Effectiveness of a Fish Passage Facility for Anadromous River Herring Recruitment

Migratory stocks of river herring, alewife (Alosa pseudoharengus) and blueback herring (A. aestivalis) have declined as a result of overfishing, poor water quality, and loss of spawning and nursery habitat. To provide access to previously blocked spawning and nursery areas, fish passage facilities have been installed at stream blockages. In 1997, a fish ladder was installed on Unicorn Lake, a tributary of the Chester River in Maryland. In 1998 and 1999, we evaluated the effectiveness of the fish ladder for passing adult river herring and surveyed the upstream habitat for juvenile recruitment. In 1998, 18 alewife and 3,800 blueback herring used the fishway. Temporary modifications made to the fishway in 1999 substantially increased passage to 1,270 alewife and 13,400 blueback herring when densities below the dam appeared to be similar between years. Besides using the fishway, an additional 3,375 alewife were manually passed into Unicorn Lake 1999. Although juvenile river herring reportedly reside in upstream areas for several months, only 1 individual was collected during 2 years of the study in Unicorn Lake. The apparent recruitment failure may be attributed to low food availability, poor water quality, early emigration, or predation on early life stages.

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