower reaches of 3 braided rivers in Canterbury, New Zealand, during periods of low, stable flows. In the Ashley, which has been least affected by floods in recent years, the standing stock of fish was severalfold higher than in the Hurunui and Rakaia Rivers, which experienced large floods over much of the time, particularly the Rakaia River. Mean abundance + 1 standard deviation (S.D.) of all species combined, amounted to 5.95 + 2.76 fish m 2 for 10 species in the Ashley, 0.59 + 0.60 fish m 2 for 7 species in the Hurunui, and 0.23 + 0.11 fish m 2for 6 species in the Rakaia. Mean biomass was 24.85 + 9.59, 2.11 + 1.19, and 2.50 ± 3.60 g m 2in the Ashley, Hurunui, and Rakaia Rivers, respectively. The more common species in the Ashley and Rakaia Rivers were torrentfish, longfinned eel, blue‐gilled bully, and upland bully. The same dominant species complex prevailed in the Hurunui River except that the common river galaxias was present instead of the bluegilled bully. The adverse effects of floods on fish, either directly, indirectly, or both, are considered to be a major limiting factor offish populations in braided rivers characterised by highly unstable flows and river beds, and a lack of suitable cover for fish.