Are Block Nets Necessary?: Movement of Stream‐Dwelling Salmonids in Response to Three Common Survey Methods

Abstract

AbstractFish movement during sampling may negatively bias sample data and population estimates. We evaluated the short‐term movements of stream‐dwelling salmonids by recapture of marked individuals during day and night snorkeling and backpack electrofishing. Bull trout Salvelinus confluentus and rainbow trout Oncorhynchus mykiss were captured in sufficient numbers to evaluate instream movements and the influence of stream habitat on movement. Salmonids moved upstream more often than downstream, which suggests that fish fled in response to sampling procedures. Fish movement rates were related to sampling method and stream habitat characteristics. The proportion of bull trout (mean total length, 148 mm) moving upstream from a 50‐m reach was 28% for day snorkeling, 25% for night snorkeling, and 18% for single‐pass electrofishing. The likelihood of rainbow trout movement upstream during sampling declined as the percentage of rubble substrate increased. Rainbow trout (mean total length, 139 mm) traveled short distances; 67% of mobile rainbow trout traveled 25 m, whereas 59% of mobile bull trout traveled more than 50 m. Our results suggest that fish movement out of sample sites during sampling may negatively bias abundance estimates and positively bias estimates of detection probability. We suggest that block nets are necessary to ensure population closure when estimating population abundance for stream‐dwelling salmonids. We further caution that, when fish flee in response to sampling, observations of habitat use may only apply to the proportion of fish that remained stationary and that were observed.