288 



Fishery Bulletin 9 1 (2), 1993 



- 5 mm 

 25 50 



150 100 50 50 100 150 



43/lOm 2 93/IOm* 



46B/I0m 2 354/IOm 2 



I89/I0m 2 123/IOm 2 



75/IOm 2 74/lOm 2 



47/IOm 2 37710m 2 



SALINlTY(psu) 

 32 00 2 4 6 6 3300 2 



I2/I0m 2 I6/I0m 2 



Figure 1 



Mean day and night vertical distribution Li «/100m') of (A) 

 haddock Melanogrammus aeglefinus and (B) cod Gadus morhua 

 size-classes of larvae collected by the 1 m- MOCNESS on 

 weakly-stratified Site 83-1 during 13-14 May 1983. Estimated 

 water column abundances (rc/lOnr) are included at bottom of 

 plots. Temperature and salinity profiles are plotted as depth 

 stratum means (•) and 95 r 7r confidence limits ( 1. 



the upper half of the water column at night, increas- 

 ing somewhat towards the surface. There were no sig- 

 nificant interaction effects in the ANOVA for either 

 cod or haddock (Table 4). 



On 18-2 2 June 1986 at a well-mixed deeper (65- 

 78 m) Site 86-1, cod juveniles 14-49 mm were caught 

 (Fig. 15). Mean day and night vertical distributions 

 show a clear pattern of larger size-classes of cod 

 located progressively deeper in the water column, 

 with the 40-49 mm cod located almost entirely be- 

 tween 50 m and the bottom. Night catches were 

 greater than day catches for all size-classes of cod. 



Night mean abundance was significantly greater 

 (ANOVA; p<0.01) than day mean abundance of the 

 20-29 mm and 30-39 mm size-classes (Table 4). 

 There was no detectable evidence of diel vertical 

 migrations. 



By July, relatively few pelagic haddock and cod ju- 

 veniles were caught. Highest densities of juvenile 

 gadids were located on eastern Georges Bank (Lough 

 et al. 1989). Three sites were sampled in mid-July 

 1985 on eastern Georges Bank: Site 85-1 ( 82-87 m), 

 stratified; Site 85-11 (64-72 m), weakly stratified; Site 

 85-111 (78-83m), stratified (Fig. 16). The deeper Sites 

 85-1 & 85-111 both had a gradual thermocline at 15- 

 30 m depths. Only a few pelagic cod juveniles were 

 caught. Pelagic cod ranging 30-59 mm were caught 

 mostly at night, although a day tow was not made at 

 Site III. Relatively few fish were caught in the upper 

 30 m, and there was a tendency for the highest densi- 

 ties to be located in the bottom third of the water 

 column. Cod were caught in day tows at Site 85-1 in 

 the bottom 60-80 m stratum. 



Diel migration of recently-settled juveniles 



In 17 bottom trawls (with rollers) at Site 86-1, 25- 

 26 June 1986, demersal cod juveniles with a mean 

 length of 4-5 cm were collected (Table 5). Few were 

 caught in day trawls, but at least an order of mag- 

 nitude more cod (x 103/30 min trawl) were caught 

 in night trawls. During 17-18 July 1985 at Site 85- 

 IV, nine bottom trawls (with rubber disk sweep 

 chain) were made (Table 6; Fig. 17). Both haddock 

 and cod appeared to be caught more abundantly in 

 day trawls than at twilight. Only one haddock ju- 

 venile was caught in the single night trawl. The 

 mean length of haddock was 7.1cm (range 4-10 cm), 

 and 5.5 cm for cod (range 2-12 cm). Mean lengths 

 of both species were the same in day and twilight 

 trawls. The high daylight catches of cod juveniles 

 in this series are in contrast to the high night 

 catches of cod of the same average size, 4-5 cm, in 

 June 1986 using a bottom trawl with rollers. Gear 

 difference (rubber disks vs. rollers) may have been 

 the cause of the different day-night catches, but 

 conclusions cannot be made because in July 1985 

 only one night trawl was made. 



During 16-17 August 1985, 12 bottom trawls (with 

 rubber disk sweep chain) on southeastern Georges 

 Bank (63-71 m), Site 85- V, caught relatively high num- 

 bers (210-257/30 min trawl I of 9-19 cm haddock 

 juveniles (Table 7; Fig. 18). The mean number and 

 lengths of fish per trawl were not significantly differ- 

 ent (p>0.05) by paired /-tests for the night, day, and 

 twilight catches. 



