Human transformation of aquatic systems and the introduction of nonnative species increasingly threaten the persistence of imperiled freshwater fishes. In response, large‐scale mechanical removal of nonnative fishes has been implemented throughout parts of the Colorado River basin to aid recovery of endangered fishes, but the effects of these efforts can be difficult to quantify. Fisheries population models for predicting outcomes of harvest regulations have been widely used to prevent overfishing of commercial and game stocks. Here, we used population models to investigate size‐specific removal efforts needed to overfish a nonnative population of Channel Catfish Ictalurus punctatus and thereby aid recovery of endangered fishes in the San Juan River, New Mexico and Utah. The minimum size of fish that were efficiently captured with electrofishing gear was 280 mm TL, and annual removal rates increased with fish size, ranging from 0.10 for 200‐mm fish to 0.44 for 600‐mm fish. Model results suggested that removal rates should be increased from 0.14 to a range of 0.21–0.34 to cause growth overfishing and should be increased to a range of 0.26–0.29 to cause recruitment overfishing at a minimum electrofishing size limit of 280 mm TL. However, model results indicated that overall population abundance and biomass are being substantially reduced compared to an unmanaged population. In concordance, long‐term monitoring data from 1991 to 2015 demonstrated a decrease in Channel Catfish TL and mass as well as an increase in catch rate variability since removal efforts intensified in 2006. Overall, current rates of removal will probably not achieve collapse of the nonnative Channel Catfish population in the San Juan River, but the reduction in size structure indicates that the population has responded to these efforts.
Reservoirs and associated river fragments are novel ecosystems not experienced by fishes in their evolutionary history, yet they are now commonplace across the globe. Understanding how fishes use these novel habitats is vital to conservation efforts in contemporary riverscapes. Movement patterns of the endangered razorback sucker (Xyrauchen texanus) synthesized from tagging efforts in the upper Colorado River basin, USA, illustrate the applications of tagging technology and data sharing by multiple agencies to better understand the spatial ecology of large river fishes. Tagging studies between 2014 and 2018 in Lake Powell and its two main tributary rivers, the Colorado (unfragmented) and San Juan (waterfall‐fragmented), were used to quantify movement of razorback sucker within this river–reservoir habitat complex. In addition, facilitated translocations of fish upstream of a waterfall barrier in the San Juan River were assessed in 2016–2017. Extensive movement of fish occurred within and across river and reservoir habitats. Of 722 fish captured in the Colorado River arm of Lake Powell, 36% of re‐encounters occurred upstream in the Colorado or Green rivers, or fish dispersed through the reservoir and were detected in the San Juan River arm. Fourteen fish moved more than 600 km. In the San Juan arm of the reservoir, 29% and 20% of fish in 2017 and 2018, respectively, had moved ~30–40 km upstream below the waterfall in the San Juan River within a year. In 2016–2017, 303 fish were translocated upstream of the waterfall into the San Juan River, but 80% were re‐encountered downstream of the waterfall within a year. Long‐distance movements by razorback sucker were common within and among rivers and reservoirs illustrating how large river fish, in general, might maintain population connectivity in highly altered ecosystems.
The establishment of nonnative predators can have devastating consequences for native fish communities, but predation rates are often difficult to quantify due to spatial and temporal variation in predator foraging behavior. Predation by Channel Catfish Ictalurus punctatus throughout the Colorado River basin potentially threatens the recovery of native fishes. Because Channel Catfish are highly opportunistic feeders, an understanding of how piscivory by this species impacts prey populations should help to guide management in invaded systems. We used laboratory observations to model temperature‐dependent stomach evacuation rates and combined those estimates with field‐collected diet data to derive annual consumption across a 152‐km reach of the San Juan River (New Mexico, Colorado, and Utah). In the field, stomach fullness increased with water temperature while the probability of observing fish prey in the diet increased with TL of Channel Catfish and water turbidity. Based on estimates of daily ration, diet composition, and Channel Catfish population demographics, we estimated the San Juan River population’s fish consumption to be 4.9 kg·ha−1·year−1 (95% CI = 4.0–6.1 kg·ha−1·year−1) in 2018 and 2.3 kg·ha−1·year−1 (95% CI = 1.8–2.8 kg·ha−1·year−1) in 2019. Native fishes accounted for 54% of the fish biomass consumed, which included two incidents of endangered Colorado Pikeminnow Ptychocheilus lucius consumption. Although these estimates should help managers to assess the predatory threat of Channel Catfish, additional information, such as the efficacy of nonnative control and prey population compensatory responses, will likely be necessary to develop robust management strategies aimed at reducing the predatory impacts of this nonnative species on native fish assemblages.
River‐reservoir interfaces have been described as aquatic ecotones and contain strong environmental gradients of depth, turbidity and trophic resource abundance. These transitional habitats have traditionally been excluded by riverine and reservoir management schemes despite their prevalence in modern riverscapes. By systematically sampling shoreline habitats along a river‐reservoir interface gradient from riverine to lacustrine zones with trammel nets in 2018–2019, strong patterns were identified in total species captured and individuals captured in the San Juan River‐Lake Powell inflow, USA. Changes in assemblage structure were driven mainly by increases in relative abundance of benthic omnivores towards the riverine zone, including imperiled razorback sucker, Xyrauchen texanus (Abbott), but also by increases in predatory species, such as striped bass, Morone saxatilis (Walbaum). Inter‐annual variation in species distributions along the river‐reservoir interface gradient was likely influenced by variation in reservoir water level that differed by nearly 12 m between years. River‐reservoir interfaces provide high‐quality feeding and potentially spawning areas for both benthic omnivores and piscivores, and these areas should be considered in management and conservation efforts for species using these habitats.
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