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We present a graph theoretic model of analysing food web structure called regular equivalence. Regular equivalence is a method for partitioning the species in a food web into ''isotrophic classes'' that play the same structural roles, even if they are not directly consuming the same prey or if they do not share the same predators. We contrast regular equivalence models, in which two species are members of the same trophic group if they have trophic links to the same set of other trophic groups, with structural equivalence models, in which species are equivalent if they are connected to the exact same other species. Here, the regular equivalence approach is applied to two published food webs: (1) a topological web (Malaysian pitcher plant insect food web) and (2) a carbon-flow web (St. Marks, Florida seagrass ecosystem food web). Regular equivalence produced a more satisfactory set of classes than did the structural approach, grouping basal taxa with other basal taxa and not with top predators. Regular equivalence models provide a way to mathematically formalize trophic position, trophic group and trophic niche. These models are part of a family of models that includes structural models used extensively by ecologists now. Regular equivalence models uncover similarities in trophic roles at a higher level of organization than do the structural models. The approach outlined is useful for measuring the trophic roles of species in food web models, measuring similarity in trophic relations of two or more species, comparing food webs over time and across geographic regions, and aggregating taxa into trophic groups that reduce the complexity of ecosystem feeding relations without obscuring network relationships. In addition, we hope the approach will prove useful in predicting the outcome of predator-prey interactions in experimental studies. r

Abstract.-Sounds produced by spawning fishes in Pamlico Sound, North Carolina, have been recorded both under captive conditions and in hydrophone and sonobuoy field surveys. These sounds, produced by males, are species specific, are associated with spawning, and are most likely used for advertisement to attract females. Sounds can be discriminated by use of spectral analysis (oscillograms and spectrograms) of recordings, and the peak frequencies produced by each species can be correlated with species and fish size. Sonobuoys were used for passive acoustic surveys, which were ''sound truthed'' from recordings of captive fishes to determine the timing and location of spawning sites for four species in the family Sciaenidae: red drum Sciaenops ocellatus, spotted seatrout Cynoscion nebulosus, weakfish C. regalis, and silver perch Bairdiella chrysoura. During May-September 1998, sounds were first detected in the early evening, increased in loudness after sunset, and ended by sunrise. Weakfish and silver perch were heard predominantly at inlet locations in May and June, whereas spotted seatrout (peak drumming in July) and red drum (peak drumming in September) were heard predominantly at lower-salinity river mouth locations in western Pamlico Sound. Passive acoustic surveys can be used to determine critical spawning habitats of sciaenid fishes; such surveys have revealed interesting insights into fish behavior and should be integrated into ocean observing systems.

Abstract.-Many fishery biologists that are interested in documenting fish habitat and following the movements and behavior of fishes use acoustic tags. Because over 700 fish species naturally produce lowfrequency, species-specific sounds, these can be used as natural acoustic tags. Passive acoustic approaches (monitoring sound-producing fishes with hydrophones) show great promise for gathering data in a noninvasive and continuous manner. In this special section, authors review past studies and contribute new findings based on the concept of passive acoustics, in which the sounds produced by fish are used to identify the species present and quantify their relative abundance. Fish have long been known to produce lowfrequency sounds, especially members of the families Sciaenidae, Gadidae, Ictaluridae, Cyprinidae, Batrachoididae, Haemulidae, Lutjanidae, and Serranidae. Passive acoustic methods include the use of lowfrequency hydrophones, digital recorders, autonomous recording sonobuoys and data loggers, and towed hydrophone arrays to record fish sounds. The sounds of fishes that have been recorded so far have been described in monographs, scientific papers, and online digital libraries; in most cases, the recordings are species specific and can be used to identify fish. Work is progressing in using the passive acoustic approach along with traditional fisheries sampling methods (net and active acoustic surveys) to identify habitat use, spawning areas, and relative abundances. The authors in this special section present new passive acousticsderived data on sciaenids, batrachoidids, and ictalurids. They outline the methods currently being used and discuss their limitations, provide examples where passive acoustics has been employed successfully, warn of pitfalls in interpreting acoustic data, and lay the groundwork for future studies.

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