increasingly difficult to identify year-marks. 

 This causes a greater amount of error in the 

 determination of age of older fish. 



A slowdown in fish growth may occur dur- 

 ing a growing season, and the resulting check 

 in scale growth sometimes resembles a year- 

 mark. Where such checks are counted as year- 

 marks, the age determined from the scale will 

 be greater than the true age. These false year- 

 marks are sometimes associated with reduced 

 growth-rate at spawning time or with shock, 

 such as injury or disease. 



Growth in length of a fish scale is propor- 

 tional to the growth in length of the fish itself. 

 Because this is true, the past growth history 

 of a fish can often be worked out from its scale 

 through a technique called back-calculation. 

 If the length of a fish at capture is known, it 

 is possible to calculate length at earlier ages 

 from measurements of the scale at each year- 

 mark. The increase in length of a haddock 

 in each year, in relation to increase in scale 

 length, is illustrated in Figure 5. Lengths at 

 each of the three year-marks were determined 

 from back-calculations. 



1 YEAR 



YEARS 



2.0 



YEARS - I 7.0' 



Figure 5. — Illustration showing how the past growth 

 history of a fish may be determined from its scale. 

 The 18%-inch haddock was in its fourth season 

 since it has three year-marks on its scale and the 

 beginnings of fourth-year growth on the scale edge. 



The back-calculation method is of importance 

 in fisheries studies since it permits an evalu- 

 ation of growth rate of fish in all years of life. 



Otoliths. — In fishes that do not have scales, 

 or where annual zones are not clearly shown on 

 scales, it is often possible to determine age from 

 seasonal bands laid down in otoliths. Otoliths, 

 or earstones, structures formed of calcium in 

 the heads of bony fishes, function as organs 

 of balance. Although there are three pairs 

 of otoliths altogether, only one pair is large 

 enough to be of use in age determination. 

 Otolith form varies in diflferent species from a 

 flat oval to spindle shape. Growth, as in the 

 scale, is concentric around a central kernel or 

 nucleus (Figure 6). Factors, such as water 



Figure 6. — An otolith from a 4-year-old yellowtail 

 flounder. The fish grew most rapidly during its 

 second year, and the otolith band in that year is 

 therefore the broadest one. 



temperature, that affect fish growth cause 

 seasonal changes in the density of layers laid 

 down in otoliths, and in some cases it is pos- 

 sible to determine fish age from the banding 

 that results. When otoliths are viewed under 

 a low-power microscope, the layers making up 

 spring and summer growth appear as a white, 

 opaque band. Layers laid down in the fall, and 

 also in the winter in some fishes, appear as a 

 dark translucent band. A light and a dark 



5 — 



