136 



PROBLEMS OF LAKE BIOLOGY 



of populations of certain species of fish in 

 small bodies of water. The lake trout is one 

 of these species. In Ontario and probably 

 elsewhere small lakes of about 30 acres 

 which contain lake trout usually harbor 

 a small race of the species with a high 

 population density. Occasionally such a 

 lake will be found to contain large trout 

 but then the density will be extremely low. 

 It is easy to believe that in such lakes the 

 total lunuber of trout may fall to so low a 

 figure that some accident could wipe out the 

 race. Thus there may be a selection mecha- 

 nism at work which prevents the mainte- 

 nance of lake trout in small lakes in which 

 conditions favor their growth, unless some 

 influx of lake trout from neighboring waters 

 could reseed the lake after such an accident. 

 Because of the complexity of the relation- 

 ship which exists between food and feeder, 

 the relation between production of fish and 

 what may be termed "basic productivity" 

 is equally complex. With all other condi- 

 tions favorable, a limit to the production of 

 fish would finally be set by the "basic pro- 

 ductivity. ' ' In practice, however, this limit 

 may not be even closely approached. Even 

 in the ocean where conditions are more uni- 

 form, the communities have not evolved to 

 a point where food is always fully utilized. 

 The fluctuations in the size of year classes, 

 and the effect of these fluctuations on the 

 density of the standing population, prove 

 that food is not always the limiting factor. 

 Dominant year classes also occur in fresh 

 water (Hart 1932, Tester 1932, Hile 1936). 

 Although the phenomenon of dominant year 

 classes is evidence against the belief that 

 food always controls the production of fish, 

 in times of maximum abundance it may 

 become the limiting factor. This is well 

 shown by the work of Foerster (Ricker 

 1937 ) on the sockeye salmon in Cultus Lake. 

 Foerster found that when the number of 

 seaward migrants was 1.5 millions or more, 

 the size of the individuals was considerably 

 below that attained in years for which the 

 number of migrants was a million or less. 

 Ricker (1937) calculates from Foerster 's 

 data that the density of sockeye fingerlings 

 in Cultus Lake in 1932 was great enough 



to bring about an appreciable reduction in 

 the entomostracan food supiDly. 



The variation in average size and popula- 

 tion density which is found in lake trout 

 populations shows the complexity of the 

 relations between various factors of growth, 

 reproduction and predation. The average 

 size of lake trout taken by anglers in a 

 number of Algonquin Park lakes and the 

 relative abundance of these fish as expressed 

 by the number of fish captured per 100 boat 

 hours of fishing effort, are plotted in Fig. 2. 

 There are wide variations in the ease with 

 which lake trout can be captured which are 

 related to the size of the fish that are taken, 

 but the relationship is not a simple propor- 

 tionality. Lakes in Algonquin Park in 

 which all the lake trout are 13 inches or 

 less in length offer practically no fishing at 

 all. Lakes in which the average length of 

 the trout captured lies between 14 and 18 

 inches may contain either an abundance of 

 fish of catchable size or practically none. 

 Lakes in which the average length is greater 

 than 18 inches contain moderate numbers of 

 larger fish ; the number taken per unit effort 

 being inversely proportional to the average 

 length of the fish captured. 



This relationship is apparently the out- 

 come of the interaction of at least three 

 factors: (1) the effect of predation by man, 

 (2) variations in the size at which lake trout 

 reach maturity, and (3) the influence of the 

 presence of large trout upon the density of 

 their own populations. It is only the second 

 of these factors which is directly related to 

 food and feeding. Since the tackle used in 

 fishing for lake trout is not efficient for 

 taking them smaller than about 14 inches, 

 trout smaller than that do not enter into 

 the production as far as man is concerned, 

 although there may be large numbers of 

 them present. Among the poiDulations of 

 lake trout in different lakes in Algonquin 

 Park there are some in which the females 

 are only 13 or 14 inches long when they 

 reach maturity, others in which the females 

 are 18 inches long before maturity, and 

 others which lie between these extremes. 

 Thus populations producing fish with an 

 average length between 14 and 18 inches 



