Estimation of native fish density in lowland streams by repeated electric fishing during the day and following night

Abstract

Single reaches in three lowland streams in Canterbury, New Zealand, were enclosed with stop-nets and fish removed by repeated electric fishing passes through the day and following night during base flow conditions in late summer. The proportion of fish caught in each pass varied between species and increased with fish size. Electric fishing efficiency declined in the second and third daytime passes, to 0.73 and 0.79 of the first pass respectively, but increased in the first and second night-time passes to 1.87 and 2.02. Population estimates, based on the first two or three daytime passes, accounted for only 62% and 82% of the population estimated using all five day and night passes. A high percentage of small benthic species remained hidden within the substrate during the day but emerged at night and became more vulnerable to electric fishing. Single or multiple daytime passes can be used to estimate the approximate density of most native fish species and size classes in small lowland streams and rivers provided calculations take into account fish hidden within the substrate and marginal vegetation.