The Prodiictirily of Fish Fond in Ouclda Lake 207 



mussels, lobsters, etc., with the ox, sheep, and \n^ is very 

 suggestive and interesting in this connection. When com- 

 l>ared in terms of tiie percentages of carbohydrates, fats, and 

 proteids, the two classes of foods (of the sea and of the land) 

 do not diifer greatly in comjxjsition. 



Annual Production of Inverti-:brates 



A large percentage of the invertebrate animals in Lower 

 South Bay are annuals, completing their life cycles in one 

 year. Such for example are the majority of the insects. In 

 some animals, as some of the small Cladocera (Needham, 'i6, 

 p. i86) there are many generations in a season, each genera- 

 tion living but a comparatively short time. In these animals 

 the birth rate is very high, some insects, as Hexagenia (Morgan, 

 '13, p. 377) require two years to attain maturity. In the Cope- 

 pods, Amphipods and Isopods there are also several genera- 

 tions (four in the Amphipods, Embody, '12, pp. 25-29). Many 

 mollusks, however, live two or more years (the larger mussels 

 live for a longer period). All mollusks lay a large number 

 of eggs and in summer the young and immature individuals 

 greatly outnumber the mature individuals. It is safe to assume, 

 perhaps, as Petersen has done for the Danish marine waters 

 ('11, p. 68) that the bottom fauna reproduces its own mass 

 each year. Some fresh-water groups may do better than this 

 and may be increasing in annual numbers. Aquatic animal 

 life is probably more abundant during the spring and summer 

 months than at other times of the year. Beginning in the earl)' 

 s[)ring the over- wintering individuals emerge (or the over- 

 wintering eggs hatch) lay their eggs and die. The number 

 of living animals increases as one generation succeeds another, 

 and reaches maturity during the summer and declines toward 

 the fall and winter. It is possible that most fish find less food 

 during the winter and this is the reason they are easily caught 

 through the ice. The amount of food in a region will, there- 

 fore, vary with the time of year, and can not be expected to be 

 the same at all times, either numerically or in kind. A group 

 of animals may be abundant in- July and not available in Octo- 



