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

Abstract

Fish 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.

Fish catchability and comparison of four electrofishing crews in Mediterranean streams

“…First, similarly to Simonson and Lyons (1995) we observed increased catch rates (10-40%) in a single pass with the use of block nets (compared to a single-pass without block nets) but no clear effects on observed richness and relative abundance. Although a single pass without nets may sometimes be appropriate to estimate abundance (given the correlation with removal estimates) and richness, block nets are necessary for removal estimates (Peterson et al, 2005) and differences amongst reaches may introduce biases in abundance comparisons (Pusey et al, 1998;Rosenberger and Dunham, 2005). The use of block nets is probably not cost effective in standard surveys (Kruse et al, 1998;van Liefferingere et al, 2010) and impossible to apply in large rivers or during high flows, making it difficult to correct for capture efficiency along rivers.…”

Evaluating Two Observational Sampling Techniques for Determining the Distribution and Detection Probability of Age‐0 Smallmouth Bass in Clear, Warmwater Streams

“…However, several biases can exist in data collected via snorkeling. The magnitude of bias depends on several factors, including the number of fish moving and the distance moved (Peterson et al 2005), the size and cryptic color of fish examined (Petty and Grossman 1996;Thurow and Schill 1996), the behavioral responses of fish to the observer (Thurow et al 2006), the accurate identification of surveyed species (Thurow 1994), and the habitat characteristics at the surveyed sites (Thurow and Schill 1996).…”

Fish community response to evolving channel complexity in an agricultural headwater system

“…The stationary seine was then turned on a 90-degree pivot through the lower end of the sample segment to capture any fish missed by the sweep seine. This technique is similar to the one described by Peterson et al (2005). All fish were then identified by species in the field in accordance with identification characteristics in Fishes of Ohio (Trautman 1981), aged into common sizeage classes (adult, subadult, juvenile), assessed for spawning condition, and released downstream of the newly placed stationary seine at the midpoint of the segment.…”