Monitoring and Managing Fishes that Are Invisible and Keep Moving Around: Influences of an Invasive Species and Environmental Factors on Capture Probability

Abstract

Understanding fish population status and trends are fundamental to effective research and management. Challenges in understanding population status include recognizing and accounting for sources of variation in capture probability (truep^) that can obscure patterns in count data and bias inferences about the population. In systems where management actions such as invasive species removals are implemented based on population triggers, errors in abundance estimation can propagate to missed management opportunities simply due to variation in truep^, rather than actual population status. We assessed the relative importance of individual heterogeneity (species and fish size), environmental (stream discharge, temperature) and spatial variation, and interspecific interactions (density) on electrofishing truep^ for native (Speckled Dace Rhinichthys osculus, Bluehead Sucker Catostomus discobolus, Flannelmouth Sucker Catostomus latipinnis) and invasive (Brown Trout Salmo trutta, Rainbow Trout Oncorhynchus mykiss) fishes. We fit closed‐population depletion models with combinations of covariates using 6 years of data collected during invasive salmonid suppression efforts. We found that the relative importance of spatially and temporally varying environmental influences on truep^ differed between native and introduced species. Temperature was important for explaining variability in truep^ for two of three native species, and discharge had a significant influence on truep^ for both Brown Trout and Rainbow Trout. We also found that truep^ declined with increasing Brown Trout density, which would limit precision and potentially bias abundance, for two of the three native species. These results illustrate the potential importance of interspecific interactions in influencing truep^ and understanding implications of management actions, such as invasive species suppression. Our results demonstrate the need for cautious interpretation of uncalibrated catch‐per‐unit‐effort data for abundance‐triggered management actions, including those designed to promote conservation of endangered or economically valuable species.

Improving Salmonid Monitoring by Nocturnal Counting in Rivers

“…2018; Healy et al. 2022)—for example, a nocturnal activity pattern is hypothesized to be an adaptation to reduce predation risk in several temperate fish species (Emery 1973; Hanych et al. 1983; Culp 1989).…”

Exploring metapopulation‐scale suppression alternatives for a global invader in a river network experiencing climate change

“…Bright Angel Creek subpopulation suppression included life stage and electrofishing pass‐specific $$\hat{p}$$ for each subpopulation (MR) (Table 1) estimated from 3‐pass electrofishing (Healy, Moore, et al., 2022). We also included a scenario with eradication of the BACU subpopulation via chemical piscicides and interception of migrants at weir operations (Healy et al., 2020).…”

“…We focused perturbation analysis on the CR because different techniques may be available to target different life stages (e.g., dam operations to target incubating eggs [Korman et al., 2011] vs. electrofishing for older life stages). All life stages are susceptible to electrofishing in BAC (Healy, Moore, et al., 2022). Although the DyHDER accounts for some environmental and demographic stochasticity (i.e., vital rate temporal variance) (Murphy et al., 2020) (Appendix S1; Figures 4, 5, & 7), we acknowledge our use of mean values for other parameters (e.g., $$\hat{p}$$ for electrofishing suppression) causes underrepresentation of error in our results; thus, scenario outcomes should be interpreted relative to each other (Morris & Doak, 2002).…”

“…For example, invasive amphibians have complex life cycles that may include aquatic egg or larval stages, metamorphosis to a juvenile stage, and sometimes a transition to upland adult habitats, all of which vary in vulnerability to removal techniques (Govindarajulu et al., 2005 ). Fishing gears used to control invasive fishes, such as electro‐fishing or netting, also select for larger (and thus older) individuals (Healy, Moore, et al., 2022 ; Koel et al., 2020 ). Species with complex life histories, including a partial or fully migratory stage, may also require a landscape‐scale approach that controls explicitly for dispersal between populations (Day et al., 2018 ; Milt et al., 2018 ).…”

Impeding access to tributary spawning habitat and releasing experimental fall-timed floods increase brown trout immigration into a dam’s tailwater

“…We assumed that capture probability, p, for electrofishing in the Colorado River was constant at the site scale and scaled p to the state scale based on this effort calculation. Sampling of brown trout in Bright Angel Creek occurred during a trout suppression project (Healy et al 2020), involved three-passes of depletion backpack electrofishing, and has previously been shown to include substantial interannual variation in capture probability (Healy et al 2022c). We summarized the catch of adult brown trout per pass via this effort and fit depletion models to estimate the year-specific p-star (i.e., cumulative p across three passes; eight parameters) for brown trout that were in Bright Angel Creek during the winter of years with removals within the model.…”