Fish density variations in the braided Ashley River, Canterbury, New Zealand

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Abstract
To determine variation in abundance (m‐2) and biomass (g m‐2) of fish populations in the lower Ashley River, New Zealand, a representative reach was sampled by electric fishing during periods of low, stable flows in two consecutive summers (1981 and 1982). For nine species of fish combined, abundance and biomass in riffles differed by 50% and 41% respectively between the two summers. Size frequency distributions for three of the most common migratory species differed significantly between summers and may partly explain the variation in fish standing stock. Abundance of fish, mostly under 150 mm body length, was highest in riffles (3.0 m‐2), whereas biomass was highest in pools (46.5 g m‐2) owing to the presence of a few large eels and trout. Within riffles, there was no significant difference in the longitudinal and cross‐sectional distributions of the more common fish species present. Both abundance and biomass of fish were lowest in runs, the habitat with the least apparent cover for fish. Limiting sampling of fish populations in braided rivers to riffles, which are easily sampled by electric fishing, provides adequate representation of species composition and temporal variation of their abundance and biomass.

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