SMITH ET AL: DIEL MOVEMENTS OF LARVAL FLOUNDER 



100 



Figure 4. — Percent of yellowtail 

 flounder larvae by depth and time (up- 

 per graph); and percent of larvae with 

 visible gut contents by depth and time 

 (lower graph). Figure represents aver- 

 aged results from 3-day study. 



the small larvae exhibited similar behavior in 

 both studies. Although their larvae were concen- 

 trated at shallower depths than ours, in both cases 

 the temperature was about 10°C where larvae <4 

 mm were most abundant. See Sette (1943) for 

 temperature profile pertaining to data presented 

 by Royce et al. (1959). 



A ^test on our adjusted catch data from the 15 

 daylight tows and 9 night tows indicated that the 

 catch at night was significantly greater than the 

 daytime catch. Some of this difference might re- 

 sult from avoidance during daylight but, based on 

 our fast towing speed, which would curtail avoid- 

 ance, and results of gear performance tests by 

 Bjdrke et al. ( 1974) and Posgay et al. (see footnote 

 2), which showed the 20-cm bongo to be an effec- 

 tive sampler, we concluded that the greater catch 

 at night was largely attributable to a change in 

 the vertical distribution of larvae and our sampl- 

 ing depths. Comparisons of day might catch ratios 

 of daily catches and catches at 20 m support our 

 conclusion. Whereas the daymight catch ratios of 

 the adjusted catch (larvae per cubic meter) were 

 1:1.56, 1:2.04, and 1:2.66 on days 1 through 3, 

 respecitvely, the reverse was true at 20 m, where 

 the ratios were 2.30:1, 2.57:1, and 2.10:1. Night 

 catches were greater than day catches because 

 most larvae migrated towards the surface at 

 night, where two nets fished. The resultant con- 



centration of larvae in a confined depth stratum, 

 and the "extra" net fishing within the stratum 

 where larvae were concentrated, accounted for the 

 significantly greater catch with less sampling ef- 

 fort at night. After descending during the early 

 morning hours, larvae were largely subjected to 

 capture at 20 m, where the daytime catch was 

 more than twice as great as the catch at night. If 

 avoidance were the principal factor in the 

 day:night differences, we would expect larger 

 catches at all depths at night. 



Both Bridger ( 1958 ) and Wood ( 197 1 ) found that 

 the daytime distribution of herring, Clupea ha- 

 rengus, larvae depended on light conditions. Their 

 larvae were nearer the surface on cloudy days 

 than on sunny days. Although weather conditions 

 changed from partly cloudy to sunny, followed by 

 fog and rain, and sea conditions changed from 

 moderate to calm as winds diminished, yellowtail 

 flounder larvae showed little variation in their 

 diel movements during our 3-day study. We 

 caught only two larvae at the surface during day- 

 light hours. On all 3 days larvae began to ascend 

 after 1900 h and were at the surface in greatest 

 numbers at 2200 h. During the early morning 

 hours of darkness their numbers decreased at the 

 surface but the young fish did not disappear from 

 the surface until sometime between 0400 and 0700 

 h. Judging from our results and those of Royce et 



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