We recorded grass carp locations in rkm upstream from the dam to characterize large-scale upstream movements. We associated locations in wider parts of the reservoir or adjacent coves to the closest point to the river channel using the rkm at that point. This method assumes the shortest distance a fish could have traveled, although the distance could be longer if a fish traveled in a nonlinear path. Distance between locations was calculated as the shortest path a fish could take through the reservoir and tributaries upstream.
We cannot confirm grass carp that made large-scale upstream movements spawned or were intent on spawning, but seasonal upstream migrations to areas suitable for spawning has been observed in grass carp [39 (link)] and other invasive carp species [40 (link)]. As such, we assumed that upstream movements during the spring/summer seasons were associated with spawning activity in our analyses. Time between detections was irregular, making it difficult to statistically compare movement behavior among individual fish. Therefore, we scaled individual fish movement to regular 12-h time steps and estimated the distance moved (rkm) using the ‘redisltraj’ function in the package ‘adehabitatLT’ [41 (link)] in R [42 ]. We used 12-h steps because consecutive detections at different locations were rarely < 12-h and intervals greater than this may miss quick movements associated with the start of a potential migratory run. Following Acre et al. [43 (link)], we identified individual 12-h steps as part of a migration when distance moved was ≥ 85th percentile of all observed distances moved over a 12-h period. Each movement step was then binomially categorized as part of a migration (1) or not (0).
We quantified river conditions based on data from the Osage River gage at rkm 116.7 (USGS gage 06918250). Temperature (°C), discharge (m3/s), and stage (m) are measured every 15 minutes at this location. We are aware that conditions were unlikely to be exactly homogeneous throughout the study area; however, as it was not feasible to measure the entire river, we make the necessary simplifying assumption that measurements at this location were representative of general conditions of discharge and temperature. River conditions along movement paths were based on gauge readings at the time of each 12-h interpolated location. The importance of these variables for successful spawning of grass carp is well documented, including ideal temperatures for optimal ripening and development of eggs post fertilization as well as higher velocities and turbulence associated with increased discharge and river stage to keep eggs suspended prior to hatching [36 (link), 44 (link)]. In addition, these variables have been associated with grass carp spawning movements in previous studies [45 , 46 ]. Although other environmental variables may play a role in grass carp movements (i.e., turbidity, food availability), it was not feasible to quantify them on a large spatial scale.
We cannot confirm grass carp that made large-scale upstream movements spawned or were intent on spawning, but seasonal upstream migrations to areas suitable for spawning has been observed in grass carp [39 (link)] and other invasive carp species [40 (link)]. As such, we assumed that upstream movements during the spring/summer seasons were associated with spawning activity in our analyses. Time between detections was irregular, making it difficult to statistically compare movement behavior among individual fish. Therefore, we scaled individual fish movement to regular 12-h time steps and estimated the distance moved (rkm) using the ‘redisltraj’ function in the package ‘adehabitatLT’ [41 (link)] in R [42 ]. We used 12-h steps because consecutive detections at different locations were rarely < 12-h and intervals greater than this may miss quick movements associated with the start of a potential migratory run. Following Acre et al. [43 (link)], we identified individual 12-h steps as part of a migration when distance moved was ≥ 85th percentile of all observed distances moved over a 12-h period. Each movement step was then binomially categorized as part of a migration (1) or not (0).
We quantified river conditions based on data from the Osage River gage at rkm 116.7 (USGS gage 06918250). Temperature (°C), discharge (m3/s), and stage (m) are measured every 15 minutes at this location. We are aware that conditions were unlikely to be exactly homogeneous throughout the study area; however, as it was not feasible to measure the entire river, we make the necessary simplifying assumption that measurements at this location were representative of general conditions of discharge and temperature. River conditions along movement paths were based on gauge readings at the time of each 12-h interpolated location. The importance of these variables for successful spawning of grass carp is well documented, including ideal temperatures for optimal ripening and development of eggs post fertilization as well as higher velocities and turbulence associated with increased discharge and river stage to keep eggs suspended prior to hatching [36 (link), 44 (link)]. In addition, these variables have been associated with grass carp spawning movements in previous studies [45 , 46 ]. Although other environmental variables may play a role in grass carp movements (i.e., turbidity, food availability), it was not feasible to quantify them on a large spatial scale.
Free full text: Click here
