290 



Fishery Bulletin 9 1(2). 1993 



55/IOm z 10/lOm 2 236/lcW 168/IOm 2 59/l0m z 67/lOm z 



13/lOm 2 7/lOm z 



I4/I0m z 13/lOm? 



SALINITYtp.su) 

 3200 Z A 6 6 3300 2 



Figure 1 2 



Mean day and night vertical distribution ix n/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 83-11 during 15-16 May 1983. 

 Estimated water column abundances In/lOnr) are included 

 at bottom of plots. Temperature and salinity profiles are plot- 

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



cantly greater than day catches. Their average night- 

 day catch ratio for 16-45 mm cod was 3.0 (range 1.6- 

 4.7), which is similar to the ratios estimated in this 

 study of cod and haddock pelagic juveniles. 



In an extensive study of the northeast U.S. conti- 

 nental shelf ichthyoplankton (an 8yr time-series of 

 data using a 61cm diameter bongo net, standard 

 MARMAP double-oblique tows), Morse (1989) found 



Figure 13 



Time-series of abundance weighted-mean depths of 

 larval haddock Melanogrammus aeglefinus and cod 

 Gadus morhua size-classes on weakly-stratified Site 

 83-11 during the period 15-16 May 1983. In the had- 

 dock time-series, boundary temperatures are re- 

 corded beside the brackets; for cod, the rate of 

 temperature change (dt/dz) over the thermocline 

 region is specified. 



larval haddock to be the exception among most taxa in 

 that day catches exceeded night catches for all lengths 

 of larvae in the range 4-15 mm. Day catches were some- 

 what greater than night catches for 4-12 mm cod, but 

 for the larger fish ( 13-20 mm) night catches were equal 

 to or greater than day catches. 



Haddock larvae, as well as cod, may remain still 

 during the day to evade predator detection by visual 

 or mechano-reception (Zaret 1980). When both had- 

 dock and cod reach a larger size and their sensory 

 systems become more developed, they exhibit a startle 

 response and evade the attacking predator (in this case 

 a net) (Blaxter & Fuiman 1990). Flexion and fin ray 

 development is complete for both species at 15-20 mm 

 (Auditore et al. 1993) and they would be more capable 

 of a powerful darting speed. Perhaps at night without 

 visual stimuli, only the larger juveniles exhibit a startle 

 response to the net-mouth pressure stimulus. Light- 

 aided avoidance is expected to decrease with water 

 depth as a function of light attenuation and the fishes' 

 visual threshold (Blaxter & Fuiman 1990). Pressure 



