Habitat selection and movements of diploid and triploid grass carp in a large reservoir
Authors:   , Hessler Tyler M. 
Published: December 2023

Grass Carp, an herbivorous fish introduced in North America to control aquatic vegetation, has become an increasing ecological threat to aquatic ecosystems they invade. Grass Carp have the potential to cause declines and alterations in aquatic vegetation communities, which in turn can have negative impacts on native species reliant on aquatic vegetation. In the last ten years, frequent captures and observed spawning of wild fish have increased concerns of Grass Carp establishment in novel waterways. A particular concern is the potential establishment of fish in Lake Erie and its tributaries. Understanding Grass Carp habitat selection might be useful in population control by guiding harvest actions. In addition, understanding movement ecology of Grass Carp might help to predict where Grass Carp go and what cues initiate those movements. Identifying predictable movement behaviors to heavily used sites might also be useful in control efforts. In this study, I tracked Grass Carp to evaluate winter habitat selection and to determine whether using tagged (Judas) fish is effective in removing wild fish during winter. In addition, this study aimed to characterize upstream migrations into lake tributaries and determine if diploid and triploid fish exhibit similar behaviors. From 2017-2019, I tracked 86 Grass Carp tagged with acoustic transmitters in Truman Reservoir, Missouri to answer these questions. I employed Bayesian discrete choice models to determine winter habitat selection of tagged fish. Generalized linear mixed models were used to determine environmental conditions associated with upstream migrations. Tagged Grass Carp showed strong selection for shallow littoral habitats, and 75 percent of locations were in waters [less than or equal to] 3 m deep. Attempts to capture fish with trammel nets using the Judas method proved difficult with only 1.9 fish caught/netting attempt versus 1.2 caught/netting attempt when selecting areas with suitable Grass Carp habitat for harvest. Fish were often observed near inundated vegetation, with food material collected in 29 of the 31 guts I sampled from fish harvested during capture attempts, which is indicative of overwinter feeding. Eleven fish (6 diploid and 5 triploid) exhibited upstream migrations over the 2018 and 2019 spawning seasons on the Osage River above Truman Reservoir, with distances between 13.6 - 108.1 river km observed. Three of the ten fish in 2019 (2 diploid and 1 triploid) made two migrations and one diploid made three migrations. Upstream migrations were observed from late April to early July and were strongly associated with river temperatures between 15 - 26[degrees] C when river levels were rising at a rate > 0.9 m/ 12 h. Five lake resident fish made upstream movements into tributaries during rising river events, indicative that fish residing in the lake proper respond to cues to move upstream. Winter habitat selection suggests that efforts to reduce Grass Carp populations via harvest may be difficult in large lake systems when fish are dispersed throughout the available littoral habitat. Upstream migrations were made by both diploid and triploid fish during conditions suitable for spawning, possibly explaining a motive for these large movements. These results may need to be considered with efforts to remove Grass Carp in large lake systems and for the utilization of triploid fish in observing Grass Carp movement behavior in natural systems.