FISHERY BULLETIN: VOL. 79. NO 4 



The average volume of water filtered during the 

 hauls was 82.9 m^ (range 58.6-109.2 m^). All 

 numbers of eggs and larvae were adjusted to 

 numbers per 100 cubic meters. Using UCLA BMD 

 computer program 02V (Dixon 1973), analyses of 

 variance were performed for several species after 

 the data had been transformed to logio iX + 1) to 

 normalize the distribution and homogenize the 

 variances, which were proportional to the means 

 before transformation. The factorial design of the 

 first experiment had the following factors: three 

 24-h days, four sampling depths (0, 4, 15, 30 m), 

 and day and night. Each factor combination had 

 three replicates. Because the cruise was in mid- 

 summer, by sampling every 3 h, three tows were 

 taken each night and five each day. To equalize 

 the number of day and night tows for the analyses 

 of variance, only the first, third, and fifth daytime 

 tows were used. We performed similar analyses on 

 the data associated with the second experiment 

 when collections were also made at the 2 and 6 m 

 depths. For these data the factors were: three 

 times of day (evening — 1800 h; night — 0000 h: 

 and day— 0600 h and 1200 h), and six depths (0, 2, 

 4, 6, 15, 30 m). Each of these factor combinations 

 had two replicates. 



RESULTS 



The drogue drifted about 11 nmi ( 20.4 km) to the 

 west-southwest of its original position during the 

 72-h sampling period. Three circular patterns that 

 corresponded to a diurnal tidal cycle were evident 

 within the overall drift (Figure 1). During the first 

 half of the experiments the wind was generally 

 southerly at 5-25 kn (9.3-46.3 km/h). The skies 

 were cloudy and a thundersquall occurred around 

 0300 h on the first night. Around 2100 h on the 

 second night there was another thundersquall, 

 and at 0300 h the wind shifted to north-northwest 

 at 20 kn (37.0 km/h) and the skies cleared. 

 The wind diminished during the third day and by 

 2100 h it had shifted to southeast at only 2 kn (3.7 

 km/h). The sky remained clear. The sun rose at 

 0545 h and set at 2016 h during the experiments. 

 The new moon rose and set during twilight or 

 daylight hours throughout the sampling period, so 

 there was no moonlight at night. 



The water column represented three water 

 types characteristic of the continental shelf of the 

 Middle Atlantic Bight. Coastal water above the 

 thermocline at 8 m was isothermal at 22.2°- 22. 8° 

 C and had salinities of < 33.61, (Figure 2). Below 



TEMPERATURE C 



SALINITY °/oo 



Figure 2. — Mean temperature (line) and salinity (dashed line) 

 profiles at drogue stations during the vertical distribution study 

 of ichthyoplanklon in the Middle Atlantic Bight. July 1974. 



the thermocline, where temperatures dropped 

 from 21.5° C at 10 m to 8.5° C at 30 m and salinities 

 exceeded 33.61 , was shelf edge water (Wright and 

 Parker 1976). Near the bottom, where tempera- 

 tures were <10° C and salinities were 34.5-34.6 1,, 

 the water was part of the cool pool that occurs over 

 the middle of the shelf off the Middle Atlantic 

 Bight in summer (Bowman and Wunderlich 1977). 

 The diversity of ichthyoplankton collected was 

 probably due in part to our sampling in these three 

 water types. 



A total of 61,534 eggs (an overall arithmetic 

 mean of 562 eggs/100 m^), most of which were 

 identified to species, were taken during the exper- 

 iments (Table 1). Most numerous were Atlantic 

 whiting and Gulf Stream flounder. Butterfish, 

 Peprilus triacanthus; fourspot flounder, Hippo- 

 glossina oblonga; hakes, Urophycis spp.; snake 

 eel; and cunner, Tautogolabrus adspersus . eggs 

 were also taken in significant numbers. No blue- 

 fish eggs were taken during the cruise. 



Throughout the cruise, egg numbers of all 

 species decreased with depth. Data from the sec- 

 ond experiment was similar to that from the first, 

 and showed that the catches at 6 m more closely 

 resembled those at the surface and 4 m than those 

 at 10 and 30 m. These findings indicate that the 

 planktonic eggs of all species taken behave in 

 much the same way in the water column, and have 

 similar specific gravities. 



A total of 68,840 larvae (an overall mean of 629 

 larvae/100 m^), of which most were identified to 



708 



