Environmental and ecological conditions surrounding the production of large year classes of walleye ( Sander vitreus ) in Saginaw Bay, Lake Huron
The Saginaw Bay walleye population ( Sander vitreus ) has not fully recovered from a collapse that began in the 1940s and has been dependent on stocking with only limited natural reproduction. Beginning in 2003, and through at least 2005, reproductive success of walleye surged to unprecedented levels. The increase was concurrent with ecological changes in Lake Huron and we sought to quantitatively model which factors most influenced this new dynamic. We developed Ricker stock-recruitment models for both wild and stock fish and evaluated them with second-order Akaike's information criterion to find the best model. Independent variables included adult alewife ( Alosa pseudoharengus ) abundance, spring water temperatures, chlorophyll a levels and total phosphorus levels. In all, 14 models were evaluated for production of wild age-0 walleyes and eight models for stocked age-0 walleyes. For wild walleyes, adult alewife abundance was the dominant factor, accounting for 58% of the variability in age-0 abundance. Production of wild age-0 fish increased when adult alewives were scarce. The only other plausible factor was spring water temperature. Predictably, alewife abundance was not important to stocked fish; instead temperature and adult walleye abundance were more significant variables. The surge in reproductive success for walleyes during 2003–2005 was most likely due to large declines in adult alewives in Lake Huron. While relatively strong year classes (age-1 and up) have been produced as a result of increased age-0 production during 2003–2005, the overall magnitude has not been as great as the initial age-0 abundance originally suggested. It appears that over-winter mortality is higher than in the past and may stem from higher predation or slower growth (lower condition for enduring winter thermal stress). From this it appears that low alewife abundance does not assure strong walleye year classes in Saginaw Bay but may be a prerequisite for them.