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



293 



50 

 40 

 30 

 20 



> l0 

 i 

 S 40 



30 



20- 



10 

 



HADDOCK 



17-18 JULY 1985 



pP 



DAY 

 n = 496 



X 



TWILIGHT 

 n=40 



n 



10 12 14 2 4 6 

 LENGTH I cm) 



10 12 14 16 



Figure 1 7 



Day and twilight length-frequency distribution of haddock 

 Melanogrammus aeglefinus and cod Gadus morhua juveniles 

 collected by Yankee 36 bottom trawl on Site 85-IV during a 

 24 h study, 17-18 July 1985. No fish were caught in night 

 trawls. 



6 8 10 12 14 16 18 20 22 

 LENGTH (cm) 



Figure 1 8 



Day, night, and twilight length- 

 frequency distribution of haddock 

 Melanogrammus aeglefinus juve- 

 niles collected by Yankee 36 bot- 

 tom trawl on Site 85-V during a 

 24 h study, 16-17 August 1985. 



migrations are fairly well estab- 

 lished when larvae reach 9- 

 13 mm length and may be initi- 

 ated at a smaller size, 6-8 mm, 

 depending on the physical struc- 

 ture of the water column. This 

 pattern of residing deeper by day 

 and shoaler at night continues 

 into the older pelagic juvenile and 

 recently-settled juvenile period. 



When larvae reach a length of ~9 mm, the swim bladder may be function- 

 ally important in the vertical migration of larvae in both species (Schwartz 

 1971, Ellertsen et al. 1980, Howell 1984). Haddock and cod have similar 

 developmental patterns; however, there are some important differences 

 (Auditore et al. 1993). At 8-9 mm, haddock have larger and more developed 

 pectoral and pelvic fins than cod. The difference in development is pro- 

 nounced after 20 mm in length; at 23 mm, the pectoral fins of cod are V-2 to 3 A 

 the size of haddock, while the pelvic fins of cod are l /s those of haddock. 

 Haddock also possess more total fin rays in the caudal, dorsal, and anal fins 

 than cod. The total number of caudal-fin rays usually determines the overall 

 size of the fin, which is an important means of thrust in gadiform fishes 

 (Cohen 1984). The higher number of caudal- and dorsal-fin rays in haddock, 

 in combination with early paired-fin development, may provide greater ma- 

 neuverability for haddock. The earlier fin development of haddock larvae 

 compared with cod may allow them to more actively locate high concentra- 

 tions of prey and stay within these patches. Miller et al. (1963) studied the 

 vertical distribution of larval haddock at three sites on Georges Bank in May 

 1958. Haddock larvae were found throughout the water column, with maxi- 

 mum concentrations centered at 20-30 m. At all three sites they found peri- 

 odic changes in the depth distribution of larvae that were related to similar 

 changes in depth of the thermocline. Smaller larvae, 4-8 mm, occurred below 

 the thermocline at each site while >80% of the larger larvae, 9-19 mm, were 

 found in the thermocline. They also found no evidence of diurnal change in 

 the weighted mean depth distribution of larvae, 24 m during the day and 

 22 m by night. Their observations were in general agreement with those of 

 Frank et al. (1989) for haddock larvae on the Southwestern Scotian Shelf. 

 Frank et al. (1989), using a discrete depth sampler, BIONESS, found 

 recently-hatched haddock larvae in May 1985 and 1986 were concentrated at 

 mid-depth (~30m) in day and night, and their population centers remained 

 at this level until 20 d post-hatching. The hydrographic conditions were char- 

 acterized by a density gradient (sigma-<) that increased linearly with depth 

 both years. Whereas Miller et al. (1963) and Frank et al. (1989) found had- 



