20 American Fisheries Society 



The special difficulties of the case have been overcome in 

 indirect ways. The statistical method was popularized by 

 C. G. J. Petersen towards the close of the last century. In this 

 method the population of a certain kind of fish in a given locality 

 is examined by taking samples at random and measuring these. 

 It is found that, if the sample be comprehensive enough, the 

 individuals fall naturally into groups according to size, the groups 

 of smaller size being most distinct. Seeing that the fish has a 

 definite spawning period, which is annual, each group is to be 

 considered as consisting of fish born in a certain year, that is, 

 belonging to a certain year-class. One group will consist of those 

 individuals that are in their first year, another of those in their 

 second year, and so on. In this way the average size at any given 

 age may be found. Not only this, but we may follow the growth 

 of any group by taking samples from time to time and determining 

 the average size of that group, from which data the average increase 

 per unit of time can readily be calculated. 



This method has disadvantages. It is often difficult or even 

 impossible to obtain comprehensive samples, owing to segregation 

 of different ages. Also, if the growth be slight, compared with the 

 amount of individual variation in size of those of the same year- 

 class, the groups overlap in size and cannot be distinguished. 

 The discovery of another method has been, therefore, of the 

 greatest value- 



Over three centuries ago, when the microscope was first 

 invented, Van Leeuwenhoek, the pioneer microscopist, living 

 in the eel-eating country of Holland, discovered not only that 

 eels have scales, but that their scales show rings, which he 

 interpreted as indicating years. Only at the beginning of the 

 present century was this discovery extended and used in the study 

 of the life history of fishes. 



The scales are not the only parts of the fish that exhibit such 

 annual markings. Similar lines to those of the scales can be seen 

 in the ear-stones and in the bones, particularly the vertebrae; but 

 in the majority of fishes the scales have proven to be the most 

 valuable of all the parts in this respect. It is, indeed, astonishing 

 for one to be able, as I was recently, to say on returning home and 

 noticing a fish scale adhering to the oilcloth of the kitchen table: 

 "You have been having a five-year old whitefish to eat lately." 



