Walleye study: hatchery vs. natural reproduction

The Department of Natural Resources this week announced the results of a long-term study to determine the contributions of hatchery-reared walleyes in the Upper Peninsula’s bays de Noc and what those results mean for fisheries management activities.

The bays de Noc, consisting of Little Bay de Noc (LBDN) and Big Bay de Noc (BBDN), supported historically important walleye fisheries that declined in the 1960s. Since that time, walleyes have been rehabilitated through protective regulations, improved habitat, and stocking efforts. Although natural reproduction has been detected in LBDN since as early as 1988, stocking efforts have continued in these waters to help increase walleye numbers. In 2004 the DNR began a study to estimate the contribution by both hatchery-reared and naturally reproduced walleyes in the bays de Noc.

During the course of the study, oxytetracycline (OTC) marked-spring fingerling walleyes were stocked into the bays de Noc. The use of OTC creates a mark on the fish’s bones for future identification of fish origin by Fisheries Division staff. Approximately 832,000 walleyes were stocked in LBDN during 2004, 2006 and 2008 and about 1,017,000 walleyes were stocked in BBDN in 2005 and 2009. No walleyes were stocked in 2007 due to VHSv (Viral Hemorrhagic Septicemia virus) concerns. Juvenile walleyes were then collected during the fall and examined for the OTC mark which would indicate whether they came from hatchery ponds or natural reproduction.

Of the juvenile walleyes produced between 2004 and 2009, 76 percent in LBDN and 62 percent in BBDN were from natural reproduction.

“These results indicate that natural reproduction is the main driver of walleye stocks in the bays, but that hatchery fish are likely contributing too,” said DNR fisheries research biologist Troy Zorn.

“The success of natural reproduction is highly variable from year to year, and the relatively long lifespan of walleyes (more than 20 years) gives them the capacity to spawn many times, hopefully producing several strong crops of offspring,” he said. “Because natural reproduction is the main contributor to these populations and reproductive success varies considerably among years, we saw no significant difference between stocked and unstocked years in the catch rates of juvenile walleyes.”

During the study, the strongest year class in LBDN occurred in 2007, when no walleyes were stocked due to VHSv. In BBDN, the stocked 2005 year class produced the highest assessment catches of juvenile walleyes. Preliminary evaluations show no relationship between walleye stocking and subsequent angler harvest in BBDN, but a positive (although weak) relationship in LBDN.

The OTC study is giving Fisheries Division managers valuable insight on the contribution of stocked fish to the walleye populations in each bay.

“The information obtained from this study contributes to the walleye management plan for LBDN. We’ll use this information to discuss future management strategies with the public, and ultimately to make informed stocking decisions for hatchery-reared walleyes in northern Lake Michigan,” said Northern Lake Michigan Management Unit supervisor, Jessica Mistak.

For more information on management of walleyes in the bays de Noc, please visit http://www.michigan.gov/dnr/0,4570,7-153-10364_52259_63282-282188–,00.htmlhttp://www.michigan.gov/dnr/0,4570,7-153-10364_52259_63282-282188–,00.html?source=govdelivery ].