328 BULLETIN OF THE BUREAU OF FISHERIES 



FACTORS INVOLVED IN APPARENT DISCREPANCIES OF LENGTH COMPU- 

 y TATIONS BASED ON THE SCALES OF HERRING 



As the conclusion has been reached that computations based on the diameters 

 of the herring scales are more accurate than those based on the anterior radii, the 

 diameters only form the basis of further discussion unless the contrary is stated. 



It has been pointed out that average calculated lengths are lower, in general, 

 than corresponding measured lengths. This is seen by comparing the average 

 measured lengths shown in the third column of Table 16 with the calculated lengths of 

 fish of the same age. Thus, 50 3-year fish average 226 millimeters in length, but 

 the calculated lengths of S-j'ear fish vary from 194 to 216 millimeters (Table 16, col- 

 umn 8). The differences between actual and calculated lengths are greatest, in 

 general, for the early years of hfe. This is illustrated in Table 17, where it may be 

 seen that these differences for years II to VI, inclusive, are 24, 14, 7, 4, and 5 milli- 

 meters, respectively. The computed length values of Table 16 show another pecu- 

 liarity, which has been found in the uncorrected computed lengths of virtually all 

 species of fish. In the fourth column of Table 16 are shown calculated lengths at the 

 end of the first year of life. The values decrease from top to bottom; that is, the older 

 the fish whose scale is used for the calcidation the lower the value obtained. This 

 peculiarity is even more striking in calculated lengths for years II to V, as seen in 

 the colunms headed "on V." 



Thus, while aU computed values apparently are too low, the eiror for a particu- 

 lar year is greater the older the fish whose scales are used for the calculation. In 

 general, when we compare the computed values of the various age groups of a sample 

 for equivalent years we find, as shown in Table 16, that in each year of life these val- 

 ues tend to vary inversely with the age of the groups from which the scales are taken. 

 This characteristic of length computations based on scales usually is referred to as 

 "Lee's phenomenon of apparent change in growth rate." 



Miss Lee (1912) has suggested seven possible explanations of this phenomenon. 

 These involve either the change in the composition of the year classes with age, selec- 

 tive elimination by death, or change in the scale itself. The suggestions are listed 

 below. 



1. The samples of fish are not representative of a year group; that is, the young- 

 est year groups are represented only by their biggest individuals, and as we proceed 

 toward the older groups there appear more and more of those that had been the smaller 

 individuals in theii- earliest years, so that the average sizes of these older groups tend 

 to show a less increment of growth and a levehng in values is attained (some such 

 thing occurs, according to Lea). This Lee termed "the selective effect of size." 



2. The nets are selective, retaining only the largest fish of the youngest year 

 group and e.xcluding the largest fish of the oldest year groups. 



3. Conditions of grov/th are improving and the fish actually are growing more 

 rapidly at present. 



4. Females and males that have different growth rates are present in varying 

 proportion. 



5. The scale, especially the flexible newest part, contracts when new increments 

 are added. Miss Lee noted that the newest increment in the scale is usually wider 



