discretely for each stock, little if anything will 

 be learned about subpopulations and, in fact, if 

 the concepts of stock and subpopulation are con- 

 fused the investigator may be very seriously 

 misled. On the other hand, the characteristics 

 of a stock may well serve as natural tags and be 

 of great value in the study of availability phenom- 

 ena . It is a fair generality to state that whereas 

 most fishery biologists have been interested in 

 defining subpopulations, they have, in fact, most 

 frequently idefined stocks or groups. This may 

 be avoided only by using characteristics Known 

 to be genotypic or by using a more direct ex- 

 perimental method such as tagging. 



METHODS 



A number of diverse methods have been, 

 or could be, used in subpopulation studies. Any 

 classification or listing of the various methods 

 must, perforce, be somewhat arbitrary rather 

 than natural, since many of the categories over- 

 lap. The order in the following listing is 

 suggested somewhat, but not entirely, by the 

 historical order of their development. 



I. Anatomical studies . 



Various anatomical characteristics have 

 frequently been used in an effort to distinguish 

 subpopulations, much as they have been used to 

 distinguish subspecies, species and higher cate- 

 gories. The types of characters used may be 

 included in the following groups: 



A. Morphometric: These include (1) 

 external features such as the proportions of 

 various body parts, (2) internal features such 

 as the proportions of the various parts of the 

 viscera and (3) cytological features such as the 

 structure of various cells. 



B. Meristic: These include (1) external 

 features such as the numbers of fin rays and 

 spines and (2) internal features such as the num- 

 bers of vertebrae and gill rakers. 



C . Presence or absence of a morpho- 

 logical structure: The presence or absence of a 

 particular structure such as teeth or pseudo- 

 branchiae may sometimes be diagnostic . 



The use of anatomical characteristics in 

 defining and describing subpopulations has at 

 least two limitations. First, it is well known 

 from both empirical and experimental evidence 

 that body form, numbers of vertebrae, etc., 

 are influenced by environmental variables (such 

 as food and temperature, for example) . Ob- 

 viously, then, in using such characteristics the 

 risk exists of studying the effects of environ- 

 mental conditions rather than the effects of 

 genetic isolation. Second, even if the character- 

 istics used are genotypic, their frequency 

 distributions generally overlap, often to a large 

 degree. If significant differences are found be- 

 tween the samples, these have usually been 

 interpreted as indicating that the two samples 

 were drawn from two distinct subpopulations. 

 Actually, this indicates only that they were not 

 both drawn from the identical population; the 

 possibility of considerable intermixing still 

 exists. 



II . Tagging or marking experiments . 



Tagging or marning experiments have 

 usually been carried out in order to learn about 

 migrations or to maice estimates of population 

 size. They can also be used to learn something 

 about subpopulations, provided that the tagging 

 and recapturing are done on the spawning grounds. 



III. Physiological characteristics. 



Subpopulations may have distinctive 

 physiological as well as anatomical characteris- 

 tics. These might include, for example, inherent 

 growth characteristics. 



IV. Life history characteristics. 



Differences in life histories may also be 

 indicative of genetic differences between sub- 

 populations. Such differences might include, 

 for example, different spawning seasons or dif- 

 ferent temperature requirements at spawning. 



V. Biochemical studies. 



Recent advances in other fields are now 

 being applied to fishery problems . They include: 



