FISHERY BULLETIN: VOL. 80, NO. 3 



current speed and direction changed with depth. 

 Although the speed usually decreased with 

 depth (e.g., Fig. 6a, 0200; 6c, 2200), an increase 

 was noted on some occasions, especially around 



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Figure 6.— Mean 2-h current vectors at four depths and three 

 stations: A) Station 210, 22-23 March 1977 north of Groote 

 Eylandt; B) Station 310, 6-7 May 1977 north of Groote Eylandt; 

 C) Station 270, 27-28 March 1977 east of Mornington Island. 



the time of slack water (e.g., Fig. 6a, 1200; 6b, 

 0000). This apparent increase is believed to be an 

 artifact of the time lag in sampling with the mi- 

 grating current meter. Short-term wind events 

 can also be detected at the surface stratum (Fig. 

 6b, 2.5 m; 1800-2200). Time lag and wind events 

 alone, however, cannot explain all the variation 

 in the current vectors. Other anomalous current 

 vectors at depth (e.g., Fig. 6a, 1000, 17.5 m; 0800, 

 7.5 m) represent short-term nontidal perturba- 

 tions of unknown origin. 



Over the 24-h sampling period at Station 210, 

 the net displacement (measured as a straight line 

 from the origin to the end of the progressive vec- 

 tor) was quite variable for migratory and non- 

 migratory animals. Surface-dwelling animals 

 would have been displaced to the northwest, 

 near-bottom dwelling ones to the south, and the 

 12:12 larva almost due west. The net drift of the 

 12:12 larva was considerably less than either of 

 the nonmigratory animals. The larval move- 

 ments (Fig. 7b, c) based on sampled vertical dis- 

 tributions would have closely paralleled the bot- 

 tom currents in this case because so much of their 

 time was spent in the lower parts of the water 

 column. The slight differences in displacement 

 direction and distance between substages reflect 

 the differences in depth distribution. Because of 

 their wider excursions, the postlarvae were ex- 

 posed to more of a mixture of the bottom and sur- 

 face currents and therefore approached the 12:12 

 larva in direction of advection during that larval 

 stage. To calculate an approximate total dis- 

 placement over the whole of the larval phase, the 

 24-h picture was extrapolated by making several 

 assumptions. Firstly, the larval life span was set 

 at 14 d with 2 d for each larval substage (Zl, Z2, 

 Z3, Ml, M2, M3, PL). Secondly, within each sub- 

 stage the same vertical migratory behavior pre- 

 vailed on both days. Thirdly, the current regime, 

 seen during the 24-h sampling period, was con- 

 stant over the 14-d larval life. By doubling the 

 length of the daily resultant vectors for each lar- 

 val substage and adding them progressively, the 

 2-wk displacement is approximated (Fig. 7d). 

 The calculation of absolute distances is not pre- 

 cise because of the nature of the assumptions, 

 especially the third. The period of sampling was, 

 however, between neap and spring tides so the 

 tidal currents would have been moderate and a 

 reasonable approximation of mean velocities 

 over the tidal cycle. Under these criteria, the 

 median larva would have been displaced about 

 69 km (37 nmi), the 12:12 larva 25 km (13 nmi), 



546 



