Moltschaniwskyj and Doherty: Distribution and abundance of juvenile Photololigo 



309 



Summer 1990/91 



Photololigo sp A 



Summer 1991/92 



Photololigo sp. B 



Summer 1990/91 Summer 1991/92 



October 



>- 

 O 



z 



LU 



O 



LU 



cr 



LL 



10 15 20 25 30 35 40 '40 



li. 



10 i ^_ 



December 8 J 



_L i ml Jl_ 



-] BO -| 10 -| 



5 10 15 20 25 30 35 40 >40 



5 10 15 20 25 30 35 40 >40 



5 10 15 20 25 30 35 40 >40 



DORSAL MANTLE LENGTH (mm) 



Figure 4 



The size-frequency distribution of juvenile Photololigo sp. A and Photololigo sp. B during eight months of sum- 

 mer sampling. Size classes are mid-points of each class. Data are pooled across depths and stations. 



present study, a gradient of tur- 

 bidity across the shelf makes it 

 possible that inshore catches would 

 underestimate abundance if cor- 

 rected for diminishing light-pools. 

 However, if the error was signifi- 

 cant, it would only exaggerate, not 

 diminish, our observation that ju- 

 venile squid were more abundant 

 within the coastal lagoon. 



High catches of juvenile squid 

 in the coastal lagoon were at lo- 

 cations where discontinuities were often observed in 

 surface temperature and salinity. Hydrodynamic 

 modelling of this region suggests that the coastal 

 lagoon is often subject to velocity shear (King and 

 Wolanski, 1992). Water in the lagoon typically flows 

 southward under the influence of the poleward East 

 Australian Current, which pushes water onto the 

 outer shelf and through the reef matrix, especially 

 through channels like the Magnetic Passage. Under 

 typical south-easterly wind conditions the shallow 

 body of water trapped against the coast moves in the 

 opposite direction, northwards. The result is a ve- 

 locity shear between the two water masses and a 



zone of low residual displacement. Modelling stud- 

 ies suggest that the cross-shelf location of this fea- 

 ture, referred to as a separation front (King and 

 Wolanski, 1992), will shift seawards as the wind 

 strength increases and vice versa. This mobility of 

 the frontal region is consistent with the daily and 

 monthly variability of salinity and temperature at 

 the surface indicated by our physical monitoring 

 during the second summer. 



This low-shear zone is identified as a significant 

 place for aggregation of planktonic organisms. 

 Cross-shelf studies have shown highest abundances 

 of larval reef fishes in a similar location near the 



