ON THE COMPETITION BETWEEN WHITEFISH SPECIES 



Thorolf Lindstrom and Nils-Arvid Nilsson* 



Sotvattenslaboratoriet, Drottningholm, Sweden 



INTRODUCTION 



The concept of interspecific competition has been defined in many ways. 

 For the present purpose the following definition is adopted: competition is 

 every interaction between species populations that adversely affects natality, 

 survival and/or individual growth in these populations. This reading involves 

 slight modifications from the expositions of Darwinian competition given 

 by Elton (1946), Crombie (1947), Solomon (1949), Odum (1953), Nicholson 

 (1954), Park (1954), and others. Competition also means that the general 

 ecological features of the species are affected as ecological 'optima' move 

 apart when species compete (for this modified meaning of 'optimum* cf. 

 Nilsson, 1956 quoted by Kalleberg, 1958). 



There are many old and often repeated experiences in fisheries that makes 

 the fisheries biologist inclined to beHeve in the existence both of intraspecific 

 and interspecific competition. Such experiences are the relation between fish 

 density and growth, and the displacements affecting existing populations in 

 a water as a new species is introduced or as fishing concentrates on some of 

 the populations. More recently it has proved possible to introduce deHcate 

 species by poisoning the old populations. Carlander (1955) has shown that 

 the standing crop of some species is lower if other species on the same food 

 level exist in the lake. Most of these demonstrations are hampered by the 

 difficulties of ruling out the effects of changes in the environment other 

 than the changes in the fish fauna and the difficulties of getting a good 

 measure of 'the adverse effect' on all the competing fish populations. 



The recent advances in the study of fish behaviour have revealed that 

 many species of fish defend territories also out of the spawning time. This 

 population regulating mechanism leads to a competition that is both intra- 

 and interspecific and also influences individual growth (Newman, 1956; 

 Kalleberg, 1958). Also in schooling fish species such as herring there exist 



* The material will be more exhaustively presented elsewhere. The material concerning the 

 first-year biology is mainly treated by Lindstrom and that concerning the adults by Nilsson. 

 We acknowledge with thanks the assistance of Mr. I. Sasserson in working up some material 

 on stomach contents. 



