Sperm quality influences male fertilization success in walleye (Sander vitreus)

We examined how variation in sperm quality influences a male's success at fertilizing ova (male fertilization success) in a wild population of walleye (Sander vitreus). To do this, we conducted controlled fertilization trials using milt and eggs (ova) from wild-spawning fish and measured male fertilization success (percentage of ova fertilized) by examining eggs after 24 h of incubation. We found that both the number of sperm and sperm swimming speed (at 10 s after activation) were significantly related to fertilization success. There was, with respect to fertilization success, a relatively large return on male investment in the number of sperm, but this return diminished as the percentage of ova fertilized increased above 50%. This is in agreement with theoretical predictions based on external fertilization dynamics. When the number of sperm used in the experimental trials was kept constant, variation in sperm swimming speed (at 10 s after activation) explained approximately 90% of the variation in a male's fertilization success. These findings demonstrate that the variation in sperm quality found in wild spawning populations has the potential to dramatically influence male reproductive success.