certain anatomical adaptations of fish. Fish feeding 

 on bottom matter have soft-lip]ied sucking mouths ; 

 fisii feeding on plankton have numerous slender gill- 

 rakers : fish feeding on other fish have large mouths 

 and sharp teeth. The adults of some fish, such as the 

 cisco. gizzard shad, paddlefish, and sunfish, consume 

 large quantities of plankton. The gizzard shad also 

 feeds on bottom mud, straining organic particles out 

 of it and grinding them up in a stomach that re- 

 sembles the gizzard of a chicken. Sturgeon, white- 

 fish, buffalo fish, carp, catfish, bullheads, suckers, 

 sunfish, and many others feed largely on bottom 

 annelids, insect larvae, mollusks, and vegetation in 

 shallow waters. As many as 354 midge fly larvae 

 have been found in a single whitefish stomach: 331 

 were found in a sturgeon stomach (Adamstone and 

 Harkness 1923). Bass, crappies, perch, pike, gar, 

 and lake trout feed principally on otiier fish. The 

 bottom feeders scoop up the bottom ooze indiscrim- 

 inately; several forms maintain contact with the bot- 

 tom by means of sensitive barbels hanging from the 

 chin, but plankton-feeders and carnivorous species de- 

 pend largely on sight for seizing individual prey. 



Young fish of many species live largely on plank- 

 ton, even though as adults they feed on something 

 quite different. A 10-centimeter perch requires 130 

 mg dry weight of food per day during the summer, the 

 equivalent of about 37,300 Cyclops. The perch would 

 have to consume Cyclops at a rate of 26 per minute 

 throughout the day in order to ingest such a total. 

 A 20-centimeter perch would require 600,000 Cyclops 

 per day, ingested at a rate of 417 per minute, which 

 is doubtless beyond its efficiency of intake. By con- 

 suming only four small fish 0.3 g dry weight each, 

 the perch could obtain the same energy intake ( Allen 

 1935). 



Most lake-inhabiting birds subsist mainly on fish, 

 diving for their food. Gulls take only dead fish, which 

 they find floating on the surface or washed up on the 

 shore. Swallows skimming over the water surface 

 consume enoromus numbers of emerging adult midge 

 flies and other insects. 



BIOMASS A.\D PRODUCTIVITY 



The dry weight of total organic matter of 

 seston in 329 fresh-water lakes was found to range 

 from 0.23 to 12.0 mg/I with an average of about 

 1.36 mg/1 (Birge and Juday 1934). Of this, living 

 plankton organisms constituted an amount ranging 

 from 20 to 80 per cent. The biomass of green phyto- 

 plankton is usually, but not always, greater than the 

 zooplankton. The biomass of net plankton may be 

 only one-third to one-tenth of the total net and nan- 

 noplankton. Net plankton is generally more abundant 

 in hard water than in soft water, more abundant in 



SIZE OF FISH IN INCHES 

 3 4 5 6 7 



10 15 20 



SIZE OF FISH IN CENTIMETERS 



FIG. 6-M Change 

 crease In size (afte 



3d habifs of perch 

 I 1935). 



eutrophic lakes than oligotrophic lakes (Rawson 

 1953). The dry weight of net plankton during the 

 summer in 18 lakes of western Canada and in 2 

 lakes of Wisconsin varied from 0.9 to 17.7 mg/1, and 

 averaged 5.0 mg/m- of water surface area (Rawson 

 1955). 



The biomass of benthos varies with the nature 

 of the bottom, amount of vegetation, and depth. 

 When computed for the total bottom of 10 Canadian 

 lakes exceeding 1 1 m in depth, it was found to vary 

 from 0.07 to 2.47 g/m-, and average 0.63 g/m- dry 

 weight, not counting the shells of mollusks (Rawson 

 1955). The mean of 36 lakes in Connecticut ranging 

 in depth from 1.1 to 11.1 m varied from 1.09 to 

 34.8 g/m'-, and averaged 7.5 g/m- (Deevey 1941). 

 The bottom fauna of various European lakes has been 

 found to range from 0.69 to 5.65 g/m-. 



When lakes of different depths are analyzed, it is 

 found that the mean biomass of both net plankton and 

 benthos exist in inverse relation with mean lake depth 

 (Table 6-3). This may indicate that the morpho- 

 nietric characteristics of a lake affect its carrying 

 capacity, a factor additional to those of dissolved salt 

 content, oxygen content, and temperature. 



The biomass of plankton is generally greater than 

 the biomass of benthos. In addition to the five Cana- 

 dian lakes listed in Table 6-3, Deevey (1940) found 

 the ratio between plankton and benthos in five other 

 lakes likewise to vary from 3.8:1 to 10.0:1. In one 

 eutrophic lake in Michigan, the standing biomass of 



Lakes 75 



