BOLZ AND LOUGH: GROWTH OF ATLANTIC COD AND HADDOCK 



Table 6. — Estimated age in days based on otolith diame- 

 ters with 95% confidence limits for larval and juvenile had- 

 dock compared with observed age derived from number of 

 daily increments. 



Table 7. — Estimated age in days based on otolith diame- 

 ters with 95% confidence limits for larval and juvenile At- 

 lantic cod compared with observed age derived from num- 

 ber of daily increments. 



ness with which the larval and juvenile curves 

 flow into those independently developed for the 

 adults (Clark et al. 1982; Penttila and Gifford 

 1976). 



The predictive models for Atlantic cod and had- 

 dock have to be viewed as general in nature, and 

 the widening of the confidence intervals with in- 

 creasing length (Tables 4, 5) must be kept in 



mind. Natural variability of length-at-age and 

 difficulty in the preparation and reading of 

 otoliths increases as the fish becomes older and 

 makes precise age determinations extremely dif- 

 ficult. For example, the ability to predict correctly 

 the age of an individual haddock at the 70% con- 

 fidence level decreases from ±3 days at 5 mm SL 

 to ±2 weeks at 175 mm. In spite of this problem, 

 otolith aging of field-caught larvae and juveniles 

 provides a degree of precision not possible with 

 indirect methods based on size-frequency analy- 

 ses (Ebert 1973). Refinement of the estimated 

 means and the reduction and stabilization of the 

 variance should result as a greater number of 

 otoliths are analyzed in the future. 



Microstructural examination of larval Atlantic 

 cod and haddock otoliths clearly delineated check 

 rings related to hatching and yolk-sac absorption 

 (Bolz and Lough 1983). Both of these transitions 

 are abrupt, and the dark, thick discontinuous 

 zones readily observable on the otoliths are a re- 

 flection of metabolic disturbances undergone at 

 these times. Although additional check rings 

 were noted in 3 or 4 of the juvenile otoliths, there 

 was no regularity with respect to age of their oc- 

 currence. In these individuals the checks were 

 probably the result of physiological trauma in- 

 duced by disease or injury since calcium carbon- 

 ate secretion ceases not only with the metabolic 

 changes accompanying transitional phases but 

 during times of stress (Morales-Nin 1987). It was 

 suspected that a distinct check, similar to the set- 

 tling check found by Victor (1982) in the bluehead 

 wrasse, Thalassoma bifasciatum , would be found 

 demarcating the transition from the pelagic to 

 the demersal mode of life with its accompanying 

 changes in diet and activity levels. No check rings 

 were found in the transition period (50-100 days) 

 on the otoliths analyzed. This suggests that an 

 abrupt metabolic disturbance does not occur at 

 this phase of the fish's life and that settling near 

 the bottom takes place over an extended period of 

 time (1-2 months) even for individual fish. This 

 agrees with a preliminary finding for Scotian 

 Shelf gadoids by Campana and Neilson (1985). 

 However, in a recent study by Mahon and Neilson 

 (1987) on the gut contents of Scotian Shelf had- 

 dock, they concluded that the transition from 

 pelagic to demersal life occurred relatively sud- 

 denly, less that a month for the individual fish. 

 Apparently, change to the demersal life stage is 

 not stressful for Atlantic cod and haddock, at 

 least as a metabolic manifestation recorded in 

 their otoliths. 



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