Grass carp (Ctenopharyngodon idella) movement states and backwater use can inform removal efforts

Abstract

Expansion of grass carp (Ctenopharyngodon idella) within the Laurentian Great Lakes poses significant ecological and economic risks. Suppression efforts to remove grass carp have increased to curtail population growth, but capture efficiency remains low. One management challenge is the lack of understanding how this invasive species is using its novel habitats in the Great Lakes. This study aimed to identify spatiotemporal patterns in grass carp behavior and backwater use that could be exploited by management efforts, including removal. We used acoustic telemetry to examine when grass carp occurred in areas vulnerable to removal by boat electrofishing in the Sandusky River, Ohio, USA. Hidden Markov models were used to categorize behavioral states and evaluate their relationships with environmental factors. Best performing 2-state and 3-state hidden Markov models included proximity to backwater as a factor in state-dependent step length and transition probability between states, with a higher probability of being in less mobile states during the middle of the day and in backwaters. For the 2-state model, this less mobile state was also more common during warmer temperatures. Distinct diel patterns and backwater use identified in this study provide critical insights for improving management. Model results suggest that current removal efforts, focused primarily on the mainstem of the river, may benefit from adjustments in both timing and removal gear. The findings raise further questions about unobserved behaviors contributing to state assignment and sufficiently flexible adaptive management strategies to account for individual variation in grass carp behavior.