448 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



that these "tagging-checks" were not discovered by 

 either reader on scales of any of the other fish. 



DISCUSSION 



The formation of an annuhis on fish scales may 

 be caused by one or more extrinsic or intrinsic 

 factors such as temperature changes or physiolog- 

 ical cycles. 



Lee (1920) has suggested that haddock form 

 annuli when they migrate from one environmental 

 temperature to another. We know from marking 

 experiments that Georges Bank haddock are rela- 

 tively sedentary and the thermal milieu on 

 Georges Bank does not vary greatly through the 

 year. It is probable then that the annidi are 

 not caused by marked ditferences in water 

 temperature. 



It might be argued that the scale annuli are 

 spawning checks. The physiological drain in 

 producing large amounts of gametes may cause 

 formation of closely spaced circuli on the scales. 

 As stated, previously, this zone of circuli is the 

 annulus counted in haddock age determinations. 

 The period of annulus formation (August to 

 April) begins almost 7 months prior to the 

 beginning of spawning and 5 to 6 months prior to 

 the maturation and ripening of the sex products. 

 Annuli are also seen on the scales of small, sexu- 



ally immature fish and could not then have been 

 caused by spawning activities. 



Scale annuli may be caused by diminished feed- 

 ing; a decline in feeding during the spawning 

 season is repoi-ted by Wigley (1956). His data 

 for April (height of spawning) are for a group 

 of 256 fish whose average length is 35 cm. Such 

 fish are almost all immature (Clark, 1959), and 

 their reduced feeding intensity probably is due 

 to a cause other than spawning activity. 



What causes the formation of the annulus is 

 not defuiitely known. But not knowing the cause 

 does not remove the effect; annuli, formed once 

 each year, are present on the scales and indicate a 

 consistent annual phenomenon. 



Consistency of age determinations is sometimes 

 cited in arguments supporting validity of the 

 determinations. Two or more people reading the 

 same sample of scales and getting the same ages 

 may be a result of (1) their having been trained 

 in one set interpretation of the zones and thus 

 reading the scales "with the same eyes," or (2) 

 using independent interpretations of obvious 

 scale markings. If the readers are using the same 

 interpretation, their readings should agree. If 

 they are using somewhat different interpretations, 

 their readings will agree closely, but not com- 

 pletely. The agreement of haddock scale readers 

 is very close (89 percent to 93 percent agreement 

 in our laboratory) between individuals and with 

 repeated readings by the same individual. This 

 high degree of consistency shows only that the 

 scale markings are usually clear and easily read. 



Another consideration is the progression of 

 strong year classes through the fisheiy as shown 

 by the modes of age-frequency histograms (fig. 5). 

 When a mode advances by 1 year with the passage 

 of 1 year, it shows that the year class represented 

 by the mode has also increased 1 year in age. 



"VAHien the scales of tagged fish are studied, it 

 might be argued that tagged fish show a "tagging" 

 check on the scale presumably caused by the 

 tagging operation. Following this line of rea- 

 soning, the scale-determined age should be 1 year 

 more than the chronological age for recovered 

 tagged fish. It has been shown that most tagged 

 haddock produce one scale annulus per year,, and 

 the scales from 29 of the 32 tagged fish mentioned 

 earlier showed only one additional annulus for 

 each year at liberty. It is reasonable to assume. 



