HIGHER ANIMALS IN ECONOMY OF LAKES 



133 



within tlieir range. Unfavorable conditions 

 can be avoided by vertical mig'i-ations wliicli 

 take tlie members of a species into strata 

 where life is possible for them. When mi- 

 g'ration is impossible or of no avail, seasons 

 when complete activity cannot be main- 

 tained may be spent in quiescence or in 

 fasting. 



In this paper the interaction of the inani- 

 mate environment and the animate com- 

 munity of lakes on the welfare of the fishes 

 to be found in them, is illustrated by a short 

 discussion of some effects of temperature on 

 the feeding- of fislh The relation of these 

 effects on feeding activity to the problem 

 of production of fish are considered. In 

 conclusion the methods of attack on this 

 problem that appear to have been most suc- 

 cessful are mentioned and the part each may 

 possibly play in the future are tentatively 

 assessed. 



Fish occupy various places in the food 

 cycle which are often more than once re- 

 moved from the primary plant food. In 

 general each species has certain morpholog- 

 ical characters which are related to the na- 

 ture of its food, thus fish living to a large 

 extent on bottom food possess inferior 

 mouths ; species living on plankton, slender 

 and more numerous gillrakers ; species liv- 

 ing on other fish, large mouths and sharp 

 teeth ; but these relationships are not rigid. 

 Anglers casting a fly for trout occasionally 

 find that their efforts are rewarded by the 

 rising of a sucker. The cisco, although it 

 has gillrakers which would class it as a typi- 

 cal plankton feeder, by no means confines 

 its diet to the micro-crustacea. The lake 

 trout, whose large mouth and sharp teeth 

 are typical of an actively predaceous fish, 

 in some lakes feeds almost exclusively on the 

 larger i~)lankton, Crustacea and other small 

 aquatic invertebrates such as Coreihra. 



On the other hand fish are by no means 

 indifferent to attributes of their food other 

 than its size. Certain species, and indeed 

 certain individuals, may show a marked 

 preference for a particular food organism, 

 although other organisms may be equally or 

 more easily available. Pearse and Achten- 

 berg (1920) record the specific preferences 



of six different species of fish in Lake 

 Wingi-a. Plankton feeders do not always 

 i-emove their food from the water by ran- 

 dom straining (Battle et al. 11)30; Wagler 

 1937; Langford 1938). Ricker (1937) re- 

 lates that when a number of fish of the same 

 species, Oncorhynchus nerka, were confined 

 in a small space, certain individuals selected 

 Cyclops from the plankton present in con- 

 trast to the Dnphnia which was more 

 favored by their fellow prisoners. Thus 

 while there may be considerable latitude in 

 their choice, fish select preferred foods from 

 those offered within their range. 



To be utilized, food must not only be of 

 suitable size and quality but it must also be 

 available. This may not be the case even 

 when the fish and the food organisms are 

 present in the same lake. Langford (1938) 

 shows that during August, the cisco in Lake 

 Nipissing feeds on Mysis to a much greater 

 extent during the day than it does at night. 

 Since Mysis migrates upward at night while 

 the cisco does not, this diurnal variation of 

 the presence of Mysis in stomachs is cpiite 

 understandable. A second illustration of 

 the effect of spatial separation of food and 

 feeder is offered by the seasonal and annual 

 variation of the amount of perch in the 

 stomachs of lake trout from Lake Opeongo, 

 which is shown in Fig. 1. The number of 

 perch eaten by the lake trout in Opeongo 

 in earl}' summer is largely dependent upon 

 the sharpness of the thermocline. In June 

 of 1937, the amount of perch in the stomachs 

 of Opeongo trout was negligible, while in 

 1938 they were the most important article 

 in the diet. Langford and Martin (unpub- 

 lished) have shown that perch are distrib- 

 uted downward to strata liaving a tempera- 

 ture of about 15° C in that lake, and it is 

 probable that lake trout and perch overlap 

 in their distribution over the temperature 

 range of at least 11-17° C in summer. 

 During June this temperature range covered 

 a far greater depth of water in 1938 than 

 it did in 1937. In late July and August 

 the difference in sharpness of stratification 

 is not so marked. 



"While most species of fish do not make 

 such extensive vertical migrations in search 



