Lake sturgeon Acipenser fluvescens, which are now protected from harvest, are considered rare in the upper Mississippi River and little information is available on the remaining populations. Transmitters were implanted into 31 lake sturgeon from two sites in the upper Mississippi River to describe their habitats and movement. The areas surrounding the tagging sites were core areas for both groups of lake sturgeon based on the high use (about 50% of locations by group) and frequent return to these areas by many of the tagged fish. Core areas contained sites with unique hydraulic characteristics, such that depositional substrates were common yet flow was present; these areas probably provide important feeding habitat for lake sturgeon. Minimal geographical overlap in range occurred between groups, suggesting that river reaches and associated core areas were unique to groups or substocks of fish. Lake sturgeon exhibited complex movement behaviors and had ranges of 3‐198 km (median, 56 km) during the study. Tagged fish moved both downstream and upstream through upper Mississippi River navigation dams. However, dams appeared to be intermittent barriers to upstream passage because upstream passage events (10 fish, 19 passages) were fewer than downstream events (13 fish, 35 passages). Extensive use of the Wisconsin River by one group of lake sturgeon tagged in the upper Mississippi River has implications regarding management of a threatened population that transcends regulatory boundaries. Our study indicates that lake sturgeon in the upper Mississippi River system share many movement and habitat use characteristics with populations in other systems. However, significant data gaps preclude development of cogent management strategies, including information on population numbers and dynamics, identification of spawning areas, relations between groups, and assessment of the effects of commercial navigation.

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Major river channels have been extensively altered worldwide. The development of restoration strategies for those alterations requires fundamental information, including the use of large deep channels by fishes. We trawled within parallel paths distributed across the width of the main-channel trough of the Mississippi River to identify how the lateral distribution of fishes responds to variations in flow ranging from 50% to 200% of the annual median, water temperature ranging from 9 to 298C and commercial shipping traffic ranging from 0 to 9 vessels per day. Among the species we encountered, only shovelnose sturgeon (Scaphirhynchus platorynchus) were persistent channel residents that remained concentrated along the main-channel centreline regardless of flow, temperature and traffic. Other persistent residents showed no distinct pattern in lateral distribution, concentrated along the deep channel margins or varied in lateral distribution with flow. Surprisingly, large adult bluegill (Lepomis macrochirus), which are conventionally viewed as limnophils, were the second-most abundant species in our samples and became increasingly abundant within the deep channel trough as flow decreased below the annual median. Clearly, those fishes exploit resources contained in the main channel and are, therefore, better viewed as opportunistic limno-rheophils. Our results imply the existence of poorly understood food resources in the main channel. We conjecture that re-creation of freeflowing secondary channels and features that increase the production and transport of invertebrates in channels can help mitigate adverse effects of channel alteration and commercial shipping. Published in

Terrestrial agricultural activities strongly influence riverine nitrogen (N) dynamics, which is reflected in the δ15N of riverine consumer tissues. However, processes within aquatic ecosystems also influence consumer tissue δ15N. As aquatic processes become more important terrestrial inputs may become a weaker predictor of consumer tissue δ15N. In a previous study, this terrestrial-consumer tissue δ15N connection was very strong at river sites, but was disrupted by processes occurring in rivermouths (the ‘rivermouth effect’). This suggested that watershed indicators of N loading might be accurate in riverine settings, but could be inaccurate when considering N loading to the nearshore of large lakes and oceans. In this study, the rivermouth effect was examined on twenty-five sites spread across the Laurentian Great Lakes. Relationships between agriculture and consumer tissue δ15N occurred in both upstream rivers and at the outlets where rivermouths connect to the nearshore zone, but agriculture explained less variation and had a weaker effect at the outlet. These results suggest that rivermouths may sometimes be significant sources or sinks of N, which would cause N loading estimates to the nearshore zone that are typically made at discharge gages further upstream to be inaccurate. Identifying definitively the controls over the rivermouth effect on N loading (and other nutrients) will require integration of biogeochemical and hydrologic models.

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