HOSS and PHONLOR: SWIM BLADDER OF GULF MENHADEN LARVAE 



FIGURE 2. — Cross section view of tanks used in laboratory 

 experiments to determine if larvae of gulf menhaden, Brevoor- 

 tia patronus, fill the swim bladder at the water surface. 



RESULTS 



At Sea 



The percentage of larvae having gas in the swim 

 bladder was much greater at night than during 



the day (Fig. 3). The number of larvae with gas 

 increased within an hour of sunset and decreased 

 within an hour of sunrise (i.e., within the twilight 

 periods). Although sample size was sometimes 

 small (8-10 fish), the combined data for the two 

 cruises were consistent. 



The volume of gas in the swim bladder of larvae 

 captured at night was greater than the volume of 

 gas in larvae captured during the day (Table 1). 

 There was a significant difference between the day 

 and night volumes, where sample size was suffi- 

 cient to allow testing. Sunrise and sunset samples 

 were not significantly different U-test, P > 0.05) 

 from daylight samples. 



Gulf menhaden larvae showed a diurnal pattern 

 of depth distribution that seemed to contrast the 

 pattern of swim bladder inflation (Table 2). Night 

 sampling (1700-0600) indicated that larvae were 

 present from the surface down to at least 20 m, the 

 deepest samples taken. Maximum water depth at 



r^ ss 

 r Day— r 



or 



LU 

 Q 

 Q 



< 

 _l 



i< 

 W I 



LU t 



O 5 



LU 



o 

 a. 



LU 



O- 



1600 2000 M 0400 0800 N 1600 2000 M 0400 



Midnight Noon 



TIME OF DAY 



FIGURE 3. — Diurnal change in percent of gulf menhaden, Brevoortia patronus, larvae with gas in 

 their swim bladders (S.S. = sunset, S.R. = sunrise). Numbers offish examined are in parentheses. 

 Data from two cruises (Dec. 1981, open circles; Dec. 1982, closed circles) are combined and presented 

 in chronological order. 



TABLE 1. — Swim bladder volume of gulf menhaden, Brevoortia patronus, 

 larvae measured immediately after capture. Samples are from oblique 

 and surface net tows. 



'SE = standard error of the mean. 



515 



