advancement. In three specimens examined, the gas 
bladder averaged about 16 percent of the standard 
length of the fish. In a 2-year-old male, 69 mm in 
standard length, it was 10 mm long, 3 mm wide, and 2 
mm deep. 
POPULATION CHARACTERISTICS 
Density 
Some information on population density of P. sciera 
in the middle Embarras River was provided by the 
electric-seine sample described earlier. The approxi- 
mately 507 square yards of river (average depth 1 
foot) yielded 750 fishes of various kinds for an average 
of 1.48 fish per square yard and one dusky darter for 
25.3 square yards. However, rather than being uni- 
ormly distributed in the 507 square yards, the darters 
ended to occupy their optimal habitat, a gravelly race- 
vay that comprised about one-fourth (127 square 
yards) of the area sampled. Thus a more realistic 
igure would be one dusky darter per 6.3 square yards 
of habitat. 
~ e;e 
~omposition 
Of the 20 dusky darters secured by shocking in the 
arricaded area, 17 (85 percent) were young-of-the- 
ear and 3 (15 percent) were 1-year-old fish. The 
roportion of young to older individuals is remarkably 
imilar to the age analysis made for all specimens ex- 
mined during the 1967-1969 study period: 82.8 per- 
ent young-of-the-year, 13.7 percent yearlings, 2.7 
€rcent second-year fish, and .7 percent third-year fish. 
Approximately equal numbers of males (117) and 
males (124) were produced (Table 2), and the over- 
ll sex ratio of young can be assumed to be 1:1; how- 
ver, males appeared to have somewhat greater lon- 
evity than females. Of 35 fish examined that were 2 
r more years in age (including several not from the 
udy area), 28 (80 percent) were males and 7 (20 
ercent) were females. However, females live to an 
se of at least 3 years, a fact substantiated by the 
resence in the Illinois Natural History Survey collec- 
on of a large female slightly more than 3 years old 
om Scott Co., Missouri. The oldest specimen ex- 
nined was the previously mentioned 108-mm (stand- 
d length) male from the Little Wabash River of 
linois (414 years). 
TABLE 2.—Distribution of sexes and year classes in samples of 
rena sciera taken from the study area between August 4, 1967, 
d June 8, 1969. 
Number by Year Class 
Sex —1 1+ 2+ 3+ Total 
ules 117 8 5 Pe 132 
males 124 32 3 0) 159 
Total 241 40 8 2 291 
I] 
Although no evidence of predation on P. sciera was 
found, the primary control of population levels must 
be a combination of predation, the inherent limited 
longevity of the species, and the harshness of the phys- 
ical habitat, particularly the fluctuations in the level of 
the river. 
Migration 
Although the dusky darter was the most common 
darter in the study area and consistently easy to collect 
during summer and fall, at times Specimens could not 
be obtained. Sometimes they could not be collected 
because excessively high water prevented effective 
seining, but at other times (November of 1967, Decem- 
ber of 1968) they could not be obtained even though 
the water levels were low and habitats in which the 
dusky darter had previously been plentiful were seined 
repeatedly. 
Between October 3, 1967, when the species was 
very abundant (31 captured in 20 minutes) to Novem- 
ber 4, 1967, when only 3 darters could be captured in 
2 hours of seining, the population in the river channels 
had become greatly reduced. October 3 was clear, 
warm, and typically autumnal. By November 4. cold 
weather had begun and the air temperature was only 
33°F. During this interval an emigration from the 
river channels to another habitat had occurred. By 
March, individuals of all sizes were again present in 
the channels. 
Several species of darters are known to immigrate 
into tributaries in the spring, either to spawn and then 
immediately return to the habitat previously occupied or 
to remain throughout the summer and return to the 
river in the autumn (Lake 1936:817; Winn 1958a:163- 
164). Three tributaries of the Embarras River are 
located within 4 miles of the study area, and 18 collec- 
tions have been made in them over the last 10 years. 
Three of 11 made in the summer months contained 
specimens of P. sciera, but none of the 7 collections 
made at other seasons contained specimens even when 
extensive effort was made and all types of habitats 
vigorously sampled in over a mile of the tributary, as 
was done November 25, 1967. Specimens had been 
collected in the same area in June of 1967, and several 
were collected the following day (November 26, 1967) 
in the Embarras River proper. 
A winter movement out of the river channels and 
spring immigrations into them, followed by fall emi- 
grations from tributaries, were the only migratory 
aspects observed in this study. 
Territoriality 
The presence of pronounced sexual dimorphism 
(larger size and darker color of the male) Suggests that 
the male establishes and defends a territory. Aquaria 
used to hold dusky darters were evidently too small 
to allow the establishment of territories, and defended 
