FISHERY BULLETIN: VOL. 70, NO. 4 



flattening the body, would glide downward and 

 forward (Figure IC-H). The angle and rate 

 of descent were controlled by the position of the 

 head and body relative to the bottom as well as 

 by the position of the dorsal, anal, and caudal 

 fins. By raising the pectoral fins and posterior 

 portions of the dorsal and anal fins, while simul- 

 taneously lowering the anterior portions of the 

 dorsal and anal fins and arching the body, the 

 fish could brake its forward motion in midwater 

 (Figure II), either to change swimming direc- 

 tion or to approach a potential prey. Gliding 

 was an eff"ective means of covering distances 

 while in the water column. Starting at a height 

 of 1.8 m from the bottom, a fish could glide 4.5 

 to 6.5 m at speeds ranging from 34.0 to 64.0 

 cm/sec. 



A third type of swimming combined swim- 

 ming within 5 to 15 cm of the sand and crawling 

 ("shambling" as described by Kruuk, 1963; and 

 Verheijen and de Groot, 1967). This was most 

 often observed when the fish were actively seek- 

 ing prey. Shambling speeds ranged from 32.0 to 

 48.0 cm/sec. 



ACTIVITY CYCLE 



Based on hourly counts of the number of fish 

 swimming in the water column or moving along 

 the sand, we found the flounder to be primarily 

 day-active (Figure 2A-H) although the light 

 level at night was apparently sufficient to permit 

 swimming and feeding. 



Throughout the course of our observations, 

 there was an overall decrease in activity occur- 

 ring first at night (Figure 2B) and then during 

 the day (Figure 2E-H). During this time, the 

 photoperiod was changing, approximating the 

 natural seasonal daylength. While changing 

 photoperiod may act as an assignable cause for 

 the decline in the fish's activity, observations 

 included only a small part of the natural sea- 

 sonal photoperiod cycle, too brief a time to per- 

 mit a definitive statement as to its influence on 

 activity. 



FEEDING 



The fish could feed with as much facility on 

 the bottom as in the water column. Bottom 



X 

 I/I 



< 

 Ik 



o 



DC 



a 

 S 

 3 

 Z 



1200 2400 1200 2400 1200 2400 1200 

 TIME OF DAY (HOURS) 



Figure 2. — Number of fish swimming in the water col- 

 umn or moving along the sand. Counts were taken at 

 hourly intervals throughout the day and night under the 

 following average photoperiods: (A) 16.3 hr; (B) 16.4 

 hr; (C) 16.4 hr; (D) 16.3 hr; (E) 16.2 hr; (F) 16.0 

 hr; (G) 15.9 hr; (H) 15.8 hr. 



1130 



