In-Lake Hatch Rates and Their Implications for Managing Walleye Populations That Serve As Egg Sources for Minnesota Hatcheries

The Minnesota Department of Natural Resources routinely collects gametes from walleye spawning runs from several large lakes, then stocks fry back into those lakes.  The number of fry was equal to 10% of the egg take, ostensibly to compensate for lost reproduction from those eggs.  To help understand the effects of this long-standing policy on the recipient walleye populations, we manipulated the abundance of stocked fry in four egg-source lakes, marked them with Oxytetracycline (OTC), and resampled them by electrofishing.  High recapture rates of OTC marked YOY walleyes allowed the use of Petersen mark/recapture methods to estimate wild fry abundance; and hatch rates were subsequently estimated by comparison of wild fry abundance to estimates of total egg production.  Our initial findings indicate that system-wide hatch rates (0.04– 1.01 %) were much lower than previously thought and historical stocking levels often exceeded natural reproduction in the egg-source lakes.  These stocking levels also substantially exceeded the abundance of fry that would have hatched naturally in the absence of any spawn-take activity.  Additional results suggest density dependent growth of YOY walleyes but the effects of fry abundance on year-class strength will not be known until the walleyes have further recruited to the fishery.