SynopsisHydrophone recordings were made to document the daily and seasonal cycles of weakfish, Cynoscion regalis (Sciaenidae), sound production in the Delaware Bay . Recordings were made at three stations perpendicular to the shore and rated qualitatively on a scale of 0-4 . `Drumming' (sonic muscles, males only) and `chattering' (pharyngeal stridulation, both sexes) were tallied separately . Weakfish were collected from the field throughout the spring and summer to monitor the reproductive status of the population . A clear seasonal trend was noted in both drumming and chattering activities . Drumming activity rose abruptly in mid-May to maximal levels, which were maintained through late June, then dropped off in July and ceased in August . Chattering activity began abruptly in late June, then rose through July and remained high in August . Seasonal drumming activity was more intense at the inshore station . On a daily scale, drumming activity was at its lowest level during the early to mid-morning, then increased to maximal levels in the early evening and remained high until the following morning . There was no daily trend in chattering activity . Daily drumming activity was also most intense at the inshore station . Male and female reproductive readiness, determined by gonadosomatic indices, sperm motility and plasma androgen levels, peaked within the period of maximal seasonal drumming activity .

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Lunar‐synchronized spawning has been reported in four orders of fish. Salmoniform, atheriniform, and tetraodontiform species spawn intertidally on spring tides, leaving their eggs exposed to air between tidal inundations. This spawning mechanism may be essential to survival of species that are residents of areas such as tide marshes where dissolved‐oxygen concentrations in the water column can be near zero. Spawning cycles of both lunar and semilunar periods have been reported in coral‐reef fishes of the Perciformes. Reproduction by these fishes does not include aerial incubation of eggs. It has been hypothesized that the spawning cyclicity in these fishes synchronizes reproduction with moonlight or current conditions that enhance parental care or predator avoidance. The intertidal spawners that have been studied display cyclic changes in gonad maturity consistent with a semilunar periodicity in recruitment of oocytes into final maturation. Oocytes in the early stages of vitellogenesis are present throughout the spawning season. In the mummichog Fundulus heteroclitus, this periodicity is generated by an endogenous mechanism that operates in the absence of diurnal, lunar, or tidal periodicities in light or water movement. Indirect evidence exists for the involvement of both light‐and tide‐related phenomena in synchronization of semilunar reproductive rhythms in fishes.