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MAXIMUM AGE, YEAR 



Fna'RK 1. — Plot of instantaneous mortality rate (yr ') against maximum observed age (yr), both on logarithmic scales. 



1) A quick preliminary estimate of the mortality rate 

 can he obtained by aging just the largest tew fish. 

 This does not preclude aging the rest of the sample at 

 a later date. 



2) In. some cases, the number of age determinations 

 must be kept to a minimum because of time or cost 

 constraints; e.g., determining the ages of tropical 

 fishes may necessitate a tedious procedure of count- 

 ing daily growth rings (Brothers 1980; Brothers et 

 al. 1976). 



3) The procedure can be used when the sample is 

 not representative of the population. This can occur 

 as the result of a particular sampling scheme or if the 



animals segregate by size. 



4) An interesting application of the method is to 

 cases where recruitment is highly variable. In an ex- 

 treme case, Goldspink (1981) reported finding only 

 three or four year classes in bream, Abramis brama, 

 in three English lakes even though the maximum age 

 found was '!'■] yr. A maximum age of 23 yr would nor- 

 mally indicate a mortality rate of 0.19. However, this 

 is likely to be a maximal estimate, since older fish 

 might have been found if there had happened to be a 

 strong year class in an earlier year. 



5) The technique can also be used to obtain a rough 

 estimate of the mortality rate when the maximum age 



900 



