world whereas the amount of fresh water in lakes 

 and rivers (that which would be available as 

 fish habitat) approaches an almost negligible 

 percentage — only 0.0093 9f of the world's water 

 (van Hylckama, 1971) (Table 1). In this sense 

 then, 41.2 9f of all fish species live in less than 

 one one-hundredth of one percent of the avail- 

 able water. Table 2, which is based on Cohen's 

 (1970) data and on the data presented in Table 1, 

 shows the great disparity between freshwater 

 and marine environments in terms of the num- 

 ber of species per unit volume of water. The 

 calculations show that there are about 113,000 

 km^ of water per marine species but only about 

 15 km^* for each freshwater species, or approxi- 

 mately a 7,500-fold difference. It is, of course, 

 true that a species does not occupy a particular 

 parcel of water to the exclusion of other organ- 

 isms; nevertheless, it seems conceptually pos- 

 sible and without undue loss of reality to con- 

 sider that each species has available a certain 

 volume of water which it can occupy. It is 

 known, too, that marine habitats vary greatly 

 from high diversity in tropical shore and coral 

 reef regions to low diversity in open ocean areas 

 (including the deep ocean which constitutes most 

 of the volume of the oceans and in which num- 

 bers and biomass greatly decrease with depth) . 

 Shore and shelf fishes have about 290 km^ of 

 water per species compared with about 1,000,000 

 km^ for pelagic species (Table 2), or approxi- 

 mately a 3,400-fold diflference. If the slope and 

 deep-sea benthic species are added to the pelagic 

 figure, the unit volume of water per species be- 

 yond the continental slope is reduced to about 

 500,000 km^, which is still a relatively very high 

 figure. The volume of water per species of ma- 

 rine shore and shelf fishes is higher than the 

 freshwater figure by about 20 x (290 km^ vs. 15 

 km^) . This reflects the similarity in the degree 

 of partitioning in these two regions. Perhaps 

 the number of species per unit volume in the 

 richer tropical reefs exceeds that in a large per- 

 centage of freshwater habitats. Also, some ma- 

 rine habitats which are superficially similar to 

 certain freshwater habitats may be expected to 

 have species densities comparable to their fresh- 

 water counterparts. Examples might be 1) the 

 deep parts of oceans and lakes (low species den- 



Table 1. — Supply (km^) of water in the world available 

 as fish habitat (from data by van Hylckama (1971)). 



Item 



Volume 



Percent of total 



100.0 

 97.0 

 0.0092 

 O.OOOI 

 (remainder of total is ice, grouncjwoter, atmospheric water, etc.) 



Table 2.- 



-Volume (km3) of water available per species 

 in various habitats. 



Type of species 



Volume/species 



Total marine 1 13,000 



Marine shore and continental shelf to 200 m 290* 



Marine pelagic beyond continental shelf 1,000,000 



Marine pelagic -f- continental slope and deep sea benthic 500,000 



Total fresh wafer 15 



* The volume of water over the continental shelf was calculated by 

 considering that the shelf underlies 7.5% of the ocean surface (Emery, 

 1969) and that the average depth over the shelf is about 100 m or 2.5% 

 of the ocean's average de'pth of 4,000 m. 



sity), 2) kelp beds in coastal waters and the 

 vegetated zones of lakes (high species density). 

 It is the open ocean with its broad expanse and 

 great depth that contributes most to the overall 

 very low concentration of species and numbers 

 (discussed below) of marine fishes. 



While it is difficult to estimate the number of 

 fish species in an environment, it is much more 

 difficult to even speculate on the number of in- 

 dividuals per species in either marine or fresh- 

 water regions. GadgiP arrived at a figure of 

 4 X 10^ as the average number of individuals 

 per fish species based largely on marine data. 

 Certainly, diflferent marine habitats support 

 widely diff'ering numbers of fishes. Pelagic spe- 

 cies such as certain anchovies may attain pop- 

 ulation levels of 10'- whereas some rocky shore 

 species may be several orders of magnitude lower 

 in total numbers, perhaps near 10^ individuals 

 per species. A figure in the middle of the above 

 two estimates would be 10-', and in this discussion 

 I have considered 10 x 10^ to be the average 

 number of individuals per species in the sea. It 



* Gadgil, M. On numbers of fish. (Unpublished 

 manuscript) Biology Department, Harvard University, 

 Cambridge, Mass. Present address: Maharashtra As- 

 sociation for the Cultivation of Science, Agarkar Road, 

 Poona 4, India. 



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