AGE OF YOUNG HADDOCK 



441 



10 



"T I I I I I I I I — I — I — 1 — r 



H.W GRAHAM (1952) 



SCHUCK and ARNOLD (1951) 



NEE0LER{I930) 



4 6 8 10 



AGE IN COMPLETE YEARS 



FiGiRE 1.— Growth curves for Georges Bank haddock. 



of the metliod, although inaccuracies are bound 

 to occur." 



In the 1030 report. Needier derived a growth 

 cune for George,'; Bank haddock, bixsed on scale 

 readings from 189 fish collected during May- 

 September 1927. The growth curve is shown in 

 figure 1, and on it we have indicated age-length 

 values derived by Schuck and Arnold (1951) and 

 H. W. Graham's (1952) growth rate. Graham's 

 (1952) age-length data were taken from Februaiy 

 1 (completed year) scale readings. Both Xeedler 

 (1930) and Schuck and Arnold (1951) obtained 

 their age-length data from mid-year scale read- 

 ings, thus the length for each age includes addi- 

 tional growth. Figure 1 indicates that Xeedler, 

 Schuck and Arnold, and H. W. Graham do not 

 differ markedly in their age determinations. 



Recently, the more specific problems encoun- 

 tered in aging Georges Bank liaddock have been 

 studied l)y Jensen and Clark (195S), wlio deter- 

 mined the time of annulus formation on haddock 

 scales; Clark (1958), who reported on the consist- 

 ency with which different scale readers read the 

 same .scale samples: and Kohler and Clark ( 1958), 

 who reported comparisons of age determinations 

 from otoliths and scales from the same sample of 

 liaddock. These recent studies warrant closer 

 examination and will be discussed in detail below. 



Our haddock biostatisticnl data show that, in 

 past years more than 80 percent of Georges Bank 

 haddock are caught by the time they have com- 

 pleted their fourth year. Furthejinore, we are 

 more confident about our age detemiinations of 

 young fish. The annuli are most sharply defined 

 in the scales of haddock 2, 3, and 4 years old, and 

 the difference between summer growth and winter 

 growth is very marked (Jensen and Clark, 1958). 

 Needier (1930) noted this, too, when he stated: 

 "In the later years in the life of the haddock, the 

 annual zones become verj' narrow and the niunber 

 of circuli in each small. In many of these cases 

 the age is truly an 'estimation,' whereas the age of 

 younger individuals can more truly be said to be 

 'determined.' " 



Our own investigations bear this out, and 

 Kohler and Clark (1958) report no statistically 

 significant differences between scale and otolith 

 ages up to about 7 years: after the seventh year, 

 scale readings are consistently lower than otolith 

 readings. 



VALIDATION OF THE SCALE READING 

 METHOD 



Haddock scales are cycloid and oval to elliptical 

 in shape (fig. 2). They do not show radii or 

 transverse grooves, sometimes present on cycloid 

 .scales from other species. The external surface 

 is sculptured with concentric circuli arranged 

 about the focus, but the internal surface is rela- 

 tively featureless. The focus is usually anterior 

 to the center of the scale, but occasional scales are 

 found with the focus at the center. Periods of 

 rapid and slow growth are indicated by the 

 spacing of the circuli. In studies of several 

 species of gadids, including the haddock, J. S. 

 Thomson (1904) noted that "* * * the growth 

 of scales is cyclical or periodic, and * * * the 

 rings formed thereby are annual." 



J. S. Thomson considered temperature as a fac- 

 tor in forming annual rings only as it affects the 

 food supply. He suggested that the scale circuli 

 are widely spaced during the period of rapid 

 growth in the warm season when food is plentiful 

 and closely spaced during the period of slow 

 growth in the cold season when food is scarce. 

 Cutler (1918) believed water temjierature to be 

 the direct cause of annual ring.s. In an aquarium 

 study with flounder and plaice, he concluded the 

 food supply determined the number of circuli. 



