310 



Fishery Bulletin 92(2). 1994 



center of the Great Barrier Reef Lagoon (Thorrold, 

 in press). These catches included individuals taken 

 from reefs farther offshore, as well as piscivorous 

 larvae of various scombrids from inshore (Thorrold, 

 1993). It is not clear whether aggregation of these 

 stages is passive, due to hydrodynamics, or the re- 

 sult of attraction to the coastal boundary area by 

 enhanced secondary productivity in this frontal zone 



EARLY SHALLOW 13 TRAPS 



LATE SHALLOW 10 TRAPS 



> 



a 



Z 

 HI 

 D 

 O 

 UJ 



or 



DORSAL MANTLE LENGTH (mm) 



Figure 5 



Size-frequency distributions of juvenile Photololigo sp. A from the two 

 inshore stations at two sampling depths (pooled across the summer 

 months 1991/92), captured early (before 2400 hr) and late (after 2400 

 hr) in the night. 



(Thorrold and McKinnon, 1992). This discontinuity 

 may be a mechanism separating the two Photololigo 

 species geographically. The separation of juvenile 

 cephalopod species in the Gulf Stream east of New 

 England is thought to be closely related to meso- 

 scale hydrological features (Vecchione and Roper, 

 1986). The importance of hydrological features in ag- 

 gregating juvenile squid has been identified in a 

 number of species (Rodhouse and 

 Clarke, 1985; Brunetti and Ivanovic, 

 1992; Rodhouse et al.„ 1992). This 

 suggests that these areas are ecologi- 

 cally important for juvenile squid. 



The second way in which shelf-scale 

 hydrodynamics affects the stability of 

 the water column is the intrusion of 

 upwelled waters from the shelf-break 

 driven onto the shelf by variations in 

 the speed and position of the East 

 Australian Current. These cold intru- 

 sions can be tracked into the Great 

 Barrier Reef Lagoon (King and 

 Wolanski, 1992) and the strong ther- 

 mal stratification observed in Janu- 

 ary 1992 was consistent with an in- 

 trusion at this time. A cold bottom 

 layer at 33 km was evident on one 

 night in November, but the inner sta- 

 tions were not stratified. The pres- 

 ence of juvenile Photololigo at most 

 stations in all months, despite a range 

 of physical conditions, suggests juve- 

 nile Photololigo can tolerate substan- 

 tial environmental variation. This tol- 

 erance is consistent with a nonsea- 

 sonal reproductive strategy, which is 

 essential for a species that lives for only four months. 

 During the night there was little evidence of a 

 pronounced vertical migration such as the mass 

 aggregations of juvenile Loligo spp. on the benthos 

 (Vecchione and Gaston, 1985) or the general move- 

 ment to the surface by juvenile L. pealei (Vecchione, 

 1981). The absence of vertical movement during the 

 night suggests that the observed ontogenetic shift 

 of Photololigo sp. B farther offshore and deeper is 

 real and not a product of location confounded with 

 time of night when sampling occurred. However, as 

 was noticed in the catch-per-unit-of-effort values, 

 both species are caught in relatively low numbers; 

 hence, conclusions based on small differences that 

 are not significantly different are limited. There was 

 a problem with low numbers in all spatial and tem- 

 poral trends described. However, this was a prelimi- 

 nary study with just two hours of sampling at each 

 station per night. More intensive sampling in bound- 



