It is necessary to realize the limitations of zone classifl ication due 

 to historic, geoaraphic, and climatic influences, however. Generally, the 

 greater the distance from the original streams studied, the more the original 

 scheme of zonation needs to be modified to meet local conditions. Pollution 

 can change zonation in localized areas. 



The zonal distribution of fish in North American rivers has been demonstrated 

 by a succession of workers. Shelford (1911) studied the distribution of fish 

 in a number of Lake Michigan tributaries and concluded that fish have definite 

 habitat preferences which cause them to be definitely arranged in streams 

 which have a graded series of conditions from source to mouth. Burton and 

 Odum (1945) and Funk and Campbell (1953) all report fish distributed in zones 

 in North American streams. 



From these studies in different parts of the world, it is evident that 

 in general there is a longitudinal distribution of fish species in rivers 

 in which a succession of different fish populations occurs from source to 

 mouth. Other generalizations regarding the pattern of this distribution 

 are more difficult to make. Funk and Campbell (1953) report that succession 

 is by gradual transition; other workers report a zonal distribution in which 

 there is a sharp border between zones. 



To what extent do fish zones represent different river biocoenoses? 

 Numerous studies have been conducted on the longitudinal distribution of 

 different benthic invertebrates in rivers. Again, the earliest research 

 occurred in Europe, but studies have taken nlace throughout the world 

 (Beauchamp and Ullyott 1932, Carpenter 1928 , Chandler 1966). The longitudinal 

 distribution of several insect orders has been investigated (Dodds and Hisaw 

 1925, Ide 1935, Hynes 1941 and 1948, Macan 1957). 



Past studies of the longitudinal distribution of aquatic insects have 

 found them be be disturbuted zonally along the length of rivers. It appears 

 that each taxon exhibits a zonal distribution of its different species along 

 the length of a river. Within taxa some species have a restricted distribution, 

 especially those in the upper reaches, while others extend over a long stretch 

 of river: therefore, over some distances, there may be little change in species 

 present. Relative abundance changes along the length of river, reflecting 

 a change in the ecological structure of the community (Hynes 1961). 



The conclusion may be drawn that both fish and benthic invertebrates are 

 longitudinally distributed along rivers, with particular species occupying 

 particular sections of the river. One would expect a correlation among the 

 zones of fish species and of benthic invertebrates. Some authors have 

 concluded generally that biocoenoses associated with the fish zones can be 

 recognized. Thorup (1966) is critical of these studies and suggests that 

 pollution is responsible for the observed zonation of invertebrates and fish. 

 Maitland's work (1966) supports the views expressed by Thorup. It appears 

 from available evidence that, although fish zones can be recognized, the 

 association of benthic biocoenoses with them does not always exist. 



A theory, known as the river continuum theory in Cummins (1975b), 

 has recently emerged to explain the distribution of groups of invertebrates 

 on the bottom of streams and rivers. This theory makes use of theoretical 



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