SUMMARY 
1. Ripe eggs of bluegills, red-ear sunfish, and green 
sunfish were fertilized with sperm of these species in 
the laboratory to make each of the six possible Py} 
crosses. From each of these crosses large numbers of 
F, hybrid embryos developed normally, hatched, and 
became active free-swimming fry. The fry were released 
in outdoor ponds, where they grew to sexual maturity. 
There appears to be no direct genetic isolation between 
these three species. 
2. Attempts to produce hybrids naturally were made 
by isolating males of one species (that is, bluegill, 
red-ear sunfish, or green sunfish) with females of an- 
other (six possible combinations) in ponds that con- 
tained no other fish. Each of the Py crosses was at- 
tempted two or more times. Only the G x B and RxG 
crosses produced significantly large numbers of Fy 
hybrids. Results of the other four Pj crosses were 
negative or inconclusive. 
3. Of the six F, hybrids produced, only the GR hy- 
brids exhibited an approximate 50:50 sex ratio. 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. 
4. When placed in ponds containing no other fish, 
the RG Fy and the GB Fy hybrids produced large num- 
bers of F> hybrids; the GR Fy hybrids produced only a 
few F> hybrids. The BG, BR, and RB hybrids failed to 
produce an F 9 generation. 
5. Varying numbers (200-10,000 per surface acre) 
of Fy hybrid fry were placed in ponds containing no 
other fish. These ponds differed in size, depth, shape 
of basin, and fertility; consequently, no valid compar- 
isons between the growth rates of the different Fy hy- 
brids in these ponds could be made. 
6. BG Fy hybrids and green sunfish of similar sizes 
and ages in equal numbers in a pond that contained no 
other fish increased in length at approximately the 
same rates. The BG hybrid sunfish males between the 
lengths of 4.8 and 5.7 inches weighed considerably 
more than the green sunfish males of similar sizes. 
The BG hybrids were much deeper bodied than the green 
sunfish. The population density of the fish in this pond 
was low (28 pounds per acre when the experiment was 
terminated), and the fish grew rapidly. The results of 
14 
the experiment in this pond indicate that under low pop- 
ulation densities there is not much difference between 
the growth potentials of BG F, hybrids and green sun- 
fish during the first year. Under high population den- 
sities, the hybrid sunfish might grow faster than the 
parent types because of a superior ability to compete 
for food and space. 
7. GR Fy hybrids, green sunfish, and red-ear sun- 
fish in equal numbers in a pond that contained no other 
fish increased in length and weight at approximately 
the same rates. The population density of the fish in 
this pond was low (61 pounds per acre when the ex- 
periment terminated), and the fish grew rapidly. The 
results of the experiment in this pond give additional 
support to the concept that in uncrowded ponds there 
is little difference in the growth rates of hybrid sunfish 
and their parent species. 
ACKNOWLEDGMENTS 
The authors acknowledge the field assistance of 
Messrs. John C. Cralley, William Leicht, Richard Ward, 
Ronald G. Altig, Ronnie D. Havelka, and John Schilling 
in the study reported here. 
Pond owners and fishing clubs who gave the use of 
their ponds for this study are Messrs. Henry Green, 
Everett E. Glasgow, David Malcolmson, H. C. Paterson, 
A.J. Plankenhorn, William S. Pusey, Ross Rudisill, 
John Taylor, William Utterback, Gene Ziegler, and G. 
G. Maxfield of the Fairmount Quarry Fish-Game Club, 
the St. Joseph Sportsmen’s Club, and the Farmer City 
Country Club. Professor F.H. Root and other staff mem- 
bers of Robert Allerton Park allowed the use of certain 
ponds at the 4-HMemorial Camp. The authors are grate- 
ful for the co-operation of these persons and clubs. 
The manuscript was read by Dr. R.W. Larimore and 
edited by Mr. James S. Ayars, both of the Illinois Nat- 
ural History Survey. 
The cover illustration, depicting a nesting male 
green sunfish challenging a female bluegill, was drawn 
by Mr. Edward C. Visnow, assistant technical editor 
of the Illinois Natural History Survey. 
The photographs are the work of Dr. Bennett, Mr. 
William E. Clark, and Mr. Wilmer D. Zehr; Mr. Clark 
was formerly photographer and Mr. Zehr is presently 
photographer for the Natural History Survey. 
