Environmental Biology of Fishes Vol. 24, No. 4, pp. 301-306, 1989 @ Kluwer Academic Publishers, Dordrecht. Nest defenseand aggressiveinteractions between a small benthic fish (the johnny darter Etheostoma nigrum) and crayfish Frank J. Rahel Department of Zoology and The Ohio Cooperative Fishery Research Unit’, The Ohio State University, Columbus, OH 43210, U.S.A. Present address: Department of Zoology and Physiology, University of Wyoming, Laramie, WY 82071, U.S.A. Received 8.9.1987 Accepted 4.7.1988 Key words: Parental care, Predation, Orconectes Synopsis Aggression by nest-guarding male johnny darters, Etheostoma n&urn, against intruding crayfish was investigated in laboratoy experiments and field observations. In the laboratory, darter success in chasing crayfish, Orconectes rusticus, from the nest site was inversely related to crayfish size. Small crayfish (less than 15 mm carapace length) were routinely evicted from the nest area by nips directed at the posterior end of the abdomen. Although such aggressive behavior was less successful against larger crayfish, even the largest crayfish tested (carapace length 30-32 mm) were chased from the nest area in 33% of the trials. Those large crayfish that entered nests often remained despite repeated attacks by the male johnny darter and egg predation was observed. In a small Ohio stream, openings to johnny darter nests were generally between 7 and 13 mm. Thus crayfish with a carapace height greater than 13 mm (corresponding to a carapace length greater than 29mm) would not be able to enter johnny darter nests. In field observations, male johnny darters successfully defended nests against another crayfish species (Orconectes sanborni, carapace length 12-29mm). Together, aggressive behavior and small size of nest entrances allow the johnny darter to successfully reproduce in areas with abundant crayfish. Introduction Small benthic fishes and crayfish are common co- habitants of streams and the littoral zone of lakes and reservoirs. Although both use rocky substrates either as shelter from predators (Cracker & Barr 1968, Stein & Magnuson 1976, Rahel & Stein 1988) or as spawning sites (Winn 1958, Downhower & Brown 1980), little is known about how these orga- nisms interact. One particularly interesting prob- lem is how small, cavity-spawning fish such as dar- ters (Percidae) are able to coexist with large, well- armored predators such as crayfish. Crayfish are known to consume fish eggs (Magnuson et al. 1975, Horns & Magnuson 1981) and readily evict non- breeding darters from shelter (Rahel & Stein 1988). However, in nature, darters and crayfish are often abundant in the same habitat, suggesting that 1The Unit is sponsored jointly by the United States Fish and Wildlife Service, The Ohio Department of Natural Resources, and The Ohio State University.