The Lower Fox River is a modified tributary of Green Bay, Lake Michigan that hosts a remnant Lake Sturgeon (Acipenser fulvescens) population. Reproduction has been confirmed, although annual spawning events have been described as small and concerns regarding the long‐term viability of the population remain. Contemporary and comprehensive surveys of spawning habitat, spawning stock, and production to the larval stage have not been conducted. The goal of this study was to describe spawning habitat, determine the size and demographic structure of the spawning stock, and quantify larval production (2017–2019) to identify impediments to population growth and recovery. Over 130 adults were present during annual spawning runs and spawning activity was consistently concentrated along the eastern riverbank. Adequate population structure exists to support reproductive success, but larval catch was low, ranging from 0 to 14 larvae captured annually. Evaluation of the riverbed below the De Pere Dam suggests the extent of the substrate available for spawning adult Lake Sturgeon is lower (~70% less) than previously described. Habitat deemed suitable for spawning exists offshore below the dam, but most habitat used for spawning is arranged along the eastern man‐made shoreline. However, nearshore spawning is problematic at this location due to fluctuating discharge from dam operation and periodic seiche events. Possible impediments limiting the success of Lake Sturgeon reproduction include dewatering of eggs, retention of eggs and larvae, algae colonization, and egg predation. We discuss these issues in the context of previous Lake Sturgeon research, future monitoring of spawning adults/larval production, and the likelihood that river habitat improvement will foster increased reproductive success.
The lower Fox River (LFR) is a highly urbanized and fragmented system with a legacy of pollution. Benthic dredging and sediment cleanup combined with capping have been implemented with the goal of remediating the polychlorinated biphenyl contamination and improving the health of resident fish and wildlife populations. The LFR hosts a small population of adult Lake Sturgeon Acipenser fulvescens that spawns at the base of the first barrier, De Pere Dam. Recent research suggests that larval production is limited and no juveniles have been documented residing within the system, although limited sampling has been conducted. The objectives of this study were to describe age-0 Lake Sturgeon movement patterns, quantify the duration of river residency, and identify areas of congregation in the LFR. To track fish, a total of 38 age-0 and 1 age-1 Lake Sturgeon were obtained from nearby wild populations and hatchery settings, implanted with acoustic transmitters, released below De Pere Dam, and actively tracked, daily, for ~40 d (August-October in 2018 and 2019). Sixty-nine percent of the acoustically tracked age-0 Lake Sturgeon remained in the LFR for >50% of the study period, and emigration events appear to be related to increases in river discharge. Individuals residing within the LFR tend to congregate within six areas of the LFR encompassing 6% of total river area. The results suggest that suitable habitat exists in the LFR for age-0 and age-1 Lake Sturgeon, that discharge may cue downstream movement, and that high-occupancy areas (likely areas of preferred habitat) are present in the LFR. Long-term monitoring of larval production, survival, and age-0/juvenile dynamics will continue to help identify recruitment bottlenecks, improve knowledge of juvenile ecology, and guide future restoration goals.
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