Age Determination of Fishes (Revised) 



By 



FRED E. LUX 



National Marine Fisheries Service Biological Laboratory 

 Woods Hole, Massachusetts 02543 



Introduction 



Span of life in fishes, like size, covers an 

 extremely wide range, depending upon species. 

 A tiny European goby that matures at little 

 over an inch in length is an example of an 

 "annual" vertebrate, running the course of its 

 life within a single year. Other fishes are 

 known to pass the century mark. Canadian 

 biologists in 1953 determined the age of a 215- 

 pound sturgeon, caught by a Lake of the Woods 

 fisherman, to be 152 years. 



Aside from its value in filling gaps in our 

 scientific knowledge of fishes and satisfying 

 human curiosity concerning them, age and 

 accompanying growth-rate information is of 

 vital importance for the management of fishery 

 resources. For instance, where fish are caught 

 commercially, growth rate must be known in 

 order to learn the size and age at which the 

 fish may be most efficiently harvested. A 

 further use of age information is in judging 

 the results of management practices. Knowl- 

 edge of the average size and age of fish before 

 and after management measures are put into 

 efi'ect can sometimes show whether or not such 

 plans are achieving desired ends. 



Age Determination 



Three basic methods have been used for age 

 and growth detei'mination of fishes: (1) ob- 

 servation of the growth of fishes of known age, 

 (2) study of fish size-frequencies, and (3) 

 study of seasonal ring formation in hard body 

 parts such as scales and bones. The method 

 used usually depends upon special problems 

 encountered in age determination of a given 

 species. 



' This publication is a revision of Lux, Fred E., 

 Age Determination of Fishes, U.S. Fish Wild. Serv., 

 Fish. Leaflet No. 488. 



Observation of the Growth of Fish of 

 Knov^tn Age 



Fish of known age are held in a pond or 

 aquarium for a number of years so that length 

 for a given age may be determined by simply 

 catching and measuring the fish periodically. 

 While the method is direct, it has limited use 

 since it requires raising fish under artificial 

 conditions where growth rate may diff'er from 

 that in their normal surroundings and where 

 maintaining certain species may be difficult. 

 The technique probably has its greatest use 

 in verifying ages that have been determined 

 by other means that are discussed later. 



As an extension of this technique, fish of 

 known age may be marked and released in 

 their normal environment and then measured 

 when they are recaught to learn size for the 

 known age. Some small fish may be marked 

 by the removal of certain fins so they can be 

 identified upon capture. Larger fish usually 

 are marked with small numbered tags of plastic 

 or noncorrosive metal. A major drawback to 

 this method is that the tag or mark may slow 

 down growth, as has been shown for many 

 fishes. To use the method eff"ectively, one must 

 therefore either demonstrate that the tag has 

 no efi'ect or correct for, its effect. Neither of 

 these is easily accomplished. 



Study of Fish Size-Frequencies 



The size-frequency method of age determina- 

 tion depends on the reproductive and growth 

 characteristics of fish. Most fishes breed dur- 

 ing a restricted period and only once a year 

 so that sizes within a given brood year are 

 fairly uniform and distinct from sizes from 

 other brood years. Haddock, for example, 



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