Establishment of river herrings in a southern Delaware impoundment: Evaluation of fish passage and predation

Many coastal rivers in the mid-Atlantic region have dams along their course, which limit upstream migration of anadromous fish, including river herrings (alewife and blueback herring). River herrings, which are important as food for piscivores and avian predators, have declined in number. To increase population size, fish ladders have been installed at some of these dams; however, their effectiveness has not been extensively studied. In 1997 and 1998, we studied a fishway at Wagamons Pond on the Broadkill River in Milton, Delaware. In spring, observations were made of adult herring using the ladder to evaluate daily-use patterns. In 1997 over 10,000 and in 1998 over 2,200 herring migrated over the ladder. Passage rates were highest during the day, at higher water levels, at higher herring densities, and at water temperatures of 16.5–19.0°C. In 1997, juveniles in the pond were significantly smaller than those in the river and vice versa in 1998. Predation of juveniles by resident pond fish was quantified using bioenergetics modeling. Consumption of herring by largemouth bass and yellow perch was low and constituted less than 5% of the total prey biomass consumed and about one-quarter of the juvenile population in the pond. The installation of ladders on other systems has merit because our findings suggest that (1) herring used the fishway, (2) juveniles have the potential for increased growth in the pond, and (3) predation on juveniles constituted only 25% of the population.