Effects of Large Bluegill Stocking on Male Size at Maturity

Authors: Richard J. Walsh and James T. Levitt

As part of its urban fishing program, the Minnesota Department of Natural Resources has stocked large bluegill Lepomis machrochirus into several small fishing ponds (5 to 25 acres) in the Twin Cities metropolitan region to provide “bonus” angling opportunities.  We investigated maturity of male bluegill to see if size at 50% maturity differed between stocked and unstocked ponds.  It has been suggested that stocking large male bluegill into established bluegill populations could cause a shift toward later maturation by means of size-dependant, competitive reproductive behavior.  Large bluegill, averaging approximately 190 mm in length and 156 g in weight, were stocked into 10 ponds between 2001 and 2005.  Stocking frequency varied among ponds, but most were stocked at least three of the five years.  Stocking rates fluctuated based on availability, with rates ranging from 4 to 45 fish/acre/year, averaging of 19 fish/acre/year.  In 2005 and 2006, bluegill were collected between late April and early July from eight of these ponds, and from six similar ponds that had not stocked with large bluegill.  Natural bluegill reproduction occurred in all ponds, and all had fish communities typical of small lakes in the region.  Although we detected differences in size at 50% maturity among ponds, there was insufficient evidence to suggest that stocking large bluegill delayed maturation in resident males.  We did find some evidence that factors such as angling pressure, and possibly available spawning habitat, influenced size at maturity in these ponds more than stocking of large bluegills.