FISHERY BULLETIN: VOL. 74, NO. 4 



led Ida to conclude that the diurnal change was 

 caused by the vertical migration of the larger 

 larvae (10 to 15 mm). 



The diel vertical movements that appear in 

 larval anchovy at the time of swim bladder 

 inflation probably persist into adult life. The 

 adults, however, are quite variable in their 

 behavior which changes with size of school and 

 season (Mais 1974). Vertical migration is most 

 noticeable in large schools which are deep during 

 the day (119 to 220 m) and rise to the surface and 

 disappear as sonar targets at dusk. These schools 

 reform and descend at first light in the morning 

 (Mais 1974). 



Possible Adaptive Advantages 



Inflation of the swim bladder reduces the energy 

 required for maintaining a position in the water 

 column. This reduction in sinking speed could 

 represent an important energy savings for larval 

 anchovy because they do not feed at night and 

 swimming can not be used in the search for food. 

 The major energy cost of a diel rhythm of swim 

 bladder inflation is the required vertical migration 

 to the surface. Laboratory work suggests that 

 anchovy larvae, by modification of swimming 

 speed and direction of turning, are able to find and 

 remain in area of high food density (Hunter and 

 Thomas 1974). Thus, it is possible that a larva could 

 follow an upward and downward movement of 

 food at dusk and dawn. In this case the added cost 

 for vertical movements would be slight since the 

 energy spent in swimming could be used in 

 searching for food. It is unlikely, however, that 

 this condition could always be met. Thus, the 

 energy saved at night by inflation of the swim 

 bladder should exceed that used in vertical migra- 

 tion. Assuming the energy used per centimeter 

 swum is the same for vertical migration as for 

 maintaining a position in the water at night, the 

 energy used in a round trip vertical migration of 

 100 m would be equivalent to that used to maintain 

 a position for 10 h at night when the sinking speed 

 was 0.28 cm/s. Thus, the difference between day 

 and night sinking speeds would have to exceed 0.28 

 cm/s before a 100-m round trip could be considered 

 an energy sparing mechanism. The difference in 

 sinking rates exceeds 0.28 cm/s for larvae 13.5 mm 

 and larger (Figure 6). This difference increases 

 with larval length suggesting that the vertical 

 range of migration over which energy savings are 



854 



possible increases with length. In addition, the 

 difference between day and night sinking speeds 

 may be underestimated because sinking speeds 

 were measured at the surface. If larvae descend 

 during the day the gases in the swim bladder 

 would be compressed, increasing body density and 

 thereby increasing the sinking speed for larvae in 

 the day. 



These calculations are, of course, a great 

 oversimplification, but they do illustrate that the 

 energy saved by inflation of the swim bladder at 

 night could exceed the cost of a vertical migration 

 and that the possible range of migration could be 

 greater for larger larvae. 



The energy costs of maintaining a position in 

 the water column for fish with and without swim 

 bladders have been calculated by Alexander 

 (1972). His calculations are not appropriate for 

 anchovy larvae at night because he considered fish 

 without a bladder to be continuously swimming 

 and gaining lift from the pectoral fins. The 

 behavior of an anchovy at night that failed to 

 inflate the swim bladder would probably resemble 

 one with an inflated bladder. It would sink motion- 

 less at an oblique angle to the water surface and 

 interrupt sinking by bursts of near vertical swim- 

 ming. To maintain a position, these bursts of 

 swimming would have to be of longer duration or 

 of greater frequency than if the swim bladder 

 were filled. 



In addition to an energy sparing mechanism, a 

 nightly pattern of swim bladder inflation could 

 possibly reduce predation. Some predators of 

 larval fishes, for example chaetognaths and 

 medusae, use the movement or turbulence 

 produced by prey for detection and attack 

 (Horridge 1966; Newbury 1972). Thus, the reduc- 

 tion of activity produced by slower sinking speeds 

 could reduce predation. The vertical migration of 

 the larvae could also result in exposure to different 

 and possibly less hazardous predators at night. It 

 would also serve to aggregate larvae, thus facili- 

 tating social contacts necessary for the develop- 

 ment of schooling which begins at about 15 mm. 



ACKNOWLEDGMENTS 



Harold Dorr and Sharon Hendrix assisted in the 

 laboratory work. James Zweifel provided statis- 

 tical advice and Reuben Lasker and Paul Smith 

 reviewed the manuscript. E. H. Ahlstrom allowed 

 us to present original data on vertical distribution 

 of anchovy larvae. 



