TJic Produclk'Uy of Fish Food in Oucido Lake 211 



here. These are omnivorous or herbivorous fish. Fish that 

 are strictly j^redators often have empty canals. This we would 

 expect on account of the ".greater difficulty of getting- food. 

 I have no data on the frecjuency with which a fish fills its 

 digestive canal or the amount it eats in a day. I doubt if any 

 rule applying to fish generally can be found. Some fish like 

 perch and some catfish, at least, appear to feed at all times of 

 the year, and others like smifish ai)pear to take little food in 

 the winter." 



F.xperiments on the rate of digestion in fish and other cold- 

 blooded vertebrate animals (r/. Riddle, '09, piJ. 450-457) show 

 that in fish (Aniiahis calznis Linn.) the rate of digestion is 

 highest, and in turtles (Fniydoidca) digestion is the lowest. 

 Temperature is shown to modify the rate of digestion in a 

 marked degree. In .iniiafiis only a trace of digestion took 

 place during 168 hours at a temperature of 2.5 C, 36.5 F. 

 (November) w^hile at a temperature of 29.8 C, 85.6 F. 

 (April) a considerable amount of food was digested in 48 

 hours (p. 450). The influence of season, which is mainly the 

 influence of temperature, is shown to be marked, the digestive 

 powers being reduced by about one-third between November 

 and March. Riddle states ('09, p. 454) : " Midwinter to 

 March is the period when digestion capacities are at the lowest 

 point; while midsummer (July) is the season most favorable 

 to digestion. It thus appears that the period of low digestion 

 power falls wnthin the fasting period of these animals, and the 

 maximum of digestion power is attained during those months 

 when a maximum of feeding is the rule." Fish are able to 

 modify the efi^ect of temperature to some extent in the winter 

 by migrating from shallow^ cool waters to deeper w^armer water, 

 but this probably does not affect the rate of digestion to a 

 large degree. 



During the 191 7 field season (August to October) many 

 fish were caught in a trap net for the purpose of making 

 studies relative to their infection by parasite worms, and inci- 

 dentally a number of these fish were examined as to the food 

 contents of their digestive organs. The trap net was emptied 

 after intervals of 24, 48 and 72 hours. Upwards of seventy 



