GROWTH THROUGH THE FIRST SEX MONTHS OF ATLANTIC COD, 



GADUS MORHUA , AND HADDOCK, MELANOGRAMMUS AEGLEFINUS, 



BASED ON DAILY OTOLITH INCREMENTS^ 



George R. Bolz and R. Gregory Lough^ 



ABSTRACT 



Daily growth increments of otoliths from larval and juvenile Atlantic cod and haddock were enumer- 

 ated, and growth curves were derived describing the first six months of life. Growth for both species 

 was best described by Gompertz-type curves. Inverse regressive methods were employed to construct 

 general models with confidence limits for predicting age (days) for given standard lengths (mm) from 

 hatching through the juvenile period. Microstructural analysis of the otoliths did not discern a 

 settling check at the time when the fish would be expected to leave the pelagic lifestyle for the 

 demersal one, which indicates that the transition is neither physiologically stressful nor abrupt. 



Fluctuations in the year-class strength of fish 

 stocks are thought to be determined by the rate of 

 mortality during the first year of life (Moser 1981; 

 Lough et al. 1985; Neilson and Geen 1986; and 

 others). Calculation of reliable mortality rates, 

 assessment of the influences of size-selectivity, 

 and establishment of precise hatching dates and 

 times during a given year when loss to recruit- 

 ment is greatest are dependent upon accurate age 

 and abundance estimates. Recently, investigators 

 have suggested that mortality during the postlar- 

 val and juvenile periods may be as critical as that 

 occurring in the egg and larval life stages (Cohen 

 and Grosslein 1982; Sissenwine 1984). Investiga- 

 tion of this hypothesis by the Northeast Fisheries 

 Center (NEFC) has been ongoing since 1984. 

 Enumeration of daily growth increments de- 

 posited on fish otoliths provides the best method 

 for the age determination of larvae and juveniles 

 needed for generating growth curves and estimat- 

 ing mortality (Essig and Cole 1986). An excellent 

 review of past and current methodologies em- 

 ployed in the study and application of otolith mi- 

 crostructure may be found in Campana and Neil- 

 son (1985). 



Atlantic cod and haddock are both spring 

 spawners on Georges Bank (Sherman et al. 1984) 

 and have pelagic eggs and larvae that undergo 

 similar development. Transformation to the juve- 



nile life stage occurs around 20-30 mm SL, or 2-3 

 months from hatching (Fahay 1983). The transi- 

 tion from the pelagic to demersal habitat of the 

 adults takes place sometime after transforma- 

 tion, usually by 6-8 cm in midsummer, and re- 

 cent field observations by the NEFC indicates the 

 transition is a gradual process with considerable 

 variability. 



In an earlier study by Bolz and Lough (1983), 

 growth curves were developed for larval Atlantic 

 cod and haddock based on otolith analysis that 

 defined growth from hatching (4-5 mm SL) 

 through the first two months of life (ca. 20 mm 

 SL). Juvenile Georges Bank Atlantic cod and had- 

 dock are not fully vulnerable to bottom-trawl 

 gear (Clark et al. 1982), and growth curves based 

 on groundfish surveys conducted by the NEFC in 

 the autumn and spring are inaccurate for fish 

 younger than about six months of age. The pri- 

 mary goal of the work reported here was to derive 

 age-at-length curves for field-caught Atlantic cod 

 and haddock describing their growth from hatch 

 until they are fully available to capture by 

 bottom-trawl survey gear. A secondary objective 

 was to determine if a check ring, a wide incremen- 

 tal band indicative of physiological or environ- 

 mental changes, was deposited during the juve- 

 nile's transition from the pelagic to the demersal 

 mode of life. 



IMARMAP Contribution FED/NEFC 87-15, Northeast Fish- 

 eries Center Woods Hole Laboratory, National Marine Fish- 

 eries Service, NOAA, Woods Hole, MA 02543. 



^Northeast Fisheries Center Woods Hole Laboratory, Na- 

 tional Marine Fisheries Service, NOAA, Woods Hole, MA 

 02543. 



Manuscript accepted December 1987. 

 FISHERY BULLETIN: VOL. 86, NO. 2, 1988. 



METHODS 



Atlantic cod and haddock larvae and juveniles 

 were collected on six cruises conducted by the 

 NEFC's RV Albatross IV and RV Delaware II on 



223 



