Some of the systems with low species diversity are 
the Edwards (52 species), Little Vermilion (of the 
north) (52 species), Plum (53 species), Bear Creek 
(54 species), and Saline (57 species). Most of these 
streams have rather small watersheds. 
LONG-TERM CHANGES 
IN ILLINOIS FISH POPULATIONS 
If freakish discoveries of aquarium escapees and 
released pets are ignored, the list of species recorded 
from Illinois waters stands at 193 (13 naturalized and 
180 native species ). 
Four of the 13 non-native species (brown trout, 
goldfish, carp, and grass carp) are exotic species. 
Six of them (rainbow trout, brook trout, coho salmon, 
chinook salmon, American smelt, and white catfish) 
are native to other parts of this continent and have 
been deliberately stocked in Illinois waters. Three 
of them (sea lamprey, alewife, and threadfin shad) 
are native to other parts of this country and have 
recently extended their ranges into Illinois. 
Of the 180 native species, 8 (silverjaw minnow, 
bigmouth shiner, red shiner, redfin shiner, fathead 
minnow, creek chub, mosquitofish, and redear sun- 
fish) have expanded their ranges in Illinois and in- 
creased in abundance. The first three cited thrive in 
small, shallow streams with sand bottoms and mod- 
erate current and are quite resilient to dredging, 
straightening, and other modifications of streams. 
It has been shown that in Champaign County, streams 
tend to be wider and shallower than formerly (Lari- 
more & Smith 1963:320), thus increasing the amount 
of preferred habitat for these mobile and aggressive 
species. Similar changes have occurred over the entire 
state. 
The redfin shiner, fathead minnow, and creek 
chub occupy quiet pools of small streams with silt 
bottoms, and they are rather tolerant of turbid waters. 
Siltation and other human alteration of watersheds 
have increased the amount of favorable habitat for 
these fishes. The mosquitofish and redear sunfish, 
native to southern Illinois, have been widely trans- 
planted in efforts to control mosquitoes and to provide 
another species of panfish in many reservoirs. Both 
are tolerant of some habitat abuse and have strong 
dispersal powers. 
For 104 of the native species little change was 
revealed in distribution and abundance. This may 
be attributed to less adequate sampling during the 
first censusing as reported by Forbes & Richardson 
(1908) or because the change in status of these 
species could not be clearly demonstrated for various 
reasons. 
Eight native species have been extirpated in Tlli- 
nois and 60 other native species show clear-cut evi- 
dence of range shrinkage and decimation. Some of 
the latter group reveal rather minor reductions in 
8 
range size and abundance; others are so severel 
decimated as to be endangered, and some of ther 
may have been extirpated in Illinois since our surve 
was completed. 
FACTORS RESPONSIBLE FOR 
CHANGES IN FISH POPULATIONS 
If the present ranges and former distributions | 
the native fish species in Illinois are carefully e 
amined, it is possible in most cases to identify tl 
factor chiefly responsible for the extirpation or decim 
tion of each species. A tally of the species affect 
by each identified factor permits an objective asse: 
ment of the environmental changes responsible ai 
brings each factor into perspective. 
Silt 
Excessive siltation ranks first and is implicated 
the principal cause for the extirpation of 2 nati 
species and the decimation of 14 others. Its effe 
include loss of water clarity and subsequent dis: 
pearance of aquatic vegetation, and the deposit: 
of silt over substrates that were once bedrock, rubt 
gravel, or sand. Feeding and spawning sites, as 
as the usual habitats for such fishes, have been reduc 
over much of the state. 
The crystal darter, described from a small tri 
tarv of the Mississippi River in Hancock Cou 
(Jordan 1878:38), once occurred also in the R 
River, Little Wabash River, and Mississippi Ri 
in Jo Daviess County (Forbes & Richardson 1908:3¢ 
but it has not been found in the state since Ii 
The gilt darter was collected in the Rock River 
early as 1877 (Forbes & Richardson 1908:289) 
found there in 1927 and 1932 (O’Donnell 1935:4¢ 
but it has not been taken anywhere in the state si 
despite careful searches in recent years. 
The bigeye shiner, bigeye chub (Fig. 3), 
pugnose minnow (Fig. 4), all of which require ¢ 
water, have been decimated primarily because of 
disappearance of aquatic vegetation. The moon 
highfin carpsucker (Fig. 5), rock bass, longear sur 
and walleye have been adversely affected by inc! 
ing water turbidity. The gravel chub (Fig. 6), O 
minnow, weed shiner (Fig. 7), western sand da 
banded darter (Fig. 8), and slenderhead darter | 
reduced ranges because they have lost extei 
gravel- and sand-substrate habitats to silt. 
Excessive siltation has adversely affected f 
over a long period of time and has drastically all 
stream habitats over the entire state with the pos 
exception of high-gradient streams in extreme 0 
western Illinois. 
Drainage 
Drainage of natural lakes, sloughs margin! 
large rivers, swamps, and prairie marshes ranks 
ond in importance and is responsible for the sk 
