Effects of seining effort and site length on variability of small-bodied fish catch-rates in a sand-bed river

Seines are a common sampling gear for monitoring small-bodied fishes in wadeable, sand-bed rivers. However, sampling effort within a site can vary among time and space, even within the same monitoring program. Determining and standardizing an adequate or optimal amount of effort within a site is an important step for designing fisheries monitoring programs. If budgets are limited, sampling intensity at a site is often a trade-off between less intense sampling at a greater number of locations or more intense sampling at fewer locations. Inadequate sampling at survey sites could introduce variability into estimates of density, species richness, or relative abundance because of within-site variability. We evaluated how variability of mean catch-rates of five species of small-bodied fishes changed with increasing effort at a fixed number of sites, and how variability changed as more sites were sampled. Focal species were Rio Grande silvery minnow, flathead chub, red shiner, fathead minnow, and western mosquitofish. We sampled 20 sites with 20 hauls in 200 m, 40 hauls in 200 m, and 40 hauls in 400 m. We observed little decrease in variability around expected catch-per-unit-effort [E & x302;mml:mfenced close=")" open="("CPUE] when 40 seine hauls were performed per site compared to 20 hauls per site, regardless of site length or species, though a 25% increase in labour was required. However, we observed considerable variability aroundE & x302;mml:mfenced close=")" open="("CPUEwhen <= 5 seine hauls were performed at each site. We recommend a minimum of 10 seine hauls per 200 m site. Our results provide guidance and starting point for monitoring programs that use seining to collect information on small fishes.