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Fishery Bulletin 91(2). 1993 



9 II 13 15 17 19 21 



Total Length (mm) 



Figure 2 



Percentage of the total number of larvae of the different 

 total length sampled for the swimbladder inflation vs. 

 light intensities experiment (Fig 3). Dashed line shows 

 larvae sampled before exposed to different light levels 

 (control; total n = 193), whereas the solid line is larvae 

 exposed to different light conditions (experimental; 

 rc=834). 



was consistent for all experiments. Since the proba- 

 bility of swimbladder inflation was not equal for 

 each size-category (Fig. 1), results will be biased 

 to responses of the most-abundant size if all size- 

 classes are grouped together. Thus, detailed analy- 

 ses should only consider individual size-classes. 

 The most-abundant size (llmmTL; Fig. 2) will be 

 used for this purpose. 



The percentage of fish inflating their swim- 

 bladders increased as they were exposed to 

 lower light intensities (Fig. 3). The highest light 

 intensity to induce a significant increase in 

 the proportion of fish with inflated swim- 

 bladders (threshold intensity) varied slightly 

 with fish size. For llmmTL larvae (Fig. 3A) 

 the threshold intensity was -6x10' ! photons 

 cnr 2 s _1 , whereas for 12-16 mmTL larvae, it 

 was 1 log unit higher (Fig. 3B). For both size- 

 groups, the proportion filling their swim- 

 bladders at ~10 13 photons cm -2 s _1 and lower 

 light intensities was not significantly different 

 from the proportion in darkness. 



The variation in swimbladder volume 

 with light intensity was considered in detail 

 for llmmTL larvae (Fig. 4). Mean volume 

 increased as light intensity decreased, but 

 the difference was not significant between the 

 initial mean volume and that in darkness 

 due to large variances (Fig. 4). Similar results 

 were also obtained for 12 and 13 mmTL 

 larvae. 



In contrast, swimbladder volume increased proportion- 

 ately with larvae size (Fig. 5). When the relationship be- 

 tween mean volume (V) in darkness and total length (L) 

 was expressed as the allometric equation (V=aL b ), the slope 

 of the regression (b) equaled 5.31 (r 2 =0.99,p<0.0001). Means 

 were calculated for larvae of each size in all conditions, 

 because volume did not change with lighting condition 

 (Fig. 4). Since volume changed with larval length, volumes 

 could not be averaged for larvae of different lengths. 



Timing of swimbladder inflation 



The timing of swimbladder inflation was measured upon 

 transfer from rearing-light intensity to darkness. By pro- 

 ducing the maximum rate of intensity change, we assumed 

 the maximum rate of inflation should be evoked. Results 

 were combined for larvae 11-16 mmTL because the pro- 

 portion inflating in darkness for 11 mm larvae was not 

 statistically different from the proportion of 12-16 mmTL 

 larvae (Fig. 3). A significant increase in the proportion 

 with inflated swimbladders was evident after 5 min in dark- 

 ness (Fig. 6). The maximum percent inflation was reached 

 within 20 min. The proportion of fish with inflated swim- 

 bladders then remained relatively constant for about the 

 next 1.5 h. 



Endogenous rhythm in swimbladder inflation 



The percent inflating prior to placement in darkness re- 

 mained low throughout the 24h sampling interval, which 



