ROTHLISRERG: VERTICAL MIGRATION OF PENAEID SHRIMP LARVAE 



for a 15-min interval. After 15 min the outlet 

 hose was quickly withdrawn, the pump speed re- 

 duced, and the hose inlet lowered to the next stra- 

 tum for 5 min of flushing before the next 15-min 

 sample was taken. The four strata were there- 

 fore sampled in a 75-min pumping series which 

 was initiated every 2 h and continued for 24-36 h. 



At the same time that the pump inlet was be- 

 ing deployed astern on the main wire, a Lerici 

 current meter (Frassetto 1967), modified for 

 deck readout of current velocity, was deployed 

 amidship on the hydrographic wire. Fifteen- 

 minute records at each depth stratum were ob- 

 tained simultaneously with plankton samples. 

 All current meter records were annotated on a 

 strip chart recorder in the deck readout unit. 



To link the vertical distribution of the shrimp 

 larvae to the current vectors at depth, the 

 median level of larval vertical distribution was 

 calculated (Cronin and Forward 1979) for each 

 of the larval substages at each 4-h time interval 

 and assigned to one of the four depth strata. The 

 intermediate (2-h) larval distributions were in- 

 terpolated from the 4-h time series. The current 

 vectors for each 2-h time interval, associated 

 with the depth stratum (levels 1-4) to which the 

 median larval depth corresponded, were added 

 progressively over 24 h for each larval substage 

 (Fig. 7b, c). In addition to the median larva, three 

 other hypothetical behavior patterns were mod- 

 elled: 1) a nonmigratory surface dwelling ani- 

 mal; 2) a nonmigratory near-bottom dwelling 

 animal; and 3) a larva (12:12 larva, Figs. 7, 8, 9) 

 that followed the behavior pattern dictated by 

 Penn's (1975) hypothesis, i.e., it moved the full 

 height of the water column on a strict 12:12 day: 

 night cycle for the entire length of the larval life. 



A photometer, with both deck and submersible 

 cells, was used to measure the ambient and sub- 

 marine irradiance (/iW/cm 2 ). Irradiance levels 

 were recorded at 2 m intervals at the start of 

 every 2-h pumping series during daylight. The 

 meter was not sensitive enough to record vari- 

 able levels of moonlight or starlight. Tempera- 

 ture profiles were obtained with both a bathy- 

 thermograph and water sampled from the pump 

 outlet. 



Plankton biomass (settled volume) from dis- 

 crete depth samples was obtained by settling the 

 fixed sample (10% formaldehyde, sodium tetra- 

 borate buffer) for 4 h in Imhoff cones (Rothlis- 

 berg and Jackson 1982). The samples were then 

 transferred to 2% 2-phenoxy ethanol for preser- 

 vation and subsequent microscopic examination. 



For economy and expediency, only every other 

 sampling series (4-h) was examined. Nosubsam- 

 pling scheme was employed. Numbers of larvae 

 in each sample were standardized to numbers 

 per cubic meter based on the calibrated pump- 

 ing rates. 



RESULTS 

 Ontogeny of Vertical Migration 



From sampling done on 22 and 23 March 1977 

 north of Groote Eylandt (Station 210), early lar- 

 vae (zoeal stages 1-3) were seen to move up into 

 the water column only at night (Fig. 3). This 

 movement was very limited and rarely extended 

 more than one-half the distance to the surface. 

 By day they were almost totally restricted to the 

 bottom stratum of the water column sampled. 

 The mysis stages extended the range of their 

 nighttime excursions to the full water column 

 without completely abandoning the lower part of 

 the water column at night. They too returned 

 almost completely to the bottom stratum by day. 

 Postlarval numbers in the samples were too few 

 to make generalizations, but they did appear to 



ZOEA 1 







5 

 10 

 15 

 20 



!!■ ■ilii ■!■!!■ 



MYSIS 1 



POSTLARVA 



BIOMASS 





 5 



10 

 15 

 20 



V/cm 2 



,1 



12 16 20 24 4 8 12 



12 16 20 24 4 8 12 

 TIME OF DAY 



12 16 20 24 4 8 12 



Figure 3.— Relative larval abundance (percent) by substage 

 and depth stratum, vertical distribution of settled plankton 

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

 ance (/iW/cm 2 ) for 22-23 March 1977 at Station 210 north of 

 Groote Eylandt. The dark horizontal bar indicates night. 



543 



