FISHERY BULLETIN: VOL. 80. NO. 3 



behavior and limited knowledge of current re- 

 gimes, Penn (1975) hypothesized that larvae of 

 Penaeus latisulcatus, off Western Australia, 

 were capable of moving onshore against prevail- 

 ing currents during certain times of the year. 



In this study I attempted to test Penn's (1975) 

 hypothesis by intensively sampling the changes 

 in vertical distribution of penaeid larvae while 

 simultaneously monitoring currents and other 

 environmental parameters in the water column. 

 I hoped to gain insight into the following: the on- 

 togeny of vertical migratory behavior, the envi- 

 ronmental factors that control the larval be- 

 havior, the current regimes and how they change 

 with depth, and the advective consequences to 

 the larvae resulting from vertical migration 

 through this variable current field. 



Staples (1979), studying the postlarvae of Pe- 

 naeus merguiensis in the Gulf of Carpentaria, 

 found discrete temporal and spatial patterns of 

 postlarval recruitment into the rivers around the 

 gulf. These patterns could not be explained en- 

 tirely by the distribution of adults and the timing 

 of spawning. He proposed that the temporal and 

 spatial patterns of recruitment were caused by 

 the different fates of larvae arising from two 

 peaks of spawning (spring and autumn). While 

 seasonal changes in the direction of larval advec- 

 tion were suggested, little was known about cur- 

 rent regimes in the gulf (Cresswell 1971) and 

 nothing known about how these currents would 

 affect the distance and direction of penaeid lar- 

 val dispersal. This study was, therefore, in- 

 tended to provide insight into the mechanisms 

 and pathways of larval dispersal and to help ex- 

 plain the variable timing and magnitude of post- 

 larval recruitment. 



MATERIALS AND METHODS 



Discrete depth sampling was conducted re- 

 peatedly at two locations (Fig. 1) during survey 

 cruises in the Gulf of Carpentaria (Rothlisberg 

 and Jackson 1982). These locations were <30 m 

 in depth and close to known concentrations of 

 adult penaeid shrimp. The ship was anchored on 

 station and a7.6cm(3in)centrifugalpump(Fig. 

 2), driven by a 6 kW (8 hp) aircooled gasoline en- 

 gine, was used to pump water from depth. The 

 end of the 10.2 cm (4 in) intake hose was clamped 

 to a weighted wire fed through a meter block. 

 The full length of the hose, in 9.2 m (30 ft) quick 

 coupled lengths, was deployed, regardless of the 

 sampling depth, to prevent variable friction 



142°E 



Figure 1.— Station numbers and locations for discrete depth 

 sampling (stars) in the Gulf of Carpentaria, Australia. 



Figure 2. — Schematic of pump, water discharge, and filtering 

 system: A) 10.2 cm (4 in) intake hose; B) 7.6 cm (3 in) self- 

 priming centrifugal pump with 6 kW (8 hp) gasoline engine; C) 

 7.6 cm (3 in) outlet hose; D) quick coupling; E) spinner drum; F) 

 tripod; G) 142 ^m mesh plankton net. 



effects. Water from the pump was discharged 

 through a 7.6 cm (3 in) diameter hose tangen- 

 tially into a drum (spinner) mounted on a tripod. 

 Upon loss of velocity the water drained gently 

 through a 56 cm diameter plankton net (142 /im 

 mesh) suspended beneath the spinner. The outlet 

 hose was coupled to the spinner in such a manner 

 that it could be inserted and withdrawn quickly 

 to allow precisely timed pumping intervals. The 

 pumping rate (up to 1,000 1/min) was monitored 

 with timed fills of a container of known volume. 

 The water column was divided into four strata, 

 and the inlet placed at the center of the stratum. 

 The pump was brought to speed, the outlet hose 

 inserted in the spinner, and the stratum sampled 



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