spawn only during the late winter and spring. 

 Baby fish from this spawning are 4 to 8 inches 

 long by the summer and fall, and their length 

 marks them as young of the current year. 

 Haddock spawned in the previous year would 

 be 10 or 12 inches long by this time and clearly 

 separable, by size, from the baby fish. 



To illustrate the technique, suppose we use 

 a net, such as an otter trawl, and obtain a 

 large catch of one kind of fish in which all 

 sizes, young to adult, are represented. Let us 

 measure the lengths of all fish in the catch and 

 count the number of fish in each length span 

 of, say, one-half inch for the entire range of 

 sizes. If we now plot the numbers of fish in 

 each of these length intervals on graph paper, 

 it is likely that more than one peak will be 

 evident in our graph because fish of certain 

 lengths occur more frequently than others. It 

 may be suspected that each clear-cut peak rep- 

 resents the average length in a separate age- 

 group because of the restricted range of lengths 

 within each age-group. 



10 



5- 



9 II 13 

 TOTAL LENGTH IN INCHES 



Figure 1. — The length-frequencies of a catch of yellow- 

 tail flounder, showing how fish age may be deter- 

 mined from fish size. 



Figure 1 shows the lengths of a catch of 

 yellowtail flounder graphed in this way. Males 

 and females have been plotted separately be- 

 cause females of this species are known to 

 grow faster than males. One-year-olds out- 



number fish of older age-groups because deaths 

 from various causes take a continuous toll. 



There are two peaks in the graph of males, 

 one at about 8I/2 inches, representing the ap- 

 proximate average length of 1-year-old fish, 

 and a second at 12 inches, marking average 

 length for 2-year-olds. For females, peaks are 

 shown at 8V2. 13, and 15 1/0 inches, representing 

 average lengths of 1-, 2-, and 3-year-olds, re- 

 spectively. The graphs indicate that few fish 

 older than these age-groups were present on 

 the fishing ground. Also, no fish in their first 

 year (0-year-olds) were caught here, for dur- 

 ing the summer when this catch was made the 

 baby yellowtail have not yet begun to school 

 with older ones. 



The size-frequency method works best for 

 young fish, generally less than 3 or 4 years old. 

 As fish grow older, the spread of sizes within 

 an age-group becomes more variable. The 

 peaks representing ages of older fish in the 

 length-frequency graph tend to blend together, 

 and it may be impossible to identify them. The 

 peaks for 2- and ;]-year-old females in Figure 

 1 already show signs of flattening out and 

 blending together. 



This disappearance of peaks with age is 

 more apparent in the length-frequency distri- 

 bution of haddock in Figure 2. This catch was 



Figure 2. — The length-frequency distribution of a 

 catch of haddock, showing the different size groups 

 of fish caught. 



obtained in the fall when baby fish are just 

 beginning to appear in otter trawl catches. 

 These averaged about 6 inches in length. One- 

 year-olds averaged about 11 inches, while 2- 

 year-olds were slightly over 16 inches in length. 

 It is clear that the peak for 2-year-olds is not 

 as sharply defined as that for 1-year-olds. This 

 is because it undoubtedly also includes some 



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