296 



Fishery Bulletin 91(2), 1993 



o. 

 a 



a 



Figure 20 



Generalized day-night depth distribution of cod Gadus morhua by size-class (mmSL) and 

 age (days post-hatching) from hatched larva through recently-settled juvenile in a 70m 

 water column. Mean population depth by day I ) and by night (•); vertical range between 

 day and night circles is represented by double arrow. Initial depth centers for the first two 

 size-classes (2-5. 6-8 mm I is largely determined by larval buoyancy and. therefore, whether 

 the water column is well-mixed or stratified. 



well-mixed in winter, with a seasonal thermocline de- 

 veloping by mid-May subject to perturbations. When 

 larvae reach a length of 9-13 mm, they appear to make 

 day-night migrations regardless of the water-column 

 structure. There is some uncertainty in the day-night 

 migration range of the pelagic juveniles; however, the 

 20 m range designated is within their swimming capa- 

 bility at this size. Furthermore, we don't know pre- 

 cisely the transition period from a day to night depth. 

 It probably occurs over a 3-4 h period, but it could be 

 shorter. For example, a 10 mm fish swimming at 1 

 body length/s could transverse 20 m in 1 h. The verti- 

 cal distribution pattern presented is sufficiently gen- 

 eral to be used as a starting point for both cod and 

 haddock at any bottom depth, even though all situa- 

 tions can be interpreted from the data. These studies 

 highlight the need for investigations of behavior- 

 related sampler avoidance and further time-series of 

 observation on fish distributions in different environ- 

 mental conditions. 



Citations 



Auditore, P. J., R. G. Lough, & E. A. Broughton 



1993 A review of the comparative development of Atlan- 

 tic cod and haddock based on an illustrated series of 

 larvae and juveniles from Georges Bank. NAFO Sci. 

 Counc. Stud. (In press). 



Azarovitz, T. R. 



1981 A brief historical review of the Woods Hole lab- 

 oratory trawl survey time series. In Doubleday, W.G., 

 & D. Rivard (eds.). Bottom trawl surveys. Spec. Publ. 

 Fish. Aquat. Sci. 58:62-67. 

 Bailey, R. S 



1975 Observations on diel behaviour patterns of North 

 Sea gadoids in the pelagic phase. J. Mar. Biol. Assoc. 

 U.K. 55:133-142. 

 Beamish, F. W. H. 



1966 Vertical migration by demersal fish in the North- 

 west Atlantic. J. Fish. Res. Board Can. 23:109-139. 

 Blaxter, J. H. S., & L. A. Fuiman 



1990 The role of the sensory systems of herring larvae 

 in evading predatory fishes. J. Mar. Biol. Assoc. U.K. 

 70:413-427. 

 Bolz, G. R., & R. G. Lough 



1983 Growth of larval Atlantic cod, Gadus morhua, 

 and haddock, Melanogrammus aeglefinus, on 

 Georges Bank, spring 1981. Fish. Bull., U.S. 

 81:827-836. 

 1988 Growth through the first six months of Atlantic 

 cod, Gadus morhua, and haddock, Melanogrammus 

 aeglefinus, based on daily otolith increments. Fish. 

 Bull, U.S. 86:223-235. 

 Bowman, R. E. 



1981a Food and feeding of 0-group haddock in the 

 Northwest Atlantic. Rapp. P.-V. Reun. Cons. Perm. 

 Int. Explor. Mer 178:322-323. 

 1981b Food of 10 species of northwest Atlantic juve- 

 nile groundfish. Fish. Bull., U.S. 79:200-206. 



