American Fisheries Society. 45 



washed from the sh(3re by waves, they are, to a small extent, 

 carried in by winds and rains. Now in a very large lake the 

 proportion of the shallow water to the whole area of the lake is 

 mnch less than in a small lake. It is in this shallow water that 

 the wave action takes place which washes out from the soil the 

 plant food materials which came to be dissolved in the water of 

 the lake. This same shallow water further gives anchorage to 

 plants which furnish shelter for many fishes and for their food. 

 Consequently the shorter the shore line of a lake and the less 

 shallow water it contains, the smaller is likely to be its production 

 of fish per unif of surface area. Our Great Lakes have all a com- 

 paratively straight shore line with very little shallow water ofY 

 shore, and hence should on this accoimt alone be expected to , 

 yield a smaller proportion of fish per unit of area than smaller 

 lakes. Their drainage basin is relatively small, and conse- 

 ciuently relatively little plant food is probably brought into the 

 lakes by the tributary rivers. In general, it is true that the larger 

 the lake, the less may be expected to be its productive capacity per 

 unit of area. 



When, however, we turn to an actual measurement of the 

 productive capacity of one of our Great Lakes by the use of the 

 plankton method, we are astonished at the result. Those smaller 

 European lakes whose plankton has been measured are found 

 to fall into two classes which are called plankton rich and plank- 

 ton poor. As compared with the plankton rich lakes of North 

 Germany our own Great Lakes are found to contain only about 

 one-twentieth as much plankton per volume of water as these. 

 As compared with the plankton poor lakes the Great Lakes con- 

 tain somewhat more than one-half as much plankton per volume 

 of water. The Great Lakes are on the average, then, the poorest 

 in plankton of any lakes that have been hitherto studied. I see no 

 escape from the conclusion that they contain also a smaller pro- 

 portion of fish per unit of area or volume than would smaller 

 lakes. The great size of the lakes does not then justify us in 

 expecting larger returns from them, it rather warns us that we 

 should expect less. The conuuercial fishes of the Great Lakes 

 are taken in large numbers within restricted areas. It is natural 

 to assume that we are thus sampling what occurs on a large part 

 of the lake. We fancv that we may go on fishing indefinitely, 

 and somehow out of the huge expanse of water fish will come 

 to our nets as fish always have come. 



The capacity of a field for the production of any crop is lim- 

 ited. If we supply the field with a certain amount of fertilizer 



