A STUDY ON TROUT AND YOUNG SALMON. 291 



If we examine the groups in table VII last column, repre- 

 senting fishes from the Mandal river, it is obvious how well 

 this table agrees with those of the two northern rivers, the 

 upper group representing fishes, which are going to emigrate 

 next spring, the lower one, fishes who left the river for the first 

 time during spring 1901, returning during autumn. 



As regards the growth of the trout after it has left the river 

 a second time as „isfisk", these tables do not give exhaustive 

 information. The only fact which, upon this point is clearly 

 established by the tables, is that a definite grouping of the growth 

 after the second emigration ceases to be perceptible. Thus the 

 further growth of the fish must undoubtedly be very irregular 

 and highly varying in different individuals. 



This fact is obvious if we look at column 1 table VII, 

 Beitstadfiord May 1900, a table representing the sizes of a 

 large number of fishes caught during a short space of time in 

 one locality. The fact also is in good accordance with the 

 experience gained by Mr. A. Landmark, inspector of fisheries, in 

 experiments with the labelling of trout of similar sizes. 



This irregularity of growth, which is however observed in 

 most fishes as they grow older, may to some extent be con- 

 nected with the fact, that the trout in these stages becomes 

 more wandering and thus encounters considerably differing con- 

 ditions of nutrition. 



Also the fact, that the larger proportion of the trout attain 

 sexual maturity only after the second emigration, may to some 

 extent account for the subsequent irregularity of growth. 



As will be observed in the tables, where fishes, sexually 

 mature, are denoted by a 9, the group immigrating in autumn 

 very rarely contains mature fishes. Thus, a careful exami- 

 nation of more than 150 trout caught in the mouth of the Orkla, 

 October 23, 1898 (vide table VI) yielded only one mature fish 

 below 30 cm. length, while not a few above 30 cm. had not 

 vet attained maturity. 



