J. L. BROOKS 119 



detectible are those between species with widely divergent gill- 

 raker counts. Many of these aberrant individuals owe their inter- 

 mediate appearance to the phenomenon of "shoal-trapping." This 

 term refers to the circumstance in which an individual becomes 

 separated from the shoal of similar-sized fish of its own species, 

 with which it would normally swim, and joins a shoal of a differ- 

 ent species. A shoal-trapped fish is thus exposed to the range of 

 environmental conditions proper to the alien species comprising 

 the shoal. Because of the extreme sensitivity of the develop- 

 mental processes in the whitefish to environmental conditions, 

 the shoal-trapped individual develops a phenotype determined 

 genetically by one species, but so modified by the environment 

 characteristic of the alien species that it strongly resembles the 

 species that it has inadvertently adopted. It is only when the 

 species involved have very different numbers of gillrakers that 

 the genetic stock of the shoal-trapped individual can be identi- 

 fied. Furthermore, it is only when the trapping has occurred 

 fairly late in the life history that the foreign fish is sufficiently 

 different from its shoal-mates to attract the attention of an ob- 

 server. If the trapping occurs in the fry stage, the phenotype of 

 the trapped fish will be little different from that of its adopted 

 species, so little different that it will very likely escape the atten- 

 tion even of a trained observer. 



Not only are shoal-trapped fish difficult to distinguish from 

 hybrids, but they are likely to increase the production of hybrids 

 because such a trapped fish is likely to spawn along with the rest 

 of the shoal. The resultant mixture of gametes is likely to produce 

 hybrids, if we can judge from the high yield in cross fertilizations 

 between species of the Coregonus hvaretus group. Although Fi 

 hybrids are readily produced in artificial crosses, it has not yet 

 been established whether F2 hybrids and backcrosses are pro- 

 duced with proportionate facility. The detected natural hybrids 

 are rare and could be almost entirely of the first generation. The 

 progeny from a backcross would probably be extremely difficult 

 to detect in nature. However, if they did occur, even with a very 

 low frequency, they might account for the introgression of 

 genetic material from one species to another. Svardson remarks 



