Midwest Fish & Wildlife Conference 2019

AUTHORS. Jeffrey C. Jolley, Duane C. Chapman, Curt G. Byrd, Patrick M. Kocovský –U.S. Geological Survey

ABSTRACT. Grass carp Ctenopharyngodon idella reproduction has occurred in the Lake Erie basin; understanding spawning cues could be useful in designing control methods and determine which Great Lakes tributaries are potential spawning locations.  Grass carp broadcast spawn and require flowing water to keep their semi-buoyant eggs suspended; spawning cues are unknown. At least 633 cumulative degree days = 15 ºC (ADD15) has been reported necessary for the maturation of gonads.  To validate this estimate, we evaluated oocyte maturation of 67 and 50 females in experimental ponds in 2017 and 2018, respectively.  In 2017, the first female with ripe oocytes occurred >800 ADD15, occurring 7 days after 633 ADD15.  In 2018, the first female with ripe oocytes occurred >1,000 ADD15, over 2 weeks after 633 ADD15.  Estimated threshold for 50% of females to have ripe oocytes was 906 ADD15 in 2017 and 1,265 ADD15 in 2018.   April 2018 had record low temperatures followed by record high May temperatures, suggesting that the sequencing of accumulation of thermal units may also be important.  A pond mesocosm was used to simulate river conditions and variable spawning cues (i.e., flow, turbidity, temperature).  Spawning occurred during the first trial, when fish were injected with hormones (HCG, carp pituitary), was confirmed by spawning behavior recordings (chasing, rubbing) in high velocity areas, and collection of fertilized eggs.  Spawning also occurred when the mesocosm was held in a stagnant condition (fish were hormone injected).  Fish spawned at the deep outlet kettle where the water input was also located.  Grass carp did not spawn in additional trials, but water temperatures may have increased beyond the suitable range.  Future trials will focus on replicating the results of trial one, precise temperature control, and further variation of environmental cues.