“…Studies dealing with polluted areas (Tulkki 1960, 1964, Leppäkoski 1975, Rosenberg et cii. 1975 or river mouths (Särkkä 1969) cannot be used and studies reporting only abundance (Sjöblom 1955, Valtonen 1976: values of 1962, 1964 or carried out with other sieves (Dahlström and Sormunen 1965, Bagge and Ilus 1973: values of 1968-1970, Valtonen 1976: values of 1974 can only be used for very generalized comparisons.…”