Abstract. -Vertical distribution 

 patterns of haddock Melanogram- 

 mus aeglefinus and Atlantic cod 

 Gadus morhua are summarized from 

 eight research cruises on Georges 

 Bank, during spring and summer 

 1981-86. Eggs and larvae were 

 sampled at discrete depths with a 

 lm 2 MOCNESS, pelagic juveniles 

 with a 10 m 2 MOCNESS, and re- 

 cently-settled juveniles with a bot- 

 tom trawl supplemented with sub- 

 mersible observations. In well-mixed 

 waters during early spring, eggs and 

 larvae were distributed throughout 

 the water column. Upon stratifica- 

 tion of waters over the outer margin 

 of Georges Bank in mid-May, the lar- 

 val population was concentrated in 

 the thermocline; the stronger the 

 stratification, the more larvae were 

 confined to this depth zone. At well- 

 mixed shoal sites, pelagic larvae re- 

 mained distributed throughout the 

 water column. 



Differences in day and night dis- 

 tribution patterns provided evidence 

 to indicate that larvae began migrat- 

 ing as small as 6-8 mm; however, 

 clear sampling differences were evi- 

 dent for larvae only at body lengths 

 of >9-13mm. Larvae tended to be 

 deeper during the day and shoaler 

 by night; larger larvae had a greater 

 vertical range. Pelagic juveniles 

 (>20 mm) occurred deeper in the wa- 

 ter column as they grew. By mid- 

 July most juveniles (-40 mm) were 

 associated with the bottom. The 

 transition from pelagic to demersal 

 life i -40-100 mm) probably occurs 

 over a period of 1-2 mo. Recently- 

 settled juveniles remained demersal 

 by day and migrated upwards 3-5 m 

 at night. Their excursions off-bottom 

 decreased in amplitude with larger 

 sizes. Haddock remained closer to 

 bottom both day and night at a 

 smaller size than cod. The diel ver- 

 tical behavior of cod may be strongly 

 related to the light-dark cycle, 

 whereas behavior of haddock was 

 more variable. Some differences in 

 vertical distributions between had- 

 dock and cod can be attributed to 

 morphology and feeding behavior. 



Vertical distribution patterns and 

 diel migrations of larval and 

 juvenile haddock Melanogrammus 

 aeglefinus and Atlantic cod Gadus 

 morhua on Georges Bank* 



R. Gregory Lough 

 David C. Potter 



Northeast Fisheries Science Center. National Marine Fisheries Service, NOAA 

 Woods Hole. Massachusetts 02543 



Manuscript accepted 9 February 1993. 

 Fishery Bulletin, U.S. 91:281-303 1 1993). 



On Georges Bank off the northeast 

 coast of the United States (Fig. 1, 

 page 293), stocks of haddock Melano- 

 grammus aeglefinus L. and Atlantic 

 cod Gadus morhua L. historically 

 have been important components of 

 the American and Canadian demer- 

 sal fishery (Murawski et al. 1983, 

 Overholtz & Tyler 1985). Eggs and 

 larvae of both species are dominant 

 in the spring ichthyoplankton com- 

 munity (O'Boyle et al. 1984, Sherman 

 et al. 1984). Research survey data 

 have shown that the size of a year- 

 class is usually established by the 

 end of the first year of life (Sissen- 

 wine 1984). At the Northeast Fisher- 

 ies Science Center (NEFSC) since 

 1976, considerable research effort has 

 been focused toward describing the 

 place and time of spawning of had- 

 dock and cod, dispersal of eggs and 

 larvae, and survival and recruitment 

 of juveniles on Georges Bank. Field, 

 laboratory, and modeling results were 

 integrated to evaluate important 

 mortality processes during the first 

 year of life. Beginning in spring 1980, 

 a series of interdisciplinary, process- 

 oriented cruises used discrete depth 

 samplers to investigate mortality due 

 to starvation by describing the fine- 

 scale spatio-temporal variability of 

 gadid larvae in relation to their prey 

 during the first few weeks post- 

 hatching (Lough 1984, Laurence & 

 Lough 1985). In 1984, a series of 

 summer and early-fall surveys of ju- 



venile gadids was initiated to docu- 

 ment the distribution and abundance 

 of the 0-group fish and their transi- 

 tion to the demersal phase when pre- 

 dation may be a dominant mortality 

 factor (Cohen et al. 1988). 



Based on these studies, we now 

 have sufficient information to de- 

 scribe the life history of haddock and 

 cod during the first 6-9 mo. Both spe- 

 cies spawn on the northeast part of 

 Georges Bank (Fig. 1). Peak spawn- 

 ing time for cod is late February- 

 early March, and early April for had- 

 dock. The egg patches drift southwest 

 a few kilometers per day in the clock- 

 wise residual drift and hatch within 

 2-3 wk (Lough 1984). By May, larval 

 concentrations can be found along the 

 southern edge of Georges Bank be- 

 tween the 60 m and 100 m isobath. 

 In late-spring to fall, a seasonal ther- 

 mocline develops to a depth of 10- 

 30 m in waters deeper than -60 m. 

 Water shoaler than 60 m is vertically 

 well-mixed throughout the year by 

 the strong ( >60 cm/s ) rotary tidal cur- 

 rents. Larvae >30 d can be found 

 shoaler than the 60 m isobath as the 

 patch moves southwest, and by late 

 June some fraction of the population 

 has moved onto the shoals of west- 

 ern Georges Bank (Walford 1938, 



*MARMAP Contribution FED/NEFC 90-01. 

 Woods Hole Laboratory, Northeast Fisheries 

 Science Center, National Marine Fisheries Ser- 

 vice, NOAA. 



28 I 



