Angling as an underappreciated sampling technique for monitoring Channel Catfish populations in Grand Canyon

ABSTRACT Objective Standard sampling methods for fishes generally provide a reliable method for fish population assessments. However, they may not always be effective due to logistical, financial, or behavioral constraints. Channel Catfish Ictalurus punctatus are a nonnative species of interest in the Colorado River basin due to predation on native species. Electrofishing and hoop nets were used in a long-term monitoring program that was designed to assess the relative abundance and distribution of fishes within the Colorado River, Grand Canyon, USA, but have proven ineffective for Channel Catfish. Thus, our objective was to determine whether angling was an effective method to target Channel Catfish and whether it was useful as a supplemental sampling method for other fishes, such as Humpback Chub Gila Cypha. Methods Angling occurred within randomly selected segments of the Colorado River, Grand Canyon, USA, as part of a long-term fish monitoring project (2004–2025). Angling occurred each evening/night at camp using standardized methods for a minimum sample time of 1 h. We used generalized linear mixed-effects negative binomial models to assess whether angling catch data could detect changes in relative abundance and linear models to investigate changes in the distribution of Channel Catfish and Humpback Chub over time. Results Channel Catfish were the most abundant fish species that was captured by angling (n = 347), followed by the federally listed Humpback Chub (n = 214). Angling provided more Channel Catfish captures per sample size than hoop nets (n = 1) or electrofishing (n = 119) during our study period based on sample sizes of 566, 2,761, and 12,911 respectively. The modeled Channel Catfish relative abundance and distribution decreased substantially over time, whereas relative abundance and distribution of Humpback Chub increased substantially over time. Conclusions Using angling as a sampling method, we were able to detect changes in the relative abundance and distribution of Channel Catfish and Humpback Chub over time in the Colorado River, Grand Canyon.