Lough and Potter: Vertical distribution of Melanogrammus aeglefinus and Oadus morhua 



285 



A Haddock 



6-Bmm 



2 3 30 20 10 10 2030 6420246 



:v 



4/lOm 2 3/l0m 2 



• SINGLE HAUL 



53/l0m 2 53/l0mZ 



5/l0m 2 9/l0m 2 



03/l0m 2 08/K)m 2 

 • SINGLE HAUL 



I/I0m 2 I5/I0m 2 



SALINITY (p.su.) 

 3200 2 4 6 B 3300 2 



3/IOm 2 5/l0m 2 



TEMPERATURE ("CI 





 10 



l» 

 £30 



40- 



SALINITY 



Figure 5 



Mean day and night vertical distribution (:f n/100m 3 ) of (A) 

 haddock Melanogrammus aeglefinus and (B) cod Gadus 

 morhua size-classes of larvae collected by the lm 2 MOCNESS 

 on well-mixed Site 83-111 during 16-17 May 1983. Estimated 

 water column abundances (rc/10nr) are included at bottom of 

 plots. Temperature and salinity profiles are plotted as depth 

 stratum means (•) and 95 f; r confidence limits ( ). 



the water column (Fig. 5). The single night haul and 

 low densities preclude firm conclusions. 



Stratified water On 21 May 1981, when the water 

 column at Site 81-11 ( 75-83 m) was strongly stratified, 

 a single 1 rrr MOCNESS night tow found haddock and 

 cod larvae 2-13 mm to be confined almost exclusively 

 to the upper 20 m of the water column, and maximum 

 densities usually were within the strong thermocline 

 gradient at 10-20 m depth (Fig. 6). No recently-hatched 

 cod larvae 2-5 mm were caught in this tow. The sur- 

 face temperature approached 10°C with a strong ther- 

 mal gradient at 11-21 m, where the temperature de- 

 creased to 5.9°C. An intense storm on 21-24 May 1981, 

 with winds up to 18-21 m/s (35-40 kn) and sea wave 

 heights of 3-5 m ( 10-15 ft), destroyed the strong ther- 

 mocline shown in Fig. 6. Following the storm, the wa- 

 ter column was well mixed and the larvae were dis- 



A Haddock 



2 -5mm 6  8 mm 9- 13mm 



2 4 6 50 100 150 50 100 150 



JMj-J 



54ol 



ZJ 



B.Cod 



14 - 19 mm 



SALINITY (psuJ 

 32 00 2 4 6 8 3300 2 



TEMPERATURE TO 



K 



Figure 6 



Night vertical distribution ix /i/100m :, i of (A) 

 haddock Melanogrammus aeglefinus and (B) cod 

 Gadus morhua size-classes of larvae collected 

 by a single lm- MOCNESS tow on well- 

 stratified Site 81-11 before the storm on 21 May 

 1981. Estimated water column abundances 

 (fi/lOm-l are included at bottom of plots. Tem- 

 perature and salinity profiles are plotted as 

 depth stratum mean l#) and 959r confidence 

 limits ( 1. 



tributed broadly (Fig. 7), but about an order of magni- 

 tude more abundant relative to the collections made 

 on 21 May. Differences in mean day and night vertical 

 distributions of larvae during the well-mixed period of 

 24-26 May 1981 indicate a diel shift in distribution 

 patterns of the larger larvae (Fig. 7), with population 

 centers located deeper in the water column by day 

 (40-60 m) and shoaler by night ( 10-40 m). This diel 

 shift in vertical distribution is supported by the ANOVA 

 (Table 2B), where both haddock and cod have signifi- 

 cant (p<0.05) depth x time interaction effects. By 28 

 May 1981, near-surface insolation warming of the up- 

 per 20 m occurred, and larvae reaggregated above 20 m 

 near a weak thermocline (Fig. 8). 



Mean depths of larvae captured in tows before and 

 after the storm are plotted in Fig. 9 along with depth 

 bounds of the thermocline region. The weak thermo- 

 cline deepened after 25 May 1981. Mean depths of 

 larval abundance remained below the weak thermo- 

 cline until 28 May Tows made on 26 May showed that 

 the larger larvae were mainly located deeper in the 



