Table 3. 
bluegill (B), green sunfish (G), an 
Number Sexed Number Sexed Total Number Per Cent 
by Dissection by Observation Sexed Males 
— Sex ratios of Fj hybrid sunfishes expressed as per cent males in individual samples. Parent species were 
red-ear sunfish (R); in name of hybrid, male parent is given first. 
evidence of hybridization was found in any of these 
ponds, even though the parent fish appeared to have 
had every opportunity to produce young. 
SEX RATIOS OF F, HYBRID SUNFISH 
Sex was determined for varying numbers of indi- 
viduals of each of the six F, hybrid types. Most of 
the Fy hybrids were sexed by dissection; however, 
some were sexed by reference to color and other 
dimorphisms during their reproductive period when 
these differences were pronounced. Table 3 lists 
numbers sexed by dissection and observation, and 
the male percentage in each sample of Fy hybrids. 
Of the hybrids sexed by observation, it was possible 
to strip sperm or eggs from some. Most of the rest could 
be sexed on the basis of (1) color (male sunfish are 
usually darker and more vividly colored than females), 
(2) size of abdomen (early in the spawning season, 
the abdomens of female sunfishes often are greatly 
distended with eggs), and (3) the relative diameters 
of the analand urogenital openings (during the breeding 
season, in the sunfish male the urogenital opening is 
much smaller than the anal opening, while in the fe- 
male the reverse is true). If there was still doubt as 
to the sex of a particular individual, that individual 
was sexed by dissection. The accuracy of determining 
sex by observation during the reproductive season was 
sometimes tested by dissection. Tests made of the 
accuracy of determinations based on the characters de- 
scribed above revealed no errors. 
Of the six Fy hybrid types, only the GR hybrids ex- 
hibited an approximate 50:50 sex ratio. As table 3 
shows, the other five F) hybrid types were predomi- 
nantly males. Of the RG and GB hybrids, 70 per cent 
were males; of the BG and BR hybrids, 97 per cent 
were males; and of the RB hybrids, 100 per cent were 
males. 
Hubbs & Hubbs (1933) reported that 81 per cent of 
43 aquarium-reared BG Fy hybrids they examined were 
males. In the study reported here, 97 per cent of 142 
BG F] fish were males, table 3. 
Ricker (1948) sexed 428 BR Fy hybrids from field 
crosses and found that 98 per cent were males. In this 
study, 97 per cent of 110 BR Fy hybrids were males. 
REPRODUCTIVE SUCCESS OF F, HYBRIDS 
Each of the six types of Fj hybrids was produced 
at various times during the summers (May through Au- 
gust) of 1956-1959 and, to demonstrate its reproductive 
capacity, was placed in one or more ponds containing 
no other fish, table 4. The males of all six types con- 
structed and vigorously guarded nests. Hubbs (1955) 
noted that male Fy] hybrid sunfish constructed, fanned, 
and guarded their nests with unusual vigor over a pro- 
longed period. 
The Fy hybrids RG and GB, each group with a ratio 
of 70 males to 30 females, produced large numbers of 
F> hybrids, table 4. 
Neither the BG Fy nor the BR Fy hybrids, each 
group with a ratio of 97 males to 3 females, produced 
an F> generation in any of ourponds. In Indiana, Ricker 
(1948) reported that BR Fy hybrids successfully re- 
produced in two ponds. It is impossible at this time to 
explain why our BG and BR hybrids did not reproduce. 
The lack of reproduction was not caused by an absence 
of females; in each pond a few Fy females were cap- 
tured and released unharmed. The possibility of in- 
compatibility of mating behavior between F, males and 
females should not be overlooked as an explanation 
for the lack of production of an F> generation in our 
Table 4.— Reproductive success of six Fy hybrid types 
when placed in ponds containing no other fish. Parent spe- 
cies were bluegill (B), green sunfish (G), and red-ear sun- 
fish (R); in name of hybrid, male parent is given first. 
Fy Hybrid | Fz Generation Remarks 
Only a few F> hybrids produced 
F > hybrids very abundant 
F> hybrids very abundant 
*Ricker (1948) reported F5 hybrids produced in two ponds con- 
taining Fy hybrids and no other fish. 
