HIGHER ANIMALS IN ECONOMY OF LAKES 



135 



which the ox^-gen was too low to support 

 tliein for any length of time. A number of 

 fish make nocturnal excursions into shal- 

 lower water and feed there. 



Further, even though food and feeder 

 may be present in the same stratum there 

 may be physiological causes which affect the 

 utilization of food. The general effect of 

 temperature on metabolic rates and the 

 familiar annual cycle of growth are too well 

 known to require discussion here, but must 

 always be taken into consideration. Fish 

 do not feed over the whole range of tem- 

 perature within which they can survive. 

 Pearse and Achtenberg (1920) state that 

 the perch in Lake Wingra take verj^ little 

 food during the warmest period of the sum- 

 mer. Mackenzie (1934) and Blair (1938) 

 found that there were upper temperature 

 limits for the cod and the salmon beyond 

 which feeding decreased markedly. When 

 feeding has once been inhibited by too high 

 a temperature the fish may not start to take 

 food again innnediately upon entering more 

 favorable temperature conditions. In an 

 experiment performed witli salmon parr 

 (Fry and Kennedy unpublished) the water 

 in which they were living was warmed 

 gradually from 18° C to 28° C over a period 

 of 20 days. At 28° C the parr stopped feed- 

 ing, and, although the water temperature 

 was quickly reduced to 10° C and main- 

 tained at that low temperature, the fish did 

 not feed again until 15 days later. The 

 feeding behavior of the Lake Nipissing cis- 

 coes (Fry 1937) is probably an instance of 

 such an effect. In Lake Nipissing, ciscoes 

 remain in the epilimnion until their upper 

 feeding temperature is passed. On ceasing 

 to feed they migrate clown into the hypo- 

 limnion where they remain until early 

 autumn. Throughout their stay in the 

 hypolimnion a large proportion of them do 

 not feed at all. Further, the group which 

 does feed is not a chance section of the 

 population but is made up of individuals 

 which migrated early ; these are for the 

 most part the larger aud older members of 

 the population. Angling statistics for both 

 the lake trout (Fry and Kennedy 1937) 

 and the rainbow trout (Mottley 1937) give 



evidence that these fish behave similarly to 

 the Cisco in respect to their summer feeding 

 activity. 



From a consideration of circumstances 

 which affect the utilization of food, such as 

 the examples given above, it will be realized 

 that variations in growth are influenced now 

 by one factor and now by another. In a 

 discussion of the growth of ciscoes in cer- 

 tain Wisconsin lakes, Hile (1936) states that 

 there is a direct correlation between growth 

 in weight and the length of the growing 

 season, but there is no relation between 

 abundance of food and the rate of growtli 

 of ciscoes. He makes the following state- 

 ment, ". . . it is quite probable that the 

 cisco does not thrive in the ph3'sical and 

 chemical conditions most conducive to a 

 large population of food organisms ..." 

 Thus as Langford (1938) points out, poor 

 growth may result on the one hand from 

 sparse food, and on the other from unfavor- 

 able physical and chemical conditions. 



Differences in feeding are often strikingly 

 reflected in fish populations by dwarfing, 

 although all dwarfing is not to be inter- 

 preted as being due merely to lack of food. 

 Certain types of dwarfing are physiological, 

 since dwarf and normal races often occur 

 in the same lake. Examples of this kind 

 are to be found in the smelt (Kendall 1927), 

 the common sucker (Deuce 1927), and the 

 northern sucker. Other cases of dwarfing 

 are more closely dependent on feeding con- 

 ditions. Sometimes too high a density of 

 the feeding population results in a dimin- 

 ished food supply for each individual. 

 Sometimes food suitable to allow rapid 

 growth is absent from a given lake, or if 

 present is spatially separated from the 

 feeder during the growing season. Finally, 

 as has been pointed out the fish may not feed 

 because of adverse physical and chemical 

 conditions, even though food is present. 

 AYliatever the cause, dwarfing is usually 

 associated with a high population density. 



Poor growth, of which dwarfing is the 

 extreme, need not be looked upon as an 

 unmixed evil, even from a strictly economic 

 point of view, for it appears to be respon- 

 sible to some degree for the maintenance 



