Unusual swimbladder behavior of fish in the Cariaco Trench

An extensive acoustic and biological experiment was conducted to examine the swimbladder behavior of fish in the Cariaco Trench, off Venezuela, whose deeper waters are anoxic. Forty-eight hours of broadband (1-20 kHz) acoustic scattering measurements determined volume scattering strengths as a function of depth, frequency, and time of day. Fifty-nine successful biological trawl deployments determined species composition and their depths as a function of time of day. A swimbladder scattering model was used to correlate biological and acoustic results. The variations in swimbladder parameters required to match model results to the measured scattering strengths provide the basis of the explanation of swimbladder behavior. Adults of only two fish species, Bregmacerous cantori and Steindachneria argentea, were caught. Both species were in the upper 200 m at night and began a downward migration shortly before sunrise. S. argentea settled in the vicinity of the oxic-anoxic interface, at 300-440 m, and remained there until late afternoon. B. cantori migrated to depths greater than 800 m, deep into anoxic water. B. cantori dispersed in early afternoon and eventually congregated at depths of 360-550 m in late afternoon. Both species began an upward migration to their nighttime depths shortly before sunset. In the pre-dawn hours, scattering in the 3-8 kHz range increased dramatically. Swimbladder scattering modeling indicates that during this time swimbladder volumes expanded significantly, tension in the swimbladder walls increased, the properties of the flesh surrounding the swimbladders changed, and most importantly, the mass of gas in the swimbladders increased. The modeling further indicates that both species use the oxygen added before dawn to survive in their respective hostile environments during the day. Thus, the results of this experiment indicate that fish inhabiting the Cariaco Trench are able to live at hostile depths during the daytime because of changes they make to their swimbladders during the nighttime, before descending to those depths. (C) 2003 Elsevier Ltd. All rights reserved.