HOSS and PHONLOR: SWIM BLADDER OF GULF MENHADEN LARVAE 



how larvae of physostomatous clupeoid fishes 

 initially inflate the swim bladder (Blaxter and 

 Denton 1976). 



Under field conditions, gulf menhaden larvae 

 began to fill and empty their swim bladders during 

 the approximately 45 min of twilight preceding 

 sunset and sunrise. The numbers of fish with gas 

 in their bladders increased and decreased gradu- 

 ally (i.e., it is not an all or none phenomenon) prior 

 to darkness (or daylight). This observation 

 suggests to us that larvae are responding as indi- 

 viduals to gradually changing light levels. This 

 response is probably better developed in larger 

 larvae. 



The relation of diurnal vertical migration to 

 swim bladder inflation that we found is different 

 from the generally accepted position that larvae 

 are near the sea surface with well-inflated swim 

 bladders at night and are deeper in the water with 

 deflated swim bladders during the day. We cap- 

 tured menhaden larvae at three discrete depths at 

 night (down to 20 m) and over 95% of the larvae 

 captured at 8 and 20 m had gas in their swim 

 bladders. On four previous cruises, collections in 

 the same location also showed that menhaden lar- 

 vae were distributed throughout the water column 

 at night but concentrated at the surface during the 

 day (unpubl. data 5 ). 



In conclusion, the swim bladder of the larval 

 stages of gulf menhaden acts as a buoyancy reg- 

 ulator that allows the fish to maintain a position in 

 the water column at night without movement. By 

 day the swim bladder is deflated, and the larvae 

 must actively swim to maintain their position 

 near the water surface. At some point during de- 

 velopment, the swim bladder's primary function 

 switches to that of a pressure-adjusting 

 mechanism for the otic bullae. 



5 Sogard, Susan M., Donald E. Hoss, and John J. Govoni. In 

 prep. Density and depth distribution of larval fishes at selected 



sites in the northern Gulf of Mexico. 

 Beaufort Lab., Natl. Mar. Fish. Serv. 

 28516-9722. 



Southeast Fish. Cent. 

 NOAA, Beaufort, NC 



ACKNOWLEDGMENTS 



Robin Berry and Susan Sogard assisted in the 

 data analyses and laboratory work. John Govoni, 

 William Hettler, and Susan Sogard provided crit- 

 ical review of the manuscript, and W Hettler 

 provided laboratory-reared gulf menhaden. 

 MOCNESS sampling was conducted by Shailer 

 Cummings (Atlantic Oceanographic and Mete- 

 orological Laboratories, NOAA) with able assis- 

 tance from the crew of the RV Oregon II. This 

 research was supported in part by a fellowship to 

 the Junior author from the Brazilian National 

 Science Council. 



LITERATURE CITED 



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