SMITH ET AL: DIEL MOVEMENTS OF LARVAL FLOUNDER 



towing speed. At the end of each tow the nets were 

 retrieved as we slowed to a stop. 



All yellowtail flounder larvae from each sample 

 of <100 fish were counted and measured to the 

 nearest 0.1 mm SL. If the count exceeded 100, a 

 subsample of about 25% was randomly selected 

 and measured. Then the number of larvae in each 

 size increment was adjusted so that the sum cor- 

 responded with the total sample size. Despite our 

 efforts to minimize sampling contamination while 

 setting and retrieving the nets, subsurface nets 

 sampled more water than the surface net. To com- 

 pensate for contamination, we standardized the 

 volume of water filtered by each net by using the 

 mean amount of water filtered by the surface net 

 (88.8 m^) as the standard. We then adjusted the 

 catch of each net to correspond with the adjusted 

 amount of water filtered. These changes accounted 

 for average reductions in the catch of < 1.0% in the 

 surface net, 3.4% in the 8-m net, 4.4% in the 20-m 

 net, and 13.4% in the 48-m net or a net reduction of 

 4.7% of the total catch. 



We inspected digestive tracts of young flounders 

 for indications of a feeding pattern, i.e., presence 

 or absence of gut contents, that might be related to 

 vertical movements. We were able to make these 

 observations simply by using a microscope and 

 incident lighting. 



After grouping the adjusted larval catches into 

 four size categories, «4.0, 4.1 to 8.0, 8.1 to 10.0, 

 and >10.0 mm, we examined the data for homo- 

 geneity of sampling variance by comparing within 

 station catches by depth. Daylight tows were con- 

 sidered replicates, as were night tows. Standard 

 deviations were proportional to the means in the 

 raw data, indicating that sampling variance was 

 not homogeneous. The variance was stabilized by 

 transforming the data to log J (J (x + 1). We used the 

 UCLA BMD computer program 02 V, a multifactor 

 ANOVA program (Dixon 1973), to test for differ- 

 ences in mean catches by day, depth, time (day vs. 

 night), and size of larvae (Table 1). To meet a 

 program prerequisite, we balanced the number of 

 day and night tows used in the analysis by ran- 

 domly selecting three of the five tows for each 

 daytime period. 



RESULTS 



Light conditions and sea state varied during the 

 3-day study in response to changing weather. The 

 sky was cloudy when we began sampling on 15 

 June. Seas were moderate, stirred by 2 days of 



Table l. — Analysis of VEiriance of data collected during study of 

 diel movements of yellowtail flounder larvae. Variables include 

 days, time (day vs. night), capture depth, and length of larvae, 

 grouped into size categories of €4.0, 4.1 to 8.0, 8.1 to 10.0, and 

 >10.0mm. Data were transformed to log, gix + 1 ) and pertain to 

 3 day tows and 3 night tows taken during each day of the 3-day 

 study. 



•P«0.05. 

 "P«0 01 



brisk south to southwesterly winds of 15 to 20 kn 

 (7-10 m/s). On the 16th the sky cleared but south- 

 erly winds persisted. The 17th was cloudy with 

 intermittent periods of light rain until evening 

 when dense fog set in. We completed field work in 

 heavy rain on the morning of the 18th. There was 

 little or no measurable wind during the last 24 h of 

 sampling. 



Water temperature in the Middle Atlantic 

 Bight increases rapidly in the spring and the 

 water column becomes thermally stratified during 

 the summer (Norcross and Harrison 1967). At the 

 time and site of our study, the surface temperature 

 averaged 15.0°C, the bottom 5.7 °C. A thermal 

 gradient of about 5°C, the predecessor of the more 

 strongly defined summer thermocline, occurred at 

 depths between 10 and 20 m. A second, weaker 

 gradient existed between 30 and 40 m. Salinity 

 increased from 31.3 %o at the surface to 32.8%onear 

 the bottom. The most pronounced change in salin- 

 ity occurred at about the same depths as the shal- 

 low thermal gradient (Figure 2). 



Drift of the drogue was erratic and sluggish 

 throughout the 72-h study. In 3 days it crossed its 

 previous path 16 times, travelled a net distance of 

 only 5.4 km in a southwesterly direction, and was 

 never more than 7.2 km from the starting point. 

 Net direction of drift was into the wind and the 

 drogue travelled the greatest distance on the third 

 day, when there was little or no wind. Because the 

 drogue's direction of drift changed at approxi- 

 mately 6-h intervals, we concluded that tidal 



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