LATERAL DISTRIBUTION OF FISHES IN THE MAIN-CHANNEL TROUGH OF A LARGE FLOODPLAIN RIVER: IMPLICATIONS FOR RESTORATION

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 29 degrees C 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 free-flowing 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 2009 by John Wiley & Sons, Ltd.