BROTHERS ET AL.: DAILY GROWTH INCREMENTS IN OTOUTHS 



lOjjm 



Figure 6. — Daily growth increments from the 

 otolith of Merluccius angustimanus . Note radial 

 fibers crossing the growth layers. 



ment width were made at five or more locations 

 along a radius and then total counts were calcu- 

 lated by extrapolation. No larval or very young 

 hake were available for examination. 



For Merluccius sp., data were available for 22 

 specimens aged 1 to 7 yr from the annuli present 

 in their otoliths. Figure 7 shows the graph of age 

 by annuli against age by daily increments for this 

 species. The correlation coefficient was 0.91 (20 df, 

 P » 0.001). The slope of the regression line was 

 1.14 (99% confidence limits [C.L.], 0.81-1.46). 

 This is not significantly different from the value 



7i- 



1 2 3 4 5 6 7 



AGE BY DAILY GROWTH INCREMENTS (years) 



Figure 7. — Graph of age-by-annuli against age-by-daily- 

 growth-rings in the otoliths of Merluccius sp. The encircled point 

 represents two points at the same position. 



of 1.00 expected if age by years and by days were 

 to yield identical values. 



Data from seven specimens of M. angusti- 

 manus were available and they varied in age from 

 only 1 to 2 yr. Given the much narrower ranges 

 and the smaller sample, the results obtained were 

 acceptable: r = 0.74 (0.05 > P > 0.01) and the 

 slope of the line was 1.25 (99% C.L., 0.24-2.25); 

 i.e., the slope was significantly different from zero, 

 but not from 1.0. 



The precision of estimates of age obtained for M. 

 angustimanus was not very good, with deviations 

 of up to 0.5 yr being obtained; however, for 

 Merluccius sp. a somewhat narrower range was 

 usual, with some values differing by 0. 1 yr or less. 

 Extreme variations occurred with fish aged 7 to 13 

 yr, where errors of up to 2 to 3 yr could be obtained 

 where daily counts were made. 



The average widths of the daily bands found in 

 the hake otoliths were 3 to 4 /xm, with wider and 

 narrower bands appearing sometimes in appar- 

 ently weekly, fortnightly, and monthly units. The 

 incidence of these units has not been examined in 

 detail and requires further study, but preliminary 

 work suggests that the basic unit used in age esti- 

 mates should be the daily unit; the higher order 

 units may be of great ecological interest, but 

 should probably not be used in aging these hake: 

 Only daily increments occur with the necessary 

 consistency and regularity. 



In addition to the species mentioned above, 

 apparently daily marks have been found in a wide 

 variety of other fish, e.g., in Tilapia zilli, T. 

 nilotica, and Clarias mossambicus from Lake 

 Victoria (examined by E. B. B. and C. P. M.; 

 specimens kindly collected by John Rinne and Dr. 



