is fairly certain, I think, that there are fewer 

 individuals per species among- freshwater fishes 

 than among marine fishes. The degree of dif- 

 ference in abundance is, however, difficult to esti- 

 mate or even imagine. There are some very 

 abundant freshwater species such as certain 

 clupeids and cyprinids, but some are quite rare, 

 most notably the desert cyprinodontids of the 

 southwestern United States which may exist 

 only in the thousands or even hundreds per spe- 

 cies. 



Two values were used for the average number 

 of individuals per freshwater species — a high 

 value (10 X 10^), the same as the figure for 

 marine species, and a low value (10 x 10**) 

 which I think is a conservative minimum. A 

 range of values conveys more information in 

 comparing the marine and freshwater situa- 

 tions. The calculations in Table 3 show that 

 marine fishes have 10 x to 10,000 x more space 

 available per individual than freshwater forms, 

 depending upon which freshwater value is 

 chosen. If the lower freshwater figure (10 X 

 10^) is more nearly correct, then the degree of 

 isolation and habitat partitioning in fresh wa- 

 ters becomes even more strikingly apparent. On 

 the basis of total numbers per species, the dif- 

 ference per unit volume between the oceans and 

 fresh water is only 10-fold whereas on the basis 

 of species per unit volume, the diflference is ap- 

 proximately 7,500-fold. 



Table 4. — Net primary productivity and plant biomass 

 per unit area in three major ecosystems (from data 

 compiled by Whittaker (1970)). 



Major ecosystem Nel primary productivity 



Biomass 



Lake and stream 

 Continental shelf 

 Open ocean 



dry g/m^lyear (mean value) dry kg/m^ (mean value) 



503 

 350 

 125 



0.02 

 0.01 

 0.003 



water is about 1.5 X to 4 x as high as in the sea, 

 and plant biomass per unit area in fresh water 

 is about 2 X to 7 X as high as in the sea. These 

 figures are perhaps not in great discord with the 

 estimate above that 10 x as many fishes occur 

 per unit volume in fresh water as in the sea. 



These data serve, I believe, to illustrate the 

 quite astounding difference between the amount 

 of space available for freshwater and marine 

 fishes. As Cohen (1970) has emphasized, the 

 calculations also make apparent the need for 

 increased research on freshwater fishes since 

 their habitats are being rapidly modified. In 

 terms of conservation and economic policies, im- 

 portant studies should include those that com- 

 pare numbers of species and individuals in dif- 

 ferent local and regional environments in relation 

 to levels of productivity and other factors. 



I thank Daniel M. Cohen for reading and of- 

 fering valuable comments on the manuscript. 



Literature Cited 



Table 3. — Volume (km^) of water available per indi- 

 vidual fish in the sea and in fresh water. 



Typa of species 



Number 

 of species 



Individuals/ 

 species 



Volume/ 

 individual 



Marine 



Fresh woter (1) 



Fresh water (2) 



11,675 

 8,275 

 8,275 



10 X 109 

 10 X 109 

 10 X 10" 



1.1 X 10-5 

 1.5 X 10-9 

 1.5 X 10-6 



The above disparity would seem to be related 

 not only to the degree of isolation but to the rel- 

 ative levels of productivity and biomass in the 

 two environments. Table 4 shows net primary 

 productivity and plant biomass estimates for 

 three major ecosystems: 1) lake and stream, 

 2) continental shelf, and 3) open ocean. Net 

 primary productivity per unit area in fresh 



Cohen, D. M. 



1970. How many recent fishes are there? Proc. 

 Calif. Acad. Sci., Ser. 4, 38:341-345. 



Emery, K. 0. 



1969. The continental shelves, hi The ocean, p. 39- 

 52. W. H. Freeman, San Franc. 



VAN Hylckama, T. E. a. 



1971. Water resources. In W. W. Murdoch (ed- 

 itor). Environment, resources, pollution & society, 

 p. 135-155. Sinauer Associates, Stamford, Conn. 



Whittaker, R. H. 



1970. Communities and ecosystems. Macmillan, 

 N.Y., 162 p. 



Michael H. Horn 



Department of Biology 

 California State University 

 Fullerton, CA 92634. 



1297 



