Lough and Potter Vertical distribution of Melanogrammus aeglefinus and Gadus morhua 



287 



the thermocline region, they were not confined exclu- 

 sively to it, as in May 1981 when the water column 

 was strongly stratified. Larger larvae of both species 

 tended to be found at greater depths during the day, 

 especially the 9-13 mm larvae. 



At Site 83-1, >64% of the 9-13 mm size-class for both 

 haddock and cod was found below the thermocline by 

 day, but <28% by night (Table 3A). A significantly 

 (p<0.05) greater percentage of the 9-13 mm size-class 

 was found within and above the thermocline at night. 

 For the 2-5 mm size-classes, haddock and cod larvae 

 were found mostly within or above the thermocline; 

 <15% were located below the thermocline, and there 

 was no significant day-night difference in the profile 

 percentages. For the 6-8 mm size-classes of haddock 

 and cod, the highest percentage of larvae were located 

 within the thermocline. Of the haddock larvae, 40% 

 were found below the thermocline by day, but only 

 18% at night (p<0.005). There was no significant change 

 in the day-night percentages for 6-8 mm cod. 



On 15-16 May 1983, at Site 83-11 (88-93 m) near 

 the shelf/slopewater front, thermocline depth was some- 

 what deeper with a smaller gradient than at Site 83-1. 

 Most haddock and cod larvae were confined to the up- 

 per 40 m, the lower bound of the thermocline (Fig. 12). 

 A small percentage of larger larvae (9-13 mm) were 



located below the thermocline by day ( 13% for had- 

 dock and 20% for cod) but moved nearer the surface by 

 night (Table 3B). All size-classes of larvae were broadly 

 distributed within the upper part of the water column. 

 There were significant (ANOVA; p<0.001) effects of 

 depth, time, and size for haddock; only depth x size 

 interaction was significant (p<0.01) for cod (Table 2C). 

 Mean depths of haddock and cod were greater by day 

 for the larger larvae, especially for the 9-13 mm size- 

 classes (Fig. 13), and in all but one night tow the mean 

 depths were located within the confines of the thermo- 

 cline region. 



Vertical distribution of pelagic juveniles 



In June 1984, pelagic juvenile haddock and cod were 

 collected at a well-mixed shoal ( 40-49 m) Site 84-1 

 (Fig. 14). Cod were more abundant than haddock at 

 Site 83-1, and the larger fish (14-29 mm) were caught 

 more abundantly in night tows than day. The three 

 size-classes of haddock were similarly distributed 

 throughout the water column. Few were caught in the 

 upper 10 m; abundance usually peaked near 10-20 m 

 both day and night. Pelagic cod also were caught 

 throughout the water column during the day, with 

 abundance peaks near 10-20 m for the 14-19 mm and 

 20-29 mm size-classes. They were more abundant in 



