FISHERY BULLETIN: VOL. 74, NO. 4 



various times of day commencing at age 1 day to 

 determine if a daily rliytiim of inflation existed 

 and to determine the larval length at which the 

 swim bladder was inflated. Samples of preserved 

 specimens from California Cooperative Oceanic 

 Fisheries Investigations (CalCOFI) ichthyoplank- 

 ton collections were also examined to determine if 

 differences existed in swim bladder inflation in 

 sea-caught specimens. 



The standard length was measured to the near- 

 est 0.1 mm and the maximum width and length of 

 the swim bladder to the nearest 0.02 mm. The 

 volume of the swim bladder was calculated by 

 using the equation for a prolate spheroid, 

 V = i/STrab'-, where a is half the maximum blad- 

 der length and b is half the maximum width. For 

 larvae 16 mm and larger, the calculated swim 

 bladder volume may be converted to actual gas 

 volume by multiplying it by the coefficient 0.82 

 (Figure 1). This conversion is based on data ob- 

 tained while measuring the composition of swim 

 bladder gas. The larvae used in that experiment 

 were larger (mean length 15.6 to 29.6 mm) than 

 most of the larvae in the rest of the experiments. 

 For this reason we have used the calculated swim 

 bladder volume in all computations. 



We also sampled larvae reared in the laboratory 

 to determine the eff"ect of swim bladder develop- 



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CALCULATED VOLUME (mm^) 



Figure 1.- Relation between the volume of the swim bladder 

 calculated from the equation V = 4/3 :r ab- and the actual volume 

 of gas extracted from the swim bladder for northern anchovy 

 larvae of mean length 15.6 to 29.6 mm. Each point is the mean 

 volume of the swim bladder calculated for a sample of two to 

 eight larvae taken at night and the average volume of gas 

 extracted from that sample. Sample means were weighted by 

 their variances to calculate the regression line; intercept for line 

 did not differ from 0; and the standard error of line was 0.0428. 



ment and swim bladder inflation on sinking rate. 

 The method of Blaxter and Ehrlich (1974) was used 

 to measure sinking rates of larvae. Larvae an- 

 esthetized in MS 222^ were measured and added to 

 a 1-liter graduated cylinder without contact with 

 the air. The larvae were allowed to sink a few 

 centimeters, then the rate of descent was timed 

 with a stopwatch for a distance of 7 to 35 cm. Only 

 one measurement was made per larva and larvae 

 were reexamined after the test to determine if 

 they were still alive (dead larvae sank faster than 

 live ones) and if any gas had been lost from their 

 bladders. Fresh seawater was used in the 

 graduated cylinder for each day's run and the 

 specific gravity and temperature of the seawater 

 were measured before each larva was tested. The 

 specific gravity averaged 1.0262 and ranged from 

 1.0259 to 1.0266. The graduated cylinder was 

 immersed in a temperature-controlled water bath 

 which was maintained within 1°C of the rearing 

 temperature. One rearing group was tested at 

 15.9° ± 0.2°C and another at 18.0° ± 0.1°C. In the 

 Results section we have combined the data from 

 these two rearing groups because covariance 

 analysis indicated that the diflferences in sinking 

 speed when adjusted for swim bladder volume and 

 larval length were not significant. 



To determine if anchovy larvae filled the swim 

 bladder by gulping air at the water surface, the 

 following experiment was performed. Commenc- 

 ing 4 h after the onset of dark, larvae in a 400-liter 

 rearing tank were sampled and the lengths and 

 dimensions of the swim bladder of each larva in 

 the sample measured. Just before the onset of dark 

 on the following day, the surface of the tank was 

 sealed with a 0.5-cm layer of mineral oil. A second 

 sample was taken commencing at 2400 h, 4 h after 

 the onset of dark and ending just before the 

 beginning of light at 0800. A third sampling was 

 taken of lar\-ae in the sealed tank during the day 

 beginning at 1000 h, 2 h after the onset of light, 

 and ending at 1400. 



The gas content of the swim bladders of labora- 

 tory-reared larvae captured in the dark was an- 

 alyzed using the micro gasometric method and 

 apparatus described by Scholander et al. (1955). 

 Swim bladders were dissected from the larvae in 

 acid citrate solution, removed with a Pasteur 

 pipette, and injected into an acid citrate filled 

 capillary tube sealed at one end. After two to eight 



-Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



848 



