Fisheries (continued)

Table 11 shows the increase in harvest for select fish species in Illinois. Separate data are not available specifically for the Illinois River, however, in 1908, 57% of the fishermen of the state and nearly two-thirds of the capital expended are on the Illinois River (Forbes & Richardson 1919).

Table 11: Statistics of Fisheries in Illinois 1894-1908 (Forbes and Richardson)

Table 12 illustrates that despite the increase in the number of fishermen in the second period of nine years, the rate of increase in harvest is a little more than half as great in the second period as in the first. Forbes and Richardson suggest this is due to over-fishing during the second period. This “diminishing return” despite increased activity is evident for the carp as well. Native fish species increase at an average rate of 8% during the first period and then decline at a rate of 1% per year in the second.

Forbes and Richardson note that in comparing the two periods, conditions in 1908 are exceptional for fishing, and the yields are the greatest ever recorded for the Illinois River. The fishing season of July to November is one of low water, concentrating fish into smaller areas. This dry year follows several years of exceptionally high waters where the fish are able to reproduce in large numbers. The consequence is a fisheries yield 50% greater than in the preceding five years for which there are records: 1899, 1900, 1903, 1906, and 1907. The yields of 1908 needs to be diminished by one-third to reflect the unusual conditions that existed for fishing. In so doing, the numbers provided in Table 11 are adjusted in Table 12.

Table 12: Percentage Change between 1899 and 1908, Adjusted (Forbes and Richardson)

In addition to the enormous reproduction of the carp that occurs between 1894 and 1897, the yield of native fishes falls off by 22.2%, but from 1900 to 1907, it increases by 56%. So, as the number of carp increases, the number of native fish species decreases before the opening of the sanitary canal at a rate of 7% per year. Once the canal opens, the number of carp remains relatively constant, and the native fishes increase in seven years at an average of 8% per year. Forbes and Richardson find this difficult to explain except that the additional food supply provided by the decomposing sewage allows the native fish to compete well despite the enormous competition provided by the carp.