Estimating Fish Populations by Removal Methods with Minnow Traps in Southeast Alaska Streams

Abstract

Passive capture methods, such as minnow traps, are commonly used to capture fish for mark–recapture population estimates; however, they have not been used for removal methods. Minnow traps set for 90-min periods during three or four sequential capture occasions during the summer of 1996 were used to capture coho salmon Oncorhynchus kisutch fry and parr, Dolly Varden Salvelinus malma, cutthroat trout O. clarki, and juvenile steelhead O. mykiss to estimate population size with the Zippin or generalized removal method. More than 45% of the total catch was obtained during the first capture occasion, and in most cases, the catch during the fourth occasion was less than 15% of the total catch. In most pools, the probability of capture was greater than 0.4 but was lower for coho salmon fry than for coho salmon parr and other species. Mean population estimates for coho salmon parr made with concurrent mark–recapture and removal methods differed significantly in small streams. Estimates from mark–recapture and removal methods were not significantly different for coho salmon fry and Dolly Varden, but mark–recapture estimates were higher than removal estimates in most cases. My results show that removal estimates can be obtained with minnow traps if sampling procedures conform to the assumptions required for the method.