292 



Fishery Bulletin 9 1 (2). 1993 



14 19 mm 20 - 29 mm 30 • 39 mm 40 49 mm 



„3 2 I I 2 3 60 40 20 20 40 60 60 40 20 20 40 60 6 4 2 2 4 6 



IL 



J 



SALlNITrlpsu I 

 3200 2 4 6 8 3300 2 



TEMPERATURE (*C) 



5 10 lb 



TEMPI SAL 



2 7/l,000m 2 6 4 /1, 000m 2 79/l,O0Om 2 I34/I.000m 2 UO/I.OOOm 2 l54/l,000m 2 6.l/l,000m z 8 5/l,0O0m 2 



Figure 15 



Mean day and night vertical distribution (x /z/10,000m :j ) of cod Gadus morhua size-classes of pelagic 

 juveniles collected by the 10 nr MOCNESS on well-mixed Site 86-1 during 18-22 June 1986. Water 

 column abundances (/i/1000nr) are included at bottom of figure. Temperature and salinity profiles are 

 plotted as depth stratum means (•) and 95% confidence limits ( ). 



was reported by Walford (1938), 

 Colton (1965), and Lough (1984) for 

 haddock on Georges Bank. Their re- 

 sults and other regional studies are 

 reviewed by Frank et al. (1989) and 

 Page et al. (1989) in depth distribu- 

 tion studies of haddock eggs on the 

 Southwestern Scotian Shelf. Because 

 haddock eggs increase their specific 

 gravity with age, newly-hatched lar- 

 vae were generally found at inter- 

 mediate depths. Furthermore, these 

 studies found the degree of aggrega- 

 tion for late stage-IV eggs was sig- 

 nificantly related to the intensity of 

 stratification. 



In the present study, in well-mixed 

 waters during early spring, larval 

 haddock and cod were distributed 

 throughout the water column in the 

 same general pattern as the eggs 

 (Lough 1984), with a mode in abun- 

 dance at mid-depth, 30-40 m. By 

 mid-May, when the surface water is 

 stratified, larvae resided within the 

 thermocline region; the stronger the 

 stratification, the more closely the 

 abundance centers were confined to 

 the thermocline, especially for the 

 smallest larvae. The larval-thermo- 

 cline coupling is a dynamic process 

 that is frequently perturbed by strong 

 winds and storms but becomes rees- 

 tablished with a new thermocline. 

 Even under stratified conditions, 

 larger larvae (>9mm) tend to be 

 found at greater depths during the 

 day. Day-night shifts in their distri- 

 bution patterns indicate that diel 



