Distribution and population demographics of invasive bigheaded carp in the Lower Red River Basin

Our study objectives were to 1) determine the spatial and temporal distribution and adult population demographics of invasive Silver Carp and Bighead Carp, 2) establish baseline native and invasive fish assemblage and habitat association data, and 3) determine movement patterns of both Silver Carp and Bighead Carp in the Red River using telemetry. We sampled larger-bodied fishes using a combination of gillnets, hoop nets, and electrofishing and mini-fyke nets, seins, and larval tows. We completed sampling at 136 sites, 71 where we targeted juvenile bigheaded carp and other small-bodied fishes and 65 sites where we targeted adult bigheaded carp and large-bodied fishes in the lower Red River catchment through September 2023. We conducted 330 surveys at our juvenile sampling sites and 318 surveys at our adult sampling sites. Of the 134 sites, 40% were in the Oklahoma portion of the catchment, 35% in the Arkansas portion of the Red River, and 25% were in associated tributaries in Oklahoma and Texas. Gillnets and electrofishing were most effective at capturing larger-bodied fishes, whereas mini-fyke nets and seine hauls collected mainly smaller-bodied native fishes. For bigheaded carp occupancy modeling, we included data collected from 43 reaches (1.5-2.0 km), comprising 137 surveys (sites sampled 1-3 times each). We removed lapilli otoliths for age and growth analyses and determined body condition. We processed a subset of carp ovaries to determine if successful spawning occurred within the lower Red River. No juvenile or age-0 bigheaded carp were sampled or observed in the study area. Silver Carp were detected at 26 of the 49 mainstem Red River adult sites and 13 of the 16 adult tributary sites with an overall naïve occupancy of 0.65. Bighead Carp were detected at 11 of the 49 mainstem Red River adult sites and 10 of the 16 adult tributary sites with an overall naïve occupancy of 0.32. The occupancy models that had the most support for both species retained the covariates: presence of backwater in the reach, sinuosity, width-to-depth ratio, and chlorophyll-a (µg/L). All top ranked models included the detection covariates of water temperature (°C), Secchi depth (cm), discharge, and electrofishing effort (s). The mean age of Bighead Carp estimated using otoliths was 10.4 years, whereas Silver Carp mean age was lower (6.8 years). The oldest sampled Silver Carp and Bighead Carp were 14 and 20, respectively. For Bighead Carp, relative weight (Wr) ranged from 94.48 to 107.82, with an average of 101.13. For Silver Carp, Wr ranged from 90.05 to 106.33, with an average of 100.81. For both Bighead and Silver Carp, we observed ovaries occupying much of the body cavity and containing developed eggs (i.e., oocytes occupy most of the coelomic cavity) throughout the year. The average monthly gonadosomatic index (GSI) was highest in May for Bighead Carp (GSI: 14.95) and in June for Silver Carp (GSI: 20.39). We processed and quantified 9 Bighead Carp ovaries, and 22 Silver Carp ovaries collected from May 2023 through May 2024. We observed the presence of atretic oocytes in five female carp. Two Silver Carp and two Bighead Carp (sampled May 1, 2024) contained post-ovulatory follicles indicating at least one spawning event had occurred relatively recently. We also processed carp collected later in May that had post ovulatory follicles. Combined, these data reflect spawning is occurring within the catchment. A total of 284,194 fishes, comprising 86 species and 48 genera, were identified during sampling of the lower Red River catchment. No juvenile carp were sampled. All 38 juvenile species that were sampled had positive occupancy relationships with reaches having deep pools and slackwater habitats present, and the distance from the nearest upstream dam. Lastly, all species had a negative occupancy relationship with deeper thalwegs and the percentage of limestone within the catchment. Although species had the same relationship with thalweg depth, the effect size of these relationships differed. Several nursery habitat relationships were species specific. Like juvenile fishes, some occupancy relationships were shared between large-bodied fishes; however, others varied by species. All species were negatively related to increasing drainage area, distance from dam, and sinuosity. Large-bodied species displayed variable relationships with salinity, elevation, width-to-depth ratio, presence of blackwater in the reach, and discharge. Notably, all species other than Silver Carp were negatively related to increasing salinity. We tagged 25 Bighead Carp and 25 Silver Carp with acoustic transmitters in spring 2023. From spring 2023 through December 2024, we conducted 171 active tracking events and obtained 1,519 detections of Bighead Carp and 2,596 detections of Silver Carp using active and passive telemetry (acoustic receivers set in gates).  Our top-ranked model of carp movements contained an additive effect of coefficient of variation (CV) discharge and an interactive effect between CV water temperature and discharge. Our top ranked model explained 71% of the deviance in these data. Our movement rate model included an additive effect of CV discharge and an interactive effect between CV water temperature and discharge. This model explained 39% of the deviance in these data. We defined six irregularly spaced time periods based on water temperature and discharge patterns throughout the study period to reflect unique environmental conditions between each time period (t1, t2, t3, t4, t5, and t6). Transition probabilities from state A (tributary/backwater) to state M (mainstem river) were the highest between t1 and t2 and t4 and t5 and the lowest between t2 and t3, t3 and t4, and t5 and t6. Transition probabilities from state M to state A were highest between t1 and t2, t4 and t5, and t5 and t6 and lowest between t2 and t3 and t3 and t4. The level of uncertainty was higher for transitions estimates from state M to state A. This uncertainty also occurred in the probability of remaining in state M. Average apparent survival in state A was higher than in state M. As Bighead Carp and Silver Carp occupy the Red River catchment for longer periods of time, management strategies aimed at preventing their spread and exploiting their vulnerabilities will be key to population control. It would be beneficial for agencies to consider restrictions on locations for bait acquisition if concerned about bigheaded carp spreading to new systems. Although there is currently no documentation of juvenile carp in the Red River upstream of the LA-AR border, we did document successful spawning by adults, and regular recruitment may also occur in adjacent basins or reaches further downriver suggesting the need for managers to consider barriers if the goal is to limit increases in abundance over time. Given the costs, limitations, and risks associated with removal efforts, continued evaluation of the distribution and movement patterns of both native fishes and carp may be necessary to determine an approach to limit carp increases in abundance that may necessitate temporary or permanent barrier implementation.