CHEMICAL SUITABILITY OF SUBSTRATES FOR WALLEYE EGG DEVELOPMENT IN THE LOWER FOX RIVER, WISCONSIN

Site suitability for egg development was assessed for walleyes Stizostedion vitreum in the Fox River, Wisconsin. Water chemistry at the sediment-water interface was compared with criteria for walleye egg hatch success (oxygen and hydrogen sulfide) and chronic toxicity (free ammonia). Reduced concentrations of dissolved oxygen and elevated concentrations of ammonia-nitrogen and hydrogen sulfide occurred above some sand and all soft muck substrates. The chemical oxygen demand (COD) of the surficial sediments was a useful surrogate variable for identifying the chemical suitability of spawning sites; locations with a surficial sediment COD less than 40 mg O2/g dry weight were considered suitable. A comprehensive surficial sediment survey of the lower Fox River below the DePere Dam showed that optimal walleye spawning habitat (gravel-cobble substrate) is rare (less than 3% of the area) and that 75% of the suboptimal (sand and muck) substrate is chemically unsuitable for walleye egg development. We conclude that successful natural reproduction by walleye in this portion of the lower Fox River is limited by the availability of suitable substrate.