Larval Swimming Behavior in Grass Carp Ctenopharyngodon Idella and Bigheaded Carps Hypophthalmichthys Spp

Recruitment of Grass Carp (Ctenopharyngodon idella) has been documented in the Sandusky River and other tributaries to the Laurentian Great Lakes, and information on early life history is necessary to assess the invasion risk.  Early behavioral information, including swimming style, is lacking for the pro-larval period.  Grass Carp, Silver Carp (Hypophthalmichthys molitrix) and Bighead Carp (H. nobilis) were placed in 2-meter swimming tubes with no thermogradient. Observations were made twice daily (12-hour intervals) to find vertical position, ascent and descent rates.  Immediately after hatching, the initial swimming style in all species was upward swimming interspersed with periods of falling or downward swimming.    Grass Carp larvae were more likely to lie on the bottom of the tank between vertical swimming episodes than the bigheaded carps.  Horizontal swimming began at gas bladder emergence.  Ascent rates were slowest in Grass Carp and increased with ontogeny.  Descent rates were lowest in Grass Carp, and decreased with ontogeny.  All species varied water column position over development, but tended to be higher in the water column during daytime hours after gas bladder emergence.  Grass carp exhibited phototactic behavior after gas bladder emergence.  Behavioral information can allow species- and river-specific dispersal models to assess invasion risk.