FISHERY BULLETIN: VOL. 80, NO. 3 



extend the mysis pattern further by almost com- 

 pletely abandoning the lower parts of the water 

 column by night but still returning by day. 



Variations in the Pattern of 

 Vertical Distribution 



High light penetration was characteristic of 

 the station occupied on 22 and 23 March 1977 

 (Fig. 3). Conditions were calm, isothermal, and 

 clear with a secchi disc depth of 16 m in the 22 m 

 water column and penetration of the 1,000 /uW/ 

 cm 2 isolume to 20 m. Under these conditions, 

 movement of the larvae away from the deepest 

 stratum occurred only at night and almost all 

 stages (first zoea (Zl) through third mysis (M3)) 

 returned to the bottom stratum during daylight. 



On 6 and 7 May 1977 (Station 310), though sea 

 state and wind conditions were comparable with 

 the previous sampling session, increased turbid- 

 ity limited the 1,000 /iW/cm 2 isolume penetra- 

 tion to only 10 m or about half of the water col- 

 umn (Fig. 4). This change in light penetration 

 paralleled marked changes in the vertical distri- 

 bution of all larval stages. During daylight, early 



larval stages (zoea 1-3) were not confined to the 

 bottom stratum. The day-night differences in 

 vertical distribution, though present, were less 

 distinct than in the previous case, and these early 

 larval stages were seen even at the surface at 

 night. The day-night pattern for mysis stage 

 larvae was even less distinct. Though spread 

 throughout the water column, they appeared to 

 be slightly more concentrated at or near the sur- 

 face at night. Postlarval (PL) numbers were 

 again low, and they were near the surface dur- 

 ing the entire diel period. They were more abun- 

 dant in the surface stratum during the night. 



At the station east of Mornington Island on 27 

 and 28 March 1977 (Station 270) the wind and 

 sea conditions were extremely calm but light 

 penetration was even more diminished than in 

 the previous two cases. On this occasion the 1,000 

 juW/cm 2 isolume penetrated only about one-third 

 of the water column (Fig. 5). Further changes in 

 the patterns of larval distribution were seen. 

 Early larval stages were concentrated in the 

 middle two depth strata with nighttime move- 

 ments to the surface. The mysis stage larvae 

 were also predominantly in the middle part of 



ZOEAl 



ZOEA 1 





 7 

 14 

 21 

 28 



MYSIS 1 



MYSIS 1 







5 



10 



15 



20 



POSTLARVA 



4 8 12 16 20 24 4 



BIOMASS 



I 



4 8 12 16 20 24 4 



TIME OF DAY 



pW/cm 2 xl0 2 



4 8 12 16 20 24 4 



Figure 4.— Relative larval abundance (percent) by substage 

 and depth stratum, vertical distribution of settled plankton 

 volume (percent), and vertical profiles of submarine irradi- 

 ance (/xW/cm 2 ) for 6-7 May 1977 at Station 310 north of Groote 

 Eylandt. The dark horizontal bar indicates night. 







7 

 14 



21 

 28 



POSTLARVA 



BIOMASS 



22 2 6 10 14 18 22 22 2 6 10 14 lb 22 22 2 6 10 14 18 22 



TIME OF DAY 



Figure 5. — Relative larval abundance (percent) by substage 

 and depth stratum, vertical distribution of settled plankton 

 volume (percent), and vertical profiles of submarine irradi- 

 ance (^W/cm 2 ) for 27-28 March 1977 at Station 270 east of 

 Mornington Island. The dark horizontal bar indicates night. 



544 



