Identifying Walleye and Lake Whitefish Spawning Habitat below De Pere Dam in the Lower Fox River to Inform Habitat Improvements for Lake Sturgeon

Abstract

Lake sturgeon Acipenser fulvescens populations in the Great Lakes basin have declined considerably due to a variety of factors including habitat loss and fragmentation resulting from the construction of dams that block access to historical spawning grounds. In the Lower Fox River (LFR), lake sturgeon entering from Green Bay have access to a 12-km reach of river downstream of the De Pere Dam. Although this section contains some potentially suitable spawning habitat, reproductive success remains low, with limited egg retention and poor larval survival. To support population recovery, a spawning reef will be constructed below De Pere Dam with the goal of improving lake sturgeon reproductive success. The LFR also supports spawning populations of walleye Sander vitreus and lake whitefish Coregonus clupeformis, two species that support economically and culturally important commercial and recreational fisheries that occur in Green Bay. Both species are known to spawn below the De Pere Dam, yet limited information exists on the exact locations or environmental conditions associated with their spawning activity. Because the proposed lake sturgeon spawning reef may overlap with important walleye and lake whitefish spawning habitat, it was essential to conduct thorough pre-construction assessments to avoid unintended consequences. My study was initiated to guide the design and placement of the lake sturgeon spawning reef while ensuring that critical spawning habitat for walleye and lake whitefish is preserved. My objectives were to: 1) describe spatial variation in walleye and lake whitefish egg densities in the LFR below De Pere Dam to inform placement of the lake sturgeon spawning reef; 2) determine if spatial variation in egg densities were related to water velocity, depth, and substrate; and 3) describe the timing (e.g., start, peak, end, and duration) of walleye and lake whitefish spawning. Walleye and lake whitefish egg densities and habitat characteristics in the LFR below the De Pere Dam were assessed during the fall (lake whitefish) and spring (walleye) spawning seasons during 2023–2025. Egg sampling was conducted using a diaphragm pump across systematically arranged sites. Spatial distribution patterns of egg densities were analyzed using various geostatistical interpolation methods, and model performance was validated using leave-one-out cross-validation metrics. Spatial autocorrelation and clustering of egg densities were further evaluated using Global and Local Moran’s I statistics. Habitat data, including substrate composition, depth, and water velocity, were collected and analyzed using multiple logistic regression to model the habitat variables significantly influencing the probability of lake whitefish and walleye egg density at or above median density. Egg mats served as a secondary gear to index spawning progression, with deployments designed to minimize angling disturbance and capture short-term variations in egg deposition. Start, peak, end, and duration of each spawning period were identified. River discharge, water temperature, and water level were used to characterize annual variation in environmental conditions that may be related to egg densities using coefficient of variation. A total of 500 egg samples were collected by pump for lake whitefish and 533 samples were collected for walleye. Empirical Bayesian Kriging (EBK) outperformed other interpolation methods in predicting egg densities and was used to generate spatial heat maps. These maps consistently showed high egg deposition near the dam, especially along the eastern shoreline for both species. The proposed reef area showed low to moderate densities, particularly along its southeastern portion. Spatial analyses using Moran’s I revealed significant clustering of egg deposition within species over time. Local cluster analyses further detailed shifting high-density zones, with consistent low-density clusters observed northwest of the lower paper mill. Egg deposition was most strongly influenced by depth and substrate composition. Logistic regression models showed that probability of egg densities at or above median density decreased significantly with increasing depth (32.4% decrease per meter for whitefish; 27.8% decrease for walleye). Conversely, a 10% increase in cobble substrate raised the probability of egg densities being at or above the median (20.7% increase for whitefish; 33.1% increase for walleye). Similarly, a 10% increase in boulder substrate raised this probability (11.6% increase for whitefish; 29.3% increase for walleye), while a 10% increase in pebble substrate raised the probability (49.5% increase for walleye). Lake whitefish spawning lasted 14–16-d in November, with peaks occurring at water temperatures between ~6–8°C. Walleye spawning spanned 29- to 30-d from late-March to late-April, peaking at water temperatures between ~5–8°C. Water temperature and discharge trends aligned with spawning periods, with lower flow conditions favoring greater overall egg densities. Although egg densities in the proposed reef site were generally low, eggs were found within the boundaries in multiple seasons, particularly walleye eggs in spring 2025. Spatial analyses (Moran’s I) identified similar clustering patterns, reinforcing the reliability of observed trends. Both lake whitefish and walleye primarily spawned in shallow water (< 2 m), with lake whitefish favoring coarse substrates like boulder and cobble, while walleye used a broader range of substrates including boulder, cobble, and pebble. Exact spawning dates differed between years, but generally lake whitefish spawned in mid-November and walleye in early April. Higher lake whitefish egg densities occurred in 2023 under cooler, more stable water temperatures and lower discharge, while higher walleye egg densities in 2024 were likely due to lower, albeit more variable spring discharge, that improved egg retention. Water temperature influenced timing of spawning, but discharge better explained spatial and interannual variability in egg densities. These findings represent an application of adaptive management, incorporating consideration of the spawning habitats of multiple species rather than focusing solely on the restoration target. Mapping walleye and lake whitefish spawning areas below De Pere Dam informs placement of the proposed lake sturgeon reef to minimize overlap among species. Both walleye and lake whitefish use similar shallow, flowing, cobble-bottom habitats and could benefit from shared habitat restoration strategies in Great Lakes tributaries. Understanding spawning timing and environmental cues can improve monitoring efforts and potentially guide dam operations to support successful reproduction and long-term population sustainability.