FACTORS REGULATING RECRUITMENT OF WALLEYES AND SAUGERS IN
NAVIGATION POOL 4 OF THE UPPER MISSISSIPPI RIVER, 1965-1996.

Brian S. Ickes1, Donald L. Pereira2, and Allen G. Stevens3.  
1U.S. Geological Survey, Upper Midwest Environmental Sciences Center, La Crosse, WI 54603 
2Minnesota Department of Natural Resources, St. Paul, MN 55106  3Minnesota Department of
Natural Resources, St. Paul, MN 55155.

We analyzed stock and recruitment data for walleye Stizostedion vitreum and sauger S.
canadense in Navigational Pool 4 of the upper Mississippi River.  The primary goal of this work
was to synthesize knowledge of recruitment mechanisms in this riverine system to promote
research and assessment of long-term river impoundment effects on walleye and sauger
reproductive ecology.  We developed regression models of recruitment for each species by
considering such generalized factors as spawning stock size, river hydrology, and climate.  Our
objective was to systematically, yet conservatively, account for variability in recruitment. 
Surprisingly, density-dependent relationships between the size of the spawning stock and
subsequent recruitment accounted for the majority of the variation in walleye and sauger
recruitment, though abiotic factors also contributed.  Our results suggest that biotic factors may
be the principal determinants of year class strength for Pool 4 walleye and sauger stocks. 
Furthermore, our results suggest that simple correlation studies of the relationships between
year-class strength and myriad environmental factors (e.g., river discharge, climate, etc...) may
fail to account for the key factors driving year-class dynamics in large river systems.  We
highlight the need to investigate biotic determinants of population dynamics in large river
systems and the value that long-term standardized datasets hold for investigating these
relationships.

Keywords:  stock, recruitment, walleye, sauger, Stizostedion, Pool 4, Upper Mississippi River,
population dynamics, biotic, abiotic, density-dependence