Factors related to occupancy and detection and population demographics of adult Bighead Carp and Silver Carp in the lower Red River catchment.

Abstract

North America native fishes have declined throughout the 20th and 21st century for a myriad of reasons, including invasive species. Two emblematic invasive fishes, Bighead Carp Hypophthalmichthys nobilis, and Silver Carp Hypophthalmichthys molitrix (hereafter carp), were first introduced for management purposes (i.e., algal control in aquaculture ponds and wastewater treatment facilities) but quickly spread throughout the Mississippi River catchment and have continued to invade connected catchments. Carp were first detected in the lower Red River catchment of Texas, Oklahoma, and Arkansas in 2012. My study objectives were to determine the hierarchical factors related to warm-water occupancy and assess population demographics of both species. I sampled the mainstem Red River and several tributaries during the presumed spawning season (April-Sept) of 2021 and 2022 using gill nets and electrofishing across 58 reaches. Carp detection was positively associated with sampling effort and water temperature, and negatively associated with water clarity and discharge. Both species occupancy was positively associated with reaches containing backwater habitats and low sinuous river sections where the channel tended to be narrower and deeper than other parts of the catchment. There were species-specific differences where Silver Carp occupied reaches with higher levels of chlorophyll-a, whereas Bighead Carp had no association with chlorophyll-a concentrations. Growth by both species was positively associated with higher air temperatures and negatively associated with discharge variability; however, Silver Carp growth was also positively associated with higher discharges. Silver Carp grew quickly, had stable recruitment variability, and low mortality. Additionally, both species had relatively high theoretical maximum length. However, I did not sample any carp < age three. My results indicate that carp in the lower Red River catchment use habitats characterized by local disturbances (i.e., low sinuosity and decreased width-to-depth ratio) where mitigation efforts (i.e., experimental flows) could be used to decrease this habitat. Additionally, backwater habitat may be suitable locations for targeted mitigation; however, backwaters are important to many native fishes and may be suitable locations for trapping carp or timing removal efforts when native species survival may be higher (i.e., colder water temperatures). Experimental flows to increase discharge variability may reduce carp growth; however, caution is warranted as carp recruitment in their native range has been positively associated with discharge variability. Future efforts aimed at tracking individuals over time would be useful for assessing the timing of fish habitat use or possible congregations to aid removal efforts while minimizing effects on native fishes.