METHODS OF PLANKTON INVESTIGATION IN THEIR RELATION TO 



PRACTICAL PROBLEMS. 



BY JACOB REIGHARD, 



Professor of Zoology, University of Michigan. 



In this country the fisherman as a rule continues to fish in any locality until fish- 

 ing in that locality has become unprofitable. He then moves his operations to new 

 waters until these in turn are exhausted. He is apt to look upon each new body of 

 water as inexhaustible, and rarely has occasion to ask himself whether it is possible 

 to determine in advance the amount of fish that he may annually take from the water 

 without soon depleting it. 



On the other hand, the fish-culturist is apt to plant his fry in waters that are 

 quite unsuited to them or to plant them in numbers far in excess of what the water 

 can support. 



The fisherman proceeds as a farmer might who imagined that he could continually 

 reap without either so wing or fertilizing; while the fish-culturist proceeds often as if 

 convinced that seed might grow on barren soil or that two seeds might be made to 

 grow in place of one. 



In some regions the public is beginning, through the machinery of the State, to 

 insist that its interest in the fisheries be guarded; that neither fishing nor planting of 

 fish should be carried on in excess; and the time is fast approaching when the State 

 will everywhere exert its authority to control the fisheries. It will then become 

 necessary to determine, at least approximately, the productive capacity of any body 

 of water. 



It is the purpose of the present paper to discuss the method by which it has been 

 proposed to determine the relative productive capacities of bodies of water. This 

 method, for there is really but one, was first proposed by Hensen 1 in the sea, and is 

 based upon two principles. It is known that the many species of plants and animals 

 which inhabit a body of water are interdependent. In the final analysis all the fishes 

 are dependent, directly or indirectly, on the minute floating plants and animals which, 

 taken together, we call the plankton. The total mass of plankton is, in most bodies of 

 water, so great that, in comparison with it, it is customary to neglect the fixed plants 

 along the shore and the animals that they harbor. That the plankton lies at the base 

 of all life in the water is, then, the first principle. 



The second principle is that the plankton, considered as a whole, is uniformly dis- 

 tributed. There is no longer any doubt that some constituents of the plankton, e. g., 

 the Crustacea, may not be distributed uniformly. 2 Wherever measurements have been 



1 Hensen, Victor. Ueber die Bestinmmug des Planktons. Kiel, 1887. 



2 Marsh. On the Limnetic Crustacea of Green Lake. Transactions Wisconsin Academy of 

 Science, Arts, and Letters, vol. 11, 1897, pp. 179-224. 



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