Lough and Potter Vertical distribution of Melanogrammus aeglefinus and Gadus morhua 



291 



A Haddock 



9 -13 mm " 19 mm 



6 .12 2 4 6 9 6 3 3 6 9 



9/l,OOOm 2 4/l,O0Om 2 



16/I.OOOm 2 20/i,0O0m 2 



SALINiTV(iis.u J 

 32 00 2-4 6 6 3300 2 



TEMPtRATURECC) 

 5 10 15 



I ' I t ' ' ' I 



09/I.OOOm 2 4/l,000m 2 



B Cod 9 . 13mm 14- 19mm 



„3 2 I 12 3 30 20 10 10 20 30 



2 5/I.OOOm 2 03/l,OOOm 2 29/l.OOOm 2 54/l£00m 2 



75 50250 25 50 75 3210123 



45/I.OOOm 2 l70/t,OOOm 2 05/l,000m 2 5/I.OOOm 2 



Figure 1 4 



Mean day and night vertical distribution 

 (.r /i/10,000m 3 ) of (A) haddock Melano- 

 grammus aeglefinus and (B) cod Gadus 

 morhua size-classes of pelagic juveniles col- 

 lected by the 10 m 1 ' MOCNESS on well- 

 mixed Site 84-1 during 17-19 June 1984. 

 Estimated water column abundances 

 l/i/1000m-') are included at bottom of plots. 

 Temperature and salinity profiles are plot- 

 ted as depth stratum means (•) and 95% 

 confidence limits ( ). 



or sound avoidance are expected to be 

 the same over depth, however. 



Therefore, for the three smallest 

 size-classes of haddock and cod 

 <13mm, light-aided avoidance of the 

 lm 2 MOCNESS sampler may be rela- 

 tively small (< factor of 2) as a con- 

 tributing factor in observed day-night 

 vertical distribution patterns. How- 



ever, in fish >14 mm, night-day catch ratios are expected to increase 

 to >3 because of light-aided avoidance response. Haddock larvae 

 appear to respond differently to light than cod in their avoidance 

 behavior. A correction factor for cod was not applied from the wa- 

 ter-column total abundance ratios. The night-day catch ratios are 

 not suitable for correcting depth strata densities, since net avoid- 

 ance may be a function of light level which decreases with depth. 

 That is, avoidance may be depth-dependent. We do not have the 

 data suitable for making an appropriate depth correction. Applying 

 a constant correction factor to all depth levels of a given size-class 

 would not change the vertical distribution pattern. We need to 

 know the fishes' reaction distance to the net at the different depth 

 light levels. 



During the fishes' transition from a pelagic to demersal life at 

 -30-40 mm, they are not fully vulnerable to either pelagic or demer- 

 sal sampling gear and this contributes to the sampling variability. 

 Scott (1984) studied diel variations in juvenile haddock catch rates 

 from a 24 h bottom-trawl experiment near Sable Island on the Scotian 

 Shelf in July-August. Haddock >6 cm had a marked vertical migra- 

 tion pattern, moving off the bottom at night and returning by day- 

 light. Based on changes in mean lengths, larger fish moved off the 

 bottom in greater proportions than smaller fish. Colton (1965) exam- 

 ined data from bottom-trawl surveys conducted on Georges Bank 

 and found catch-per-tow of 0-group and 1+yr haddock to be mark- 

 edly higher during the night than day. He believed the night-day 

 variability was due to escapement from the net during the day and 

 not to avoidance. Lough et al. (1989) conducted submersible studies 

 on eastern Georges Bank in July 1987 and August 1986 and found 

 recently-settled juvenile gadids (mostly cod) 4-12 cm in length close 

 to the bottom (<0.5m) during the day and a portion of the popula- 

 tion rising 1-5 m off the bottom at sunset, drifting with the current. 

 A comparison of standardized research bottom-trawl (with rollers) 

 catches and submersible transect counts showed the extent of 

 undersampling by the bottom trawl. For 4 cm modal-size cod (1987), 

 daylight trawl abundance estimates were at least an order of magni- 

 tude lower than the submersible transects, but not significantly dif- 

 ferent than the night estimates. For demersally-oriented larger fish 

 (7 cm modal length), few fish were caught by daylight trawls, and 

 night trawl catches were still at least an order of magnitude lower 

 than night submersible counts. The larger fish appeared to stay 

 closer to the bottom both day and night, and consequently they are 

 less vulnerable to the bottom trawl with roller gear which may pass 

 over them. Some preliminary results by Cohen et al. (1985) showed 

 that more cod juveniles of 4-5 cm were caught using the bottom 

 trawl with the smaller rubber disc-covered chain sweep than with 

 the larger roller gear. Thus, the bottom trawl catches of recently- 

 settled cod are known to have a significant sampling bias with more 

 cod collected in night trawls than day, and especially for the larger 

 fish escaping the roller gear. 



Vertical distribution patterns 



The initial depth distribution of recently-hatched larvae is dependent 

 upon the late-stage egg distribution. Some limited egg profile data 



