Spawning behavior of lake sturgeon (Acipenser fulvescens)

The spawning behavior of lake sturgeon, Acipenser fulvescens, was observed and characterized over a 16-year period on the Wolf and upper Fox rivers of the Winnebago system in East Central Wisconsin. Lake sturgeon display an intricate set of sexual behavior responses to water temperature during their spawning period. When water temperatures rise to 6.6-16.0 degreesC sturgeon begin exhibiting a porpoising behavior in the vicinity of the spawning grounds. As the water continues to warm, this behavior increases in intensity and continues until slightly past the peak of spawning activity. In the range of 8.8-16.0 degreesC males move onto the spawning grounds and begin cruising, apparently searching for signs of ovulating females. Individual females will move onto a site at water temperatures of 8.8-19.1 degreesC with the maximum number of females and heaviest spawning activity on a site occurring generally within 11.5-16.0 degreesC. Lake sturgeon were observed spawning both during the day and night at wide temperature ranges, 8.8-21.1 degreesC. Whereas males arrive first at the spawning site, females ultimately determine the duration of spawning through the timing and intensity of their use of that site. During the spawning act, activity also keys of the females, with the males responding to cues from the female to participate in 2-4 s spawning bouts during which a relatively small number of eggs (estimated 947-1444 eggs per bout) are released by the female into a cloud of sperm ( estimated 200 800 billion sperm) from two to eight males. The males beat the abdomen of the female with their tails and caudal peduncles while ejaculating. While ejaculating, males emit a dull, thunderous vibrating sound which attracts other males to the area. The female initiates a spawning bout at approximately 1.5 min intervals and will continue oviposition for 8 -12 h, even if the water temperature decreases or increases outside the optimal range (11.5-16.0 degreesC). Spawning typically occurs for 2 4 days on each site, depending on the number of females utilizing the site. At cessation of the spawning season, Wolf River sturgeon quickly move back into the main river channel. While water temperature is a key environmental signal affecting the onset and duration of the spawning period, the rate of water temperature increase prior to spawning appears to influence the actual temperature at which spawning begins. Sturgeon exhibit complex polygamous mating behavior whereby several males may fertilize the eggs of a single female, and each male may participate in spawning with several females while on the spawning grounds. The breeding system is both polyandrous and polygynous, thereby maximizing the opportunities for mating with numerous individuals and subsequently maximizing the genetic diversity of the offspring.