75 
generally assumed, that stocks of the kind in question in the run of years 
at least reproduce themselves in weight once annually. 
Below Havana we lack data for carrying out computations in the 
detail or with anywhere near the exactness attempted for the 60-mile 
section to the north, particularly in the case of the bottom and weed fauna 
of the lakes, having collected in only one of these in 1920. We note, 
first, of the 4,500 odd acres of river between Havana and the dam at 
Lagrange, an apparent slight gain over 1915 in the bottom yields, which 
is, however, probably no greater than the normal error to be expected 
with the apparatus and methods used. Offsetting this gain further, to 
the point in fact of rendering it negligible, are suggested losses of 
enormous size since 1914—1915 both in the bottom and weed fauna of 
the lakes and other backwaters in the 42.5 miles, which, combined, seem 
to have amounted to fully half the total stocks in the entire river and lake 
acreage open a little more than five years ago, or to more than twenty 
times the total bottom-fauna stocks of the section. This result, or a loss 
in the lake weed- and bottom-fauna of around 9,000,000 pounds, is simply 
arrived at if we estimate the loss in both the bottom- and weed-fauna 
stocks since 1915 at only 50 per cent. If we base conclusions for the 
entire acreage between Havana and the dam, on the recorded kind and 
extent of change in Stewart Lake since 1915 we must estimate the 
combined loss in both bottom and weed animals as even greater than 50 
per cent., since it appears that on the weed-fauna side both the acreage 
and its average yield have probably been reduced that much. And as 
most of the lake acreage between Havana and the dam at Lagrange has 
since 1915 been north rather than south of the lower end of Stewart 
Lake, and therefore probably equally with it exposed to invasion by foul 
water and sediments, the estimate of a nine-million-pound loss in this 
district since 1915 may be allowed to stand as a minithum probable 
figure. 
Combining a minimum loss in the first 42.5 miles south of Havana of 
9,000,000 pounds with one of 25,500,000 pounds between Chillicothe and 
that point, we have, then, it seems, to reckon on not less than 34,500,000 
pounds total reduction in the midsummer stocks of small bottom- and 
weed-animals in five years in about 103 miles; or, putting it another way, 
on about 7,000,000 pounds potential fish-yield missing from any new cal- 
culations we wish to make about the more recent value of a 103-mile sec- 
tion of the Illinois River fishery that was, only a‘few years ago, among the 
richest of its kind in the fresh-water stream systems of the world, being 
then uninjured, so far as could be seen, by the increasing pollution from 
the population and industries to the north that has recently so nearly 
overwhelmed it. It will be noted that 7,000,000 pounds is little less than 
a third of the largest total fish-catch ever taken from the entire Illinois 
River (catch of 1908), more than half of estimated total yields in 1912 
and 1913, and not improbably more than the total yield for the whole 
year 1920. 
