Balance 125 



because of a failure in the perch crop in 1949 and 1950 which in turn was 

 believed to have been related to a shortage of aquatic vegetation during 

 those years.-^ 



In this same lake a very large yield of northern pike occurred in 1954, 

 as a result of the transference in 1953 of some 17,000 young pike 10 to 

 16 inches long into Clear Lake from Ventura Marsh lying adjacent to the 

 lake. This amounted to 5 pike per acre, or by the spring of 1954, to 7 

 pounds of pike per acre— a rather abrupt increase of at least 10 per cent 

 in the predator population of the lake. Tlie pike caught were thin and 

 later in the summer some were found dead along the shore.-^ 



Still another type of interplay of reproduction, competition, and preda- 

 tion results in a progressive increase in one or two species of fishes until 

 they become so numerous as to exceed the normal food resources for these 

 species in their habitat. These abundant species spill over their habitat 

 niches into those of other less aggressive species and crowd them suf- 

 ficiently for food and space to prevent the survival of adequate young to 

 maintain a level of population of the less aggressive fishes. It follows that 

 the latter species eventually may be represented by a few old fish, and 

 they may disappear entirely. This type of population change is non- 

 reversible and is characteristic of fish populations subjected to limited or 

 ineffectual predation. Only catastrophic changes in the habitat will modify 

 the overpopulated and stunted condition of the dominant fishes. No known 

 instances of over-use of habitat resources, followed by population col- 

 lapses, such as are cited by Errington -^ for overpopulations of deer, 

 muskrats, and some other mammals, have been reported for fishes, al- 

 though diseases or parasites sometimes wipe out or severely reduce over- 

 populations of fishes that are characteristic of hatchery ponds before fish 

 distribution is begun. 



BALANCE 



Balance is a term used by some biologists to describe natural fluctu- 

 ations of animal populations around a constant numerical level. Other 

 biologists have expressed the opinion that the term is inappropriate ^^ 

 because balance refers to a state of equipoise and is synonymous with 

 equilibrium. Nicholson ^^ believes that "balance refers to the state of a 

 system capable of effective compensatory reaction to the disturbing forces 

 which operate upon it, such reaction maintaining the system in being." 



Others, for example Swingle,"^"^ ''^ use the term balance to define fish 

 populations that yield satisfactory crops of harvestable fish in relation to 

 the basic fertilities of the bodies of water containing these fish. According 

 to Swingle, fish in a balanced population ( 1 ) must reproduce periodically, 

 (2) must produce a sustained yield (presumably by angling), and (3) 



