PREDATION BY HYDRA ON LARVAL FISH - FIELD AND LABORATORY EXPERIMENTS WITH BLUEGILL (LEPOMIS-MACROCHIRUS)

We found high population densities of Hydra canadensis (up to 30,000 m (-2)) on macrophytes in Lake Opinicon (Ontario, Canada) and tested whe ther they influenced the survival of larval bluegill, Lepomis macrochi rus. Hydra were observed to capture and ingest bluegill larvae in labo ratory trials. Individuals that ingested fish larvae turned black; tho se fed a variety of crustacean species remained brown. The ingestion r ate of Hydra on larvae in 8-liter laboratory microcosms was higher dur ing the night (0.197 larvae predator(-1) h(-1)) than during the day (0 .111 larvae predator(-1) h(-1)). Many larvae also died after escaping from the stinging tentacles of Hydra (on average 26% of larval mortali ty). Population densities of Hydra were highest on the macrophyte Myri ophyllum spicatum. Bluegill colonies were surrounded by dense beds of M. spicatum in some areas, and larvae in the colonies had to swim thro ugh the plants (with the attached Hydra) in order to reach open water. We sampled Hydra in the vicinity of bluegill colonies on the morning after larval swim-up. The number of black Hydra decreased with increas ing distance from a colony; 72% black within colonies, 45% at 0.5 m ou tside colonies, and 31% at 2 m outside of colonies. We estimate that u p to 20% of the larvae produced by a colony can be killed by Hydra wit hin this 2-m zone. Many more larvae likely die as a result of encounte rs with Hydra that are abundant throughout the lake.