Fish and Blue Crab Passage through Water Control Structures of a Coastal Bay Lake

Abstract

Coastal bay lakes in the southeastern United States may be connected to coastal waters by natural openings or artificial channels, but often access to them by aquatic organisms is restricted by water control structures (WCSs) designed to minimize saltwater intrusion into freshwater habitats. Our study examined movements of alewife Alosa pseudoharengus, white perch Morone americana, American eel Anguilla rostrata, and blue crab Callinectes sapidus through two WCSs separating Lake Mattamuskeet, North Carolina, and canals leading to Pamlico Sound. All WCSs were located on Mattamuskeet National Wildlife Refuge property. Three WCS designs were tested for fish passage efficiency. The original wooden flapgate configuration was hinged at the top, which opened the full width of the embayment and was pressure activated from lake levels. A replacement design, installed in each WCS embayment in 1987, was a wooden stopblock containing a small, hinged stainless steel flapgate mounted near the bottom. This design minimized saltwater intrusion but required a considerable lakeside head to open. High water velocities produced conditions unfavorable for alewife and blue crab passage, but some species, especially white perch, were capable of moving through the restrictive opening. Small, experimental slotted weirs installed in the stopblocks near the surface passed organisms more effectively than the stainless steel flapgates. After 10 years of steel flapgate operation, the alewife spawning run into Lake Mattamuskeet decreased from an estimated high of 199,600 fish in 1970 to only 178 fish in 1997 and 454 fish in 1998. In 1999, wooden flapgates of the original design were installed experimentally, which resulted in favorable conditions (wider aperture and reduced velocities) for fish and blue crab passage. Reverting to the original flapgate design should maintain the mixed community of freshwater and marine–estuarine species in the lake.