SMITH ET AL: DIEL MOVEMENTS OF LARVAL FLOUNDER 



circulation was largely responsible for its move- 

 ments (see Figure 1 insert). 



The analysis of variance indicated that we 

 stayed within the same patch of larvae throughout 

 the study. Daily mean differences in both the 

 number and size of larvae were not significant. 

 There was, however, a highly significant differ- 

 ence between means of day and night catches, and 

 between catches at the four depths sampled. We 

 attributed these differences to diel movements 

 and the resultant shift in the distribution of most 

 larvae toward the surface, where two nets fished, 

 at night. The diel movements were repetitious in 

 time and extent. There was no significant differ- 

 ence in means of catches within daylight and night 

 tows, or in their depth distribution at a given time 

 during each day (Table 1). 



Larvae were most abundant in the 20-m net 

 during daylight tows on the first day. None were 

 caught by the surface net and the combined catch 

 of the nets at 8 and 48 m contributed <15'7c of the 

 daytime catch. The distribution of larvae changed 

 significantly after dark. By 2200 h the catch in the 

 surface net was greater than the combined catch of 

 the other three nets and more than double that of 

 any other net. When combined, the surface and 

 8-m catches accounted for nearly 17% of the 

 2200-h catch. At 0100 h larvae remained most 

 abundant at the surface and, although the surface 

 catch was less than at 2200 h, again the upper two 

 nets accounted for >709c of the catch (Figure 2). 

 At 0400 h, the last nighttime tow, most larvae 

 were caught at 8 m (Table 2). 



The vertical movements of larvae throughout 

 the second day were similar to those on the first 

 day. Most larvae were taken at 20 m on each of the 

 five daylight tows. Except for a single specimen in 

 the 1600-h tow, none were caught at the surface 

 during daylight. By 2200 h the distribution again 

 changed significantly. Like the first night, the sur- 

 face catch was greater than the total catch of the 

 other three nets. The combined catch at the sur- 

 face and 8 m made up 88% of the 2200-h catch. 

 Unlike the first night, larvae were less abundant 

 at the surface than at 8 m at 0100 h but the upper 

 two nets again contributed >80'^ of the catch 

 (Figure 2). At 0400 h larvae reoccurred in greatest 

 numbers at the surface. This increase in the sur- 

 face catch at 0400 h did not occur on the previous 

 day (Table 2). 



Results of tows on the last day were much like 

 those on the first 2 days. Larvae were most abun- 

 dant at 20 m on all five daylight tows. Only one 



larva was taken at the surface, that at 1900 h. By 

 2000 h the distribution of larvae shifted towards 

 the surface. The young flounder repeated their 

 behavior of the previous day by descending at 0100 

 h. Most were at 8 m and, for the first time, the 20-m 

 net caught more larvae than the surface net on a 

 night tow. Despite the somewhat deeper distribu- 

 tion, the combined catch of the surface and 8-m 

 nets contributed nearly 80% of the 0100-h catch 

 (Figure 2). The distribution of larvae at 0400 h was 

 much like that at 0100 h. It differed from the other 

 two 0400-h tows in that the contribution of the 

 surface net was greatly reduced, and that of the 

 8-m net greatly increased (Table 2). 



The amplitude of diel movements increased 

 with size of larvae but, within each of the four size 

 groups, the movements were similar each day 

 (Figure 3). The vertical movements of larvae ^4.0 

 mm were relatively insignificant compared with 

 those of larger larvae. During daylight hours the 

 recently hatched larvae were at an average depth 

 of about 24 m, at night 20 m, a difference of only 4 

 m. Larvae 4.1 to 8.0 mm long were more active. 

 They moved vertically from an average depth of 24 

 m during the day to about 9 m at night. The trend 

 continued with larvae 8.1 to 10.0 mm long. During 

 the day were at an average depth of 29 m. At 

 night they ascended to an average depth of 5 m. 

 Larvae > 10.0 mm exhibited the most pronounced 

 vertical movements. During the day they were at 

 an average depth of 41 m, at night 7 m. 



By not having a net near bottom, we failed to 

 sample the entire depth range of larvae. However, 

 it appears that our nets encompassed the depth 

 distribution for nearly all larvae <10.0 mm. Only 

 5% of those < 10.0 mm were caught in the 48-m net 

 and we assume that their numbers continued to 

 decline below that depth. On the other hand, the 

 daytime distribution of larvae >10.0 mm may 

 have been deeper than our results indicate. Al- 

 most half (46%) of the daytime catch of larvae 

 >10.0 mm was caught in the deep net. None were 

 caught at depths <20 m, and most (77%) of the 

 large larvae caught at 20 m during the day were 

 collected at 0700 h, probably during their morning 

 descent. 



The incidence (percent) of larvae with visible 

 gut contents was as high as 40% at one station but 

 only 6% of the larvae caught during the 3-day 

 study contained visible gut contents. The overall 

 incidence was low, but our results indicate that 

 most feeding occurred at about the same time on 

 all 3 days. We found the highest incidence from 



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