ESTIMATING RELATIVE ABUNDANCE OF JUVENILE BROWN TROUT IN RIVERS BY UNDERWATER CENSUS AND ELECTROFISHING

Underwater census and single-pass electrofishing were compared for est imating relative abundance of juvenile brown trout in the Kakanui Rive r, South Island, NZ. Mean sampling efficiency was lower, and the varia bility of sampling efficiency was much greater, for underwater census (0+ trout: xbar = 0.38, s = 0.368; 1+ trout: xbar = 0.62, s = 0.822) t han for single-pass electrofishing (0+ trout: xbar = 0.61, s = 0.143; 1+ trout: xbar = 0.74, s = 0.171). Sampling efficiency of both methods was dependent on temperature. Electrofishing became less efficient at higher temperatures whereas underwater census became less efficient a t colder temperatures. The low, and highly variable, sampling efficien cy for underwater census of 0+ brown trout was related to substrate hi ding behaviour which is dependent on temperature. A ratio method for c omparing relative abundance estimates is presented. Minimum significan ce values for the ratio (R) were derived for 0+ trout using temperatur e adjusted sampling efficiencies. To be statistically significant, rel ative abundance estimates made by underwater census had to differ by a factor of 6-7 times, whereas those made by single-pass electrofishing had to differ only by about 2 times, depending on the number of fish counted. By confining comparisons of relative abundance estimates made by underwater census to the summer period, differences of about 3.5-4 times could be detected statistically. It was concluded that single-p ass electrofishing is superior to underwater census for estimating the relative abundance of juvenile brown trout in shallow (< 1 m) river h abitat, especially when temperature varies widely as with season and t ime of day.