AGE OF YOUNG HADDOCK 



443 



but the water temperatui-e determined the spacing 

 between circuli. Lee (1020) notes that naiTow 

 "sclerites" beo^in to form on haddocii scales in 

 August or earlier, when the water temperature 

 is increasing. She suggests that at this time the 

 fish may be migrating into the cooler offshore 

 watere where the decreased temperature produces 

 the closely spaced circuli or "winter rings.'" 



The role of food and feeding and their effects 

 on haddock scale zones have not been clearly dem- 

 onstrated, although some indirect evidence has 

 been accumulated. Xarrowly spaced circuli 

 generally form on the scales of Georges Bank 

 haddock in the period August-A]iril (Jensen and 

 Clark. 1958). For haddock from Xova Scotia 

 offsliore banks, Homans and \nadykov (1954) 

 report Xovember througli May to be a season of 

 diminished feeding. The period of least feeding 

 coincided with the spawning period (Febiiiary. 

 March, and April) while the months of June 

 tliroujrh October constituted the feeding season. 

 For Georges Bank haddock. "Wigley (1956) re- 

 ports that during the spawning period a decline 

 in feeding, rather than a complete cessation, takes 

 ]ilace. and for the period of annulus fonnation, 

 the average volume of food per haddock rose 

 from 1.8 cubic centimeters in September to 3.0 

 cubic centimeters in January' but was lowest (1.0 

 cubic cm.) in February. We can see no clearcut 

 evidence, based on "Wigley's data, that diminished 

 feeding causes the formation of narrowly spaced 

 circuli. 



There seems no reasonable doubt that widely 

 spaced circuli are formed in summer and nar- 

 rowly spaced in winter, whatever the dii'ect cause. 

 For the pui-pose of this stiuly the point of interest 

 is that zones are laid <lown on the scales at the 

 rate of one for each winter and one for each 

 summer, the two zones making a pattern which 

 indicates a single year's interaction of the had- 

 dock and its environment. 



Haddock Scale Reading Procedure 



The annulus used by us in haddock age de- 

 terminations is the zone of closely spaced circuli. 

 It has the following characteristics: 



1. It is concentric with the niiirsrin of the scale. 



2. It can be trace<l. by careful scrutiny if necessary, 

 entirely around the scale. 



3. It is clearly separated from other such zones and 

 does not ordinarily meet them at any point. 



4. If present, it is on all the normal scales of an 

 individual. 



The scales from scrod haddock are thin and 

 readily transmit light. Therefore, they are 

 placed dry on a glass slide and examined under a 

 binocular microscope with substage illumination. 

 Large haddock scales are thicker and do not 

 readily transmit light. To overcome this, large 

 haddock scales are routinely impressed in plastic 

 slides as described by Arnold (1951). The im- 

 pressions are projected with a commercial micro- 

 projector at a magnification of about -40 X. A 

 minimum of three scales are examined from each 

 fish. Regenerated and deformed scales are 

 discarded. 



The nmnber of complete annuli. from the focus 

 to the posterior edge of the scale, is counted to 

 determine the age of the fish in years. The annu- 

 lus is considered complete on February 1 (the 

 arbitrary '"birthday" of Georges Bank haddock), 

 selected because most of the haddock commence 

 spawning in February. 



Results from a study of the formation of scale 

 zones of Georges Bank haddock (Jensen and 

 Clark, 1958) have shown that one annulus is 

 formed each year. This was demonstrated by a 

 predominance of widely spaced circuli at the 

 scale edge during May-July and a predominance 

 of nairowly spaced circuli during August-April. 



Problems in interpreting scales arise because 

 of the ambiguity of some annuli. For example, 

 in a routine sample of scales from 510 Georges 

 Bank haddock, read by the senior author, only 

 100 (19.6 percent) had distinct, well-separated 

 annuli and were considered by him as easy to read. 

 The fish were 1 to 5 years old and the annuli were 

 sharply defined. The remaining 80 percent of the 

 scales required detailed study to determine the 

 age. 



The most freciuent error in scale reading prob- 

 ably arises from counting an accessoiy annulus 

 usually fomid in the firet zone of broad circuli 

 (fig. 2) . Thompson ( 1923) has termed this annu- 

 lus a "false winter appearance" and suggests it 

 occurs when the young haddock first descend to the 

 bottom and their growth is checked as they enter 

 the deep cold water. Our studies have sho^vn that 

 this accessory annulus occurs in slightly less than 

 50 percent of the scales examined for age 

 determination. 



