drifts, almost a thousand fishes and some other creatures 

 were swimming within sight. Many of the same species 

 are present in both the Hawaiian and equatorial localities. 

 The length of these creatures ranged from a few inches to 

 several feet. The observations on these drifts constitute 

 the most exhaustive study yet made on an underwater 

 community in the open sea. 



These raft studies and those described earlier have dealt 

 with individual fish or at the most a few thousand, the 

 school. Between the school and the millions of fish that 

 make up the total population of a single species lies anothei- 

 natural unit of study, the subpopulation, a group of geneti- 

 cally related fishes. Its dimensions are as yet unknown, 

 other than that they must stand somewhere between the few 

 thousands of a school and the many millions of a species. 

 The relatively recent application to fisheries of techniques 

 of genetic research that have proved their value in agricul- 

 ture and animal husbandry, as well as in the treatment of 

 human illness, is now providing a powerful tool for under- 

 standing some of the fundamental jjroblems of fishery 

 utilization. 



Blood Kin 



The water that Hows through a tap may have come from 

 a single reservoir or from one fed by half a dozen streams. 

 In a like manner, the reservoir of a species of fish that is 

 sampled by a fishery may consist of groups from many 

 sources. Such isolated, interbreeding stocks of fishes (or 

 other animals) are called subpopulations. Many lines of 

 evidence have strongly indicated that fished populations are 

 not as a rule homogeneous, like water from a reservoir fed 

 by a single stream, but consist of several subpopulations 

 that breed separately in different places and perhaps at 

 different times of the year. 



Evidence for the e.xistence of subpopulations has come 

 from morphometries, the study of the physical character- 

 istics of the fish, from mathematical treatment of statistics 



■"//«. 



i^-T^ 



FKfI KK 9. >\ hitelip sharks (Carcharbinus tttuffhiKtiius) 

 photugraphrfl from lh<* rafi .\*>fiu<'. Th*- >lript-(l fish**» a^<^ 

 pil4>tfi>li<'^ ( ytiucrntfs tluctor) thai arronipaiiv iUv sharks. 

 \ i>ibl> altai'hfil Ui ill** sharks ar«* surkcriish. Rentttra 

 rt'itiarn. Sharks fm|uenll> approa*-h«-cl thf .\**iim#'. a^ <lifl 

 a scorr <»f ijthcr (ishfs. 



on the catch, and from tagging studies. Morphometries 

 offer many interesting clues to the existence of subpopula- 

 tions but have the shortcoming of dealing with character- 

 istics that can be drastically modified by the environment; 

 size is an example, for fishes of the same sjiecies can grow 

 at different rates in waters of different temperatures, and 

 these differential rates have not been shown to he linked to 

 inheritance. 



Catch statistics, again, provide inconclusive evidence. Arid 

 tagging studies, which usually are made for estimates of 

 population size and mortality rates, rather than for sub- 

 jxiijulation research alone, are both slow and expensive. 



11 



