Field studies on the ureogenic gulf toadfish in a subtropical bay. I. Patterns of abundance, size composition and growth

Opsanus beta abundance in Biscayne Bay, Florida, varied significantly depending on site and season. O. beta catch per unit effort (cpue) was highest in southern study sites which tended to provide the combination of shallow mean depth, nocturnally high levels of dissolved oxygen, and Thalassia-dominated subtrate. Significant seasonal differences in abundance were restricted to the three sites where O. beta cpue tended to be highest; two of these sites shared the same pattern of increase during the cool-dry season (November-April), followed by a return to lower levels during warm wet season (May-September). Dramatic seasonal abundance increases could not be ascribed to large influxes of immature fish. Rather, size composition data suggested sexually mature individuals moved seasonally among habitats, possibly for reproductive purposes. Modal analysis of length-frequency data followed by nonlinear regressions yielded preliminary values for the von Bertalanffy growth parameters K and t(II) given that L(t) = 300 mm. Possible reasons for O. beta growing slower and attaining a smaller maximal size than its temperate congener, O. tau, are discussed.