FISHERY BULLETIN: VOL. 82, NO. 3 



TABLE 2. — Swim bladder volume of gulf menhaden, Brevoortia patronus, larvae captured in 

 MOCNESS tows and measured immediately after capture. Volumes were corrected for expansion 

 of the swim bladder due to the change in pressure (0.1 atm/m of water). 



1 SE = standard error of the mean 



2 Swimbladder volume of the three fish captured was not measured. 



this station varied between 23 and 27 m. During 

 the day nearly all the larvae were taken at the 

 surface and only three larvae were captured as 

 deep as 8 m. Without exception, fish examined 

 from daylight samples did not have gas in the 

 swim bladder, while almost all of the fish from the 

 night samples contained some gas. In some cases, 

 the volume of the swim bladder was such that it 

 constricted the gut (Hunter and Sanchez 1976) or 

 burst through the body wall. 



The volume of gas in the larval swim bladder 

 would, of course, be reduced due to increased pres- 

 sure as the larvae moved deeper in the water. As 

 the volume of the swim bladder decreased, its ca- 

 pacity as a buoyancy organ would decrease, caus- 

 ing the larvae to expend more energy in swimming 

 or to sink more rapidly Since our measurements 

 were all made at the surface, we corrected the 

 volumes of the swim bladders in larvae collected at 

 8 and 20 m to reflect the increased pressure at 

 these depths (Table 2). Since swim bladder volume 

 is related to size of the fish, a £-test was used to 

 compare the mean standard length of the larvae 

 from each depth. This test showed no significant 

 difference in lengths of fish captured at the three 

 depths U-test, P > 0.05). 



In the Laboratory 



Swim bladder volume was much greater in 

 tanks where the larvae had direct access to air 

 (Fig. 4). Swim bladder volume of the larvae with- 

 out access to air remained essentially the same 

 throughout the experiment. It appears from this 

 experiment that gulf menhaden larvae, like a 

 number of other clupeoid species, fill their swim 

 bladders by swallowing air at the surface. 



DISCUSSION 



Our findings for swim bladder inflation in larval 

 gulf menhaden generally agree with the findings 

 of Hunter and Sanchez (1976) and Uotani (1973) for 



other clupeoid species. Our field studies showed 

 conclusively that gulf menhaden inflate their 

 swim bladders at night and deflate them during 

 the day. Hunter and Sanchez (1976) suggested that 

 nighttime swim bladder inflation in larvae of the 

 northern anchovy is an energy-sparing 

 mechanism that allows larvae to reduce swim- 

 ming activity during nonfeeding periods while 

 maintaining their depth in the water column. 

 These authors further suggest that a reduction in 

 swimming activity may reduce predation, since 

 some predators of larval fish (e.g., chaetognaths) 

 use the water movement caused by swimming 

 activity to detect their prey. 



In the laboratory we found that larvae were 

 unable to fill their swim bladders when they were 

 prevented from reaching the air-water interface. 

 This too agrees with the previous hypothesis on 



1600 2000 2400 0400 0800 1200 1600 

 TIME OF DAY 



FIGURE 4.— Swim bladder volume (X ± 2 SE) of gulf menhaden, 

 Brevoortia patronus, held in the laboratory with access (solid 

 line) and without access (dashed line) to the air- water interface. 



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