experiment is the most direct method, but is 

 costly. On economic grounds the possibilities 

 of other methods, such as certain immunological 

 techniques, should be examined first. 



I have also included a tagging experiment 

 design which will be of use in sardine studies 

 and, perhaps, be of broader application . 



Snieszko has taken a somewhat novel 

 approach to the problem of defining subpopula- 

 tions and has done so on the basis of differences 

 in disease resistance and susceptibility. He 

 considered experimentally the effects, in terms 

 of mortalities, of furunculosis and/or ulcer 

 disease on brook trout from different localities. 

 Some of the infections occurred "spontaneously" 

 and some were induced. Trout from two local- 

 ities showed very high, although not identical, 

 mortalities, while trout from a third locality 

 showed very low mortalities. 



The implications of these studies extended 

 to the field are obvious and a broader application 

 of this experimental approach would be of great 

 interest. The explanation for some anomalies in 

 fish distribution may, in fact, be found in an 

 earlier work of Snieszko' s in which he showed 

 that disease resistance in fish (antibody forma- 

 tion) varied with temperature . 



An interesting and important problem, 

 but one perhaps not strictly pertinent here, is 

 an evaluation of the importance of pathogens as 

 a source of fish mortality in nature; especially 

 of marine fishes. 



Finally, in this collection, Eschmeyer 

 has called attention to a problem that has re- 

 ceived little attention; namely, the possible 

 existence of subpopulations of lake trout in the 

 Great Lakes. The evidence which suggests that 

 there are subpopulations of lake trout includes 

 differences in spawning time, spawning substrate, 

 spawning depth, size at maturity and fat content. 

 Also, tagging evidence indicates the existence of 

 a homing tendency. Study of this problem may 

 be particularly urgent in view of predation upon 

 lake trout by the lamprey. 



CONCLUSIONS 



The preparation of this collection, inso- 

 far as I have been responsible for it, has been 

 an extremely instructive process. To the extent 

 that the papers in this collection are representa- 

 tive of the studies now being made of subpopula- 

 tions, some generalizations about the present 

 status of such studies may be made. 



First, there appears to be a reasonably 

 widespread realization that the classical 

 meristic-morphometric approach alone will not 

 lead to a satisfactory and conclusive solution of 

 this problem. 



Second, and perhaps as a corollary of 

 the first, the genetic implications of this prob- 

 lem are coming to be more generally realized 

 and accepted. 



Third, there seems to be more effort 

 devoted to developing new and more satisfactory 

 tools and to borrowing and adapting pertinent 

 tools from other fields . 



Fourth, the desirability of a direct 

 attack (tagging) on this problem is being recog- 

 nized by more and more individuals . 



Finally, and perhaps to a lesser extent, 

 the applicability of the experimental method to 

 problems in the field is being appreciated. 



If these generalizations are valid, we 

 may look forward to an increased rate of pro- 

 gress and understanding in this field. 



