FISHERY BULLETIN: VOL. 76, NO. 1 



METHODS 



We selected the general area for the 72-h study 

 from results of the 1965-66 survey (Smith et al. 

 1975). The specific site, 98 km south of Montauk 

 Point, N. Y., was selected by making trial plankton 

 tows until we found the patch of larvae ( Figure 1 ). 

 To stay within the patch, we deployed a free- 

 drifting parachute drogue similar to that de- 

 scribed by Volkmann et al. ( 1956). The parachute 

 was attached 18 m below the staff buoy on our 

 drogue. 



We sampled at 3-h intervals, from 1000 h on 15 

 June to 0700 h (EDT) on 18 June 1972. Tempera- 

 ture and salinity observations preceded each tow 

 during the first 2 days. We continued to take tem- 

 peratures at 3-h intervals on the third day but 

 recorded salinity data at 6-h intervals. When we 

 started sampling, the summer solstice was only 6 

 days hence and a day was divided into 15 h of 

 daylight and 9 h of darkness. Sunrise and sunset 

 were at about 0530 h and 2030 h, respectively. By 

 sampling at 3-h intervals, we made five tows dur- 

 ing daylight and three tows at night during each 

 day. 



Plankton samples were taken with an array of 

 four 20-cm bongos fitted with 0.505-mm mesh 

 nets. Each tow lasted 15 min. Towing speed was 5 

 kn (3 m/s). We chose the 20-cm bongo over the 

 larger 61-cm bongo to keep both plankton volumes 

 and numbers offish larvae at levels that would not 

 exceed our laboratory capabilities. Catch com- 

 parison tests between the 20- and 61-cm nets re- 



vealed no significant differences in the catch of 

 larvae (Bjdrke et al. 1974; Posgay et al.^). 



Readings obtained from digital flow meters 

 were used to calculate the amount of water sam- 

 pled by one side of each bongo. With the exception 

 of the surface-sampling net, the bongos were at- 

 tached to the towing wire to sample near depths 

 where temperature changes were greatest. They 

 sampled at 8 m, which was just above the shal- 

 lower of the two temperature gradients; at 20 m, 

 below the shallow gradient; and at 48 m, which 

 was below the deep thermal gradient and about 17 

 to 20 m above bottom. We preserved the contents 

 from only the metered side of each bongo. Bathy- 

 kymographs (BKG) were attached above two of 

 the three subsurface nets to monitor sampling 

 depth profiles. The sequence of attachment 

 changed with each tow. Resultant BKG traces in- 

 dicated that the average towing depth of each sub- 

 surface net was ±2 m of the intended sampling 

 depth. The bongos did not have opening-closing 

 devices. We tried to minimize contamination dur- 

 ing setting and retrieval by snapping the three 

 subsurface nets onto the towing wire and lowering 

 them into the water while the ship maintained 

 just enough way to stay on course. Immediately 

 after affixing the 8-m net, vessel speed was in- 

 creased. The surface net was snapped in place and 

 lowered into the water as the ship approached 



^Posgay, J. A., R. R. Marak, and R. C. Hennemuth. 1968. 

 Development and tests of new zooplankton samplers. Int. Comm. 

 Northwest Atl. Fish., Res. Doc. 68/85, 7 p. 



Figure l.— Site of 72-h study of diel 

 movements of yellowtail flounder lar- 

 vae. Insert shows theoretical track of 

 drogue and its position at 3-h sampling 

 intervals. 



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