FISHERY BULLETIN: VOL. 82, NO. 3 



at the mouth of the Mississippi River in Decem- 

 ber 1981 (RV Oregon II cruise 123) and December 

 1982 (RV Oregon II cruise 131). On cruise 123, 

 larvae were collected by oblique or surface tows of 

 either a multiple-opening closing net and envi- 

 ronmental sensing system (MOCNESS) ( Wiebe et 

 al. 1976), a neuston net, or an opening-closing 

 paired BFN-1 net 3 . The sampling scheme was 

 modified on cruise 131 in that only the 

 MOCNESS system was used and duplicate sam- 

 ples were taken at fixed depths (1, 8, and 20 m) 

 during a 24-h period of time. On board the ship, 

 menhaden larvae were removed from the sam- 

 ples, measured to the nearest 0.1 mm SL, and 

 examined for the presence of gas in the swim 

 bladder. Gas in the swim bladder was easily 

 observed before pigmentation of larvae (Fig. 1). In 

 inflated bladders, a light-refractive bubble is ob- 

 vious while a deflated bladder appears under the 

 microscope as a flattened sac (Doroshev et al. 

 1981). Maximum width and length of the swim 

 bladder (with or without gas) was measured to 

 the nearest 0.02 mm, and volume was calculated 

 by the equation for a prolate spheroid, V = 4/3 tt 

 ab 2 , where a is half the maximum bladder length 

 and b is half the maximum bladder width 

 (Hunter and Sanchez 1976). Approximate changes 

 in volume of the swim bladder due to increased 

 pressure at increased depth of capture were cal- 



P V 

 culated from Boyle's law — - = — - where P is 



P 2 v, 



pressure and V is volume, and temperature is 

 assumed to be constant. After being measured, 

 larvae were preserved in 5% Formalin 4 . 



In the Laboratory 



Experiments to determine if larvae filled the 

 swim bladder by gulping in air at the water sur- 

 face utilized larvae hatched from eggs in the 

 laboratory (Hettler 1983). Larvae were reared on 

 the rotifer Brachionus plicatilis, also cultured in 

 the laboratory. As larvae grew older, their diet was 

 supplemented with newly hatched Artemia naup- 

 lii. Before being used, larvae were held in 80 1 

 tanks at a water temperature of 20° C, salinity of 

 ca. 25%o, and a 12 h light-12 h dark photoperiod 

 without a dawn or dusk transition. 



Three hours before the start of the experiment, 

 15-20 larvae were transferred from the rearing 

 tanks to each of eight 10 1 tanks, and 10 larvae 

 were measured and observed for the presence of 

 gas in the swim bladder. A 500 /xm screen was then 

 placed below the water surface (Fig. 2) in four of 

 the experimental tanks to prevent access of the 

 larvae to the air-water interface. In the other four 

 tanks, larvae had access to the air-water interface. 

 During the experiment, the 12-h-light photoperiod 

 was continued. Larvae sampled at ca. 1800, 2100, 

 0630, 0900, and 1230 h were measured and ob- 

 served for gas in the swim bladder. 



3 Tarez and Co., 8460 S.W. Street, Miami, FL 33143. 



"•Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



Bulla 



Gas Bubble 



ln Gut Swimbladder 



FIGURE l.— Larval gulf menhaden, Brevoortia patronus. Inflated swim bladder, gas-filled bullae, and gas bubble in foregut are 



indicated by arrows. 



514 



