5g U. S. BUREAU OF FISHERIES 



A better knowledoje of the phenomena of growth whether of the 

 entire body of the fish or of the component parts and their composi- 

 tion, is essential for a science of fish culture. For this reason four 

 species of trout — rainbow, brook, lake and brown — have been under 

 constant observation for 5 years. These four groups have been fed 

 the same diet and kept under similar conditions in regard to the 

 water supply. Growth curves for the four species under these con- 

 ditions have been very similar. Increases in body weight have con- 

 tinued throughout this period but this growth rate has declined with 

 succeeding years. This indicates that growth is determinate in fish 

 although the cessation of increase in the size of the body is in a much 

 later period of the life span than it is in higlier mammals. 



In the course of this long-time experiment certain species differ- 

 ences have been observed. All except the lake trout spawned at a 

 normal age but this species did not spawn until September of 1937 

 when nearly 5 years of age. As the groups have become older there 

 has been a tendency to develop ulcer disease and for individuals to 

 die at about the spawning period in all groups except the lake trout. 

 All of the brook trout have now died. The males of this group died 

 before the females. The eggs from all groups have been consistently 

 inferior and smaller than those from the same species held in ponds. 



Tlie question of the rate at which an animal should grow in order 

 to develop the strongest body possible is one that is attracting in- 

 creasing consideration in many fields of animal nutrition. No one 

 desires to pusli the growth of an animal to the extent that he pro- 

 duces weaklings. One of the most difficult problems in the field is 

 to determine what constitutes a weak animal in terms of the organs 

 of the body and to detect weak organs that are unable to share in 

 the growth of the body when that growtli is unduly accelerated. 

 For the above reason studies have been made of the rates of growth 

 of the organs of the bodies of four trout species during the fingerling 

 stage. 



In this study the weights of the eyes, livers, hearts, and gastro- 

 intestinal tracts of trout were determined on April 16, July 9, and 

 October 1. Part of the trout had been retarded in growth and part 

 made to grow as rapidly as possible under our conditions. It has 

 already been demonstrated by investigators working with other spe- 

 cies that the organs of trout grow at unequal rates in comparison 

 with the whole body. In cases of limited available foodstuffs certain 

 organs, such as the eyes, are able to seize a dispro])ortionate share 

 compared to organs such as the heart. Thus, in retarded trout, the 

 eyes grow more than the other organs and tend to represent a greater 

 percent of the entire body weight. Organs such as the heart, liver, 

 and the gastrointestinal tract represent about the same percent of 

 the entire body weight whether the trout has been retarded in growth 

 or not. In last analysis these organs and their composition probably 

 determine wliether or not the trout that are planted in the streams 

 survive to greet the fisherman or die from their own inherent weak- 

 nesses. The tendency of the eyes to become a larger percent of the 

 entire body weight may ultimately prove of some use as additional 

 measurements of "condition factors." 



As an additional part of the program to extend the science and 

 to determine more about the relative hardiness of trout that are 

 produced in hatcheries, the Cortland Station has continued its study 



