Sexual Dimorphism in Sound Production and Call Characteristics in the Striped Weakfish Cynoscion guatucupa

Within the family Sciaenidae, the genus Cynoscion is composed of 25 species, but sound production in the striped weakfish C. guatucupa has not been documented. The striped weakfish possesses sexually dimorphic bilateral sonic muscles used for sound production which is associated with spawning and disturbance calls. The bilaterally paired muscles lie on the inner body wall of the male, surrounding the swimbladder. Positive identification of advertisement calls in captivity were identified with spawning activity in the field. We found that advertisement calls recorded in the field were similar to voluntary calls recorded in tanks during the spawning season. In captivity, advertisement calls had an average pulse duration of 28.8 +/- 7.04 ms, an inter-pulse interval of 219.1 +/- 83.7 ms, and a dominant frequency of 457 +/- 19.6 Hz in the spawning season. Striped weakfish advertisement calls were not detected in the non-reproductive season, but fish did produce disturbance calls when exposed to disturbance stimuli. Disturbance calls consisted of a burst of pulses produced at short intervals with an average pulse duration of 7.35 +/- 1.63 ms, an inter-pulse interval of 20.2 +/- 1.75 ms, and a dominant frequency of 484 +/- 38.6 Hz. The number of pulses produced for disturbance calls was significantly greater (Mann-Whitney test, p < 0.05) than those for advertisement calls. The pulse duration and inter-pulse interval increased as the dominant frequency decreased with fish size. The relationship between the dominant frequency (of disturbance calls) and total length (TL) showed a breaking point at 17 cm TL and corresponded to sexually immature individuals of 1-2 yr old. This break point value was related to the size-age spatial segregation of the species in the study area, whereby juveniles (< 2 yr old) inhabit brackish, warmer waters in the shallowest portion of the Uruguayan coast.