Backpack electrofishing effort and imperfect detection: Influence on riverine fish inventories and monitoring

Abstract

Summary An increased electrofishing sampling effort will increase detection probabilities of riverine fishes. In this study, a repeat-sampling approach was used in small to medium-sized Ontario (Canada) rivers to estimate: (i) species-specific detection probabilities of freshwater fishes, (ii) the number of sampling events required to confidently detect species, and (iii) the power of timed-search surveys to detect future distribution (or occupancy) declines. Wadeable habitats at 36 sites were sampled with a backpack electrofisher on four separate dates during the summer low-flow period in 2013 and 2014. Forty-two species were collected, including three species of conservation concern (American eel Anguilla rostrata Lacepede, 1802, channel darter Percina copelandi Jordan, 1877, northern sunfish Lepomis peltastes Cope, 1870), and two recreationally important species (largemouth bass Micropterus salmoides Lacepede, 1802 and smallmouth bass Micropterus dolomieu Lacepede, 1802). A hierarchical Bayesian modelling approach was used to estimate detection probabilities and site occupancy for 18 species at four levels of effort: 250, 500, 750 and 1,000 s. In all cases, species detection was imperfect. Search effort had a positive effect on estimates of detection probability and site occupancy and the power to detect declines in future distribution. Detection probabilities ranged from 0.11 to 0.66 with an effort of 250 s, and 0.27 to 0.92 with an effort of 1,000 s. For 13 species, detection and power to detect changes in distribution were significantly improved by increasing sampling effort from 250 to 750 s or 1,000 s. For the channel darter and northern sunfish, three replicate sampling visits (of 750 or 1,000 s duration) are recommended for confident detection.