FISHERY BULLETIN: VOL. 80, NO. 3 



and the surface and near bottom nonmigratory 

 animals 62 km (34 nmi) and 72 km (39 nmi), 

 respectively. The larvae were displaced to the 

 southwest from the sampling station north of 

 Groote Eylandt. This trajectory would have 

 taken them in the general direction of Groote 

 Eylandt or the coastal rivers in the Limmen 

 Bight, southwest of Groote Eylandt. Greater dis- 

 tances could be attained if the pelagic postlarval 

 stage was maintained through several instars 

 before metamorphosing to the benthic-living 

 juvenile shrimp. 



Procedures for approximating displacement 

 distance and direction on the other two sampling 

 occasions were similar but the resultant dis- 

 placements were quite different because of the 

 different vertical distribution patterns seen on 

 each occasion. At Station 310, north of Groote 

 Eylandt, submarine irradiance was reduced and 

 the vertical distribution of the larval substages 

 was more varied (Fig. 4). Consequently, the hori- 

 zontal displacements of individual larval sub- 

 stages (Fig. 8b, c), though dominated by the 

 bottom currents (Fig. 6b), were quite varied. The 

 hypothetical 2-wk displacement was again in the 

 same direction as the bottom current (Fig. 8d), 

 but the distance was enhanced by the fact that 

 larvae were further off the bottom in their 

 nightly excursions for slightly higher propor- 

 tions of the time. Total horizontal displacement 

 over the 2-wk larval period, up to and including 2 

 d of postlarval life, would be about 75 km (40 

 nmi). The direction of advection of the median 

 larva in this case is to the northwest. The 12:12 

 larva would have been displaced seawards to the 

 central Gulf of Carpentaria. 



Analysis of the larval migratory patterns (Fig. 

 5) and the current regime (Fig. 6c) at Station 270 

 east of Mornington Island showed yet another 

 pattern of displacement (Fig. 9). Here, because 

 there were large numbers of larvae in the upper 

 part of the water column both night and day, the 

 displacement would have been in the direction of 

 the surface currents (Fig. 9a, b, c). Slight deflec- 

 tion away from the surface direction was seen in 

 older larvae (M 1-PL) as more of them moved into 

 the lower part of the water column (Fig. 9d). The 

 displacement distance over the 2-wk period 

 would have been 98 km (53 nmi). All advection 

 was to the west, with the 12:12 larva going to the 

 southwest towards the coast and the median 

 larva heading west-northwest, away from the 

 coast, in the general direction of the surface cur- 

 rents. 



DISCUSSION 



While it is widely thought that changes in light 

 intensity are the primary environmental cues 

 that initiate and control the diel vertical migra- 

 tions in aquatic animals (Ringelberg 1964; Thor- 

 son 1964; Boden and Kampa 1967; Hutchinson 

 1967; Segal 1970; Buchanan and Haney 1980), 

 there have been cases in which the timing of the 

 migration was not strictly in phase with changes 

 in light intensity, possibly because of changes in 

 subsurface light and/or feeding history and 

 strategy of the animals (Enright and Honegger 

 1977; Bohrer 1980). In shallow-water coastal 

 environments, factors affecting light penetra- 

 tion and therefore vertical distribution of ani- 

 mals are numerous and subject to rapid change. 

 Turbulence with concomitant turbidity caused 

 by both wind and tidally induced currents, river 

 and coastal runoff, and rapid phytoplankton 

 growth would be the more significant causes of 

 light reduction. It is these short-term changes in 

 submarine irradiance that are probably respon- 

 sible for some of the conflicting reports about 

 whether or not penaeid larvae migrate vertical- 



ly. 



The first mention of differential vertical dis- 

 tribution of penaeid larvae is by Racek (1959), 

 sampling off the eastern Australian coast. His 

 sampling was not strictly stratified, he pub- 

 lished no supportive data, and the conclusions 

 are probably drawn from a combination of field 

 and laboratory observations. He stated that nau- 

 plii as well as first and second protozoeae (= 

 zoeae) were strongly attracted to bright light. I 

 saw no evidence of this in our field collections but 

 have observed it under artificially high light in- 

 tensities in the laboratory. There may be some 

 threshold light intensity at which point penaeid 

 larvae shift their behavior from photonegative to 

 photopositive similar to that described for the 

 larvae of Uca pugilator (Herrnk'md 1968). From 

 his field sampling, Racek found that late proto- 

 zoeae and early mysis stages rose to the surface 

 at night and sunk to lower strata during day- 

 light. The vertical distribution of late mysis and 

 postlarvae were not mentioned in Racek's brief 

 account. 



In the study by Eldred et al. (1965), the day- 

 night pattern of vertical distribution was neither 

 persistent within nor consistent among genera. 

 For Penaeus duorarum only postlarvae were dis- 

 cussed. Pooled samples were dealt with, so little 

 information about station-to-station variation 



548 



