151 
the rate of increase in the product of the fisheries was but little more 
than half as great in the second period as in the first. In other words, 
the product of the fisheries fell far short of keeping pace with the increase 
in men and means devoted to fishing. This fact indicates that the stream 
was being “‘over-fished” during the second of the above periods, and that 
its yield must consequently be expected eventually to fall off if fishing 
operations are continued on the same scale as in the years just preceding 
1909. 
This “diminishing return” for increasing activity is seen especially 
in the yield of European carp, as was to be expected in view of the 
recent introduction of this fish into waters especially favorable to its 
growth and multiplication; but it is also seen in the yield of the native 
species, which after increasing at an average annual rate of 8 per cent. 
for the first or five-year period, diminished about 1 per cent. per annum 
during the second period. That is, the rapidly growing abundance of 
carp so stimulated fishing operations that the stock of native fishes was 
too heavily drawn upon and began to dwindle in yield. We see this par- 
ticularly in the species which are distinguished as bottom-feeders—the 
buffaloes, the suckers, the sturgeon, the sheepshead or drum, and the 
catfishes.* Taken together these diminished in yield about 6 per cent. per 
annum during the first period, and nearly 1 per cent. during the second. 
In comparing these two periods, account should be taken of the fact 
that 1908 was an especially favorable year for fisheries, and its yield was 
far the greatest ever known on the Illinois River. In that year the fish- 
ing season, which begins with July and extends to November, was one 
of low water, concentrating the fish in relatively close quarters, and giv- 
ing easy access to the fishing grounds, and this fishing season had been 
preceded by a long period of unusually high water, covering neafly all 
of 1907 and the early months of 1908, during which the fishes had a 
wide range over shallow waters filled with an abundant food supply. 
The consequence was a fisheries yield 50 per cent. greater than the aver- 
age of the five years next preceding of which we have records, viz., 1899, 
1900, 1903, 1906, and 1907; and to make a just comparison between our 
two periods in respect to increase or decrease of yield the yield of 1908 
should be diminished by one-third. If this be done, the ratios will stand 
as tabulated on page 152. 
Along with an enormous multiplication of the carp in 1894 to 1897 
the yield of native fishes fell off in the Illinois River by 22.2 per cent. 
(from 5,457,731 pounds to 4,234,308 pounds) ;* but from 1900 to 1907 
it increased by 56 per cent. In other words, as the number of carp 
increased the number of native fishes diminished before the opening of 
the sanitary canal at an average rate of 7 per cent. per annum; and after 
the opening of the canal, the number of carp remaining virtually constant, 
the supply of native fishes increased in seven years by an average of 8 
* The yield of these bottom-feeding species in 1894, when the number of carp 
was as yet relatively insignificant, was 81 per cent. of the total yield of native fish. 
* Alvord and Burdick, Fig. 25. 
