Ml. :. iiilipE. ilii : ^L I B R.AR.Y OF THE UNIVERSITY or ILLINOIS cop. 2 ./.lUKAL HISTORY SURVEY STATE OF ILLINOIS Otto Kerner, Governor DEPARTMENT OF REGISTRATION AND EDUCATION William Sylvester White, Director HYBRIDIZATION BETWEEN THREE SPECIES OF SUNFISH (Lepomis) WILLIAM F. CHILDERS GEORGE W. BENNEH NATURAL HISTORY SURVEY DIVISION Harlow B. Mills, Chief Urbana, Illinois Biological Notes No. 46 Printed by Authority of the State of Illinois November, 1961 .- -.fi 2 -^ c 1'e y < I HYBRIDIZATION BETWEEN THREE SPECIES OF SUNFISH (Lepomis) American agriculturists have produced many hybrids for increasing food production or for improving the qual- ity of food products. Corn and poultry hybrids are so commonplace that they are considered normal crops for progressive farmers. The use of other types of plant and animal hybrids on the farm is spreading, and it would be no great step for farmers to accept the idea of hybrid sunfish in their pasture ponds. The need for a hybrid fish in pond culture relates largely to theproblem of population control: many kinds of warm-water fishes reproduce so successfully that they create conditions of overpopulation and become severely stunted. Stunted populations of fishes are use- less for recreation or food. A hybrid fish with a reduced reproductive potential would be a great improvement over the fishes now commonly used in warm-water ponds, particularly if it combined rapid growth to a large size with other characteristics desirable for angling. Naturally produced hybrids of fresh-water fishes are not uncommon and have been reported in the salmon family (Salmonidae), sucker family (Catostomidae), minnow family (Cyprinidae), pike family (Esocidae), sunfish family (Centrarchidae), and some others. Moenk- haus (I9II) experimentally hybridized many species of teleosts and found that in the species he tested the eggs of each species could be impregnated by the sperm of any other species. He also determined that the stage to which any given hybrid would develop was correlated with the nearness of the taxonomic relationship of the two species used. Thompson (19^5) reported that with- in certain groups of fishes hybrids were common and ranged from a few to as high as 10 per cent of the pop- ulations. Hybrid sunfish have received the attention of a num- ber of biologists. Hubbs (1920) expressed the opinion that Lepomis euryonis McKay was the hybrid of the green sunfish, L. cyanellus Raf., and the pumpkinseed sunfish, L. gibbosus (L.). Hubbs & Hubbs (1931) val- idated this opinion by successfully hybridizing green and pumpkinseed sunfishes in aquaria in the laboratory. These authors (1932) also established that the name L. iscbyrus (Jordan & Nelson) was apparently based on the hybrid of the bluegill, L. macrochirus Raf., and •William F. Childers is Assistant Aquatic Biologist and George W. Bennett is Aquatic Biologist and Head of the Section of Aquatic Biology, Illinois Natural History Survey. William F. Childers George W. Bennett* the green sunfish. Hubbs & Hubbs (1933) reported that "aquarium-reared" and "natural" hybrid sunfishes grew more rapidly than their parent species, were pre- dominately of the male sex, and were apparently sterile. Luce (1937) successfully produced hybrid sunfish by manually stripping eggs from ripe females and milt from ripe males into fingerbowls containing small a- mounts of water. After the eggs hatched. Luce raised some of these hybrids to sexual maturity in aquaria in the laboratory. Ricker (1948) and Krumholz (1950) produced large numbers of hybrids by placing adult males of the bluegill and adult females of the red-ear, L. microlophus (Gunther), in ponds containing no other fish. Lagler & Steinmetz (1957) produced hybrids by placing ripe adult males of one species of sunfish with ripe adult females of another species in ponds which contained no other fish (pumpkinseed males with blue- gill females and bluegill males with pumpkinseed fe- males). The intrageneric Lepomis crosses investigated by the authors mentioned above and the results of these investigations are summarized in table 1. Each of the Lepomis hybrids except, of course, the "natural" hy- brids of Hubbs & Hubbs (1933) was produced by one of three methods: (1) isolating a ripe male of one spe- cies and a ripe female of a different species in an aquarium, (2) manually stripping eggs from ripe females and milt from ripe males into fingerbowls, and (3) iso- lating adult males of one species and adult females of a different species in a pond containing no other fish. The environments of the hybrids in the embryonic and larval stages probably differed considerably with the method used and may have affected the viability, mor- phology, and rate of growth of the hybrids. METHODS OF PRODUCING HYBRIDS In this paper, G refers to green sunfish, B to blue- gill, and R to red-ear sunfish. Matings between indi- viduals of different species are designated as Pj cross- es, and the resultant hybrids are designated as ¥\ hy- brids, fig. I. F2 hybrids are those produced by mating an Fj male with an Fj female. The kind of fish that furnished the sperm is always given first; thus, the P| cross of a male bluegill and female green sunfish is designated B x G, and the resultant hybrids are des- ignated BG Fj hybrids; GB Fj designates the recip- rocal hybrids. In the experiments reported here, two methods were employed in an attempt to produce each of the six pos- sible Fj hybrids between bluegill, green sunfish, and each species were kept separated. Eggs from one or more ripe females of one species were stripped into a damp petri dish, fig. 3/4, and milt from one or more males of another species was stripped onto the eggs. The petri dish was then shaken vigorously in order to Tabl attempts el. — Intrageneric Lepomis Pj crosses attempted experimentally by various biologists and the results of these Pi Cross (Male X Female) Reference Result Bluegill X green Hubbs & Hubbs (1932) 61 raised to sexual maturity Green x bluegill Luce (1937) Many raised to free-swimming stage* Bluegill X red-ear Ricker (1948) Many raised to sexual maturity Krumholz (1950) Many raised to sexual maturity Red-ear x bluegill Krumholz (1950) No hybrids produced Green x pumpkinseed Hubbs & Hubbs (1932) 11 raised to sexual maturity Pumpkinseed x green Hubbs & Hubbs (1932) 41 raised to sexual maturity Pumpkinseed x orangespottedt Luce (1937) Some raised to sexual maturity Orangespotted x pumpkinseed Luce (1937) Some raised to sexual maturity Green x orangespotted Luce (1937) Some raised to sexual maturity Pumpkinseed x bluegill Hubbs & Hubbs (1932) No spawning occurred Luce (1937) Many raised to free-swimming stage * Lagler & Steinmetz (1957) Many raised to sexual maturity Bluegill X pumpkinseed Hubbs & Hubbs (1932) No spawning occurred Luce (1937) Many raised to free-swimming stage* Lagler & Steinmetz (1957) Many raised to sexual maturity Longeart x green Hubbs & Hubbs (1932) Spawning occurred, but all eggs fungused Green x longear Hubbs & Hubbs (1932) No spawning occurred Pumpkinseed x longear Hubbs & Hubbs (1932) No spawning occurred Longear x pumpkinseed Hubbs & Hubbs (1932) No spawning occurred Longear x bluegill Hubbs & Hubbs (1932) No spawning occurred Bluegill X longear Hubbs & Hubbs (1932) No spawning occurred •Information from author. TOrangespotted sunfish, L. bumilis (Girard). ^Longear sunfish, L. rriegalotis (Raf.) red-ear sunfish. Each Pj cross was made in the lab- oratory by using Luce's method (referred to hereafter as laboratory cross) and each was arranged for in out- door ponds by placing males of one species with fe- males of another species (referred to hereafter as field cross). Adults used in making the P] crosses were obtained from several lakes and ponds in central Illinois, fig. 2. Bluegills were obtained from Lake Italy, Fairmount Quarry, Fairmount; from Farmer City Country Club Pond, Farmer City; and from Big Pond, Utterback's Farm, 5 miles southeast of Gibson City. Green sunfish were obtained from Allerton Lake, 4-H Memorial Camp of the University of Illinois at Robert Allerton Park near Monticello, and from Pond E2, Fairmount Quarry, Fairmount. Red-ear sunfish were obtained from Taylor s Pond, Taylor's Farm, 3 miles southwest of Fairmount; from Lake Italy; and from Big Pond. Laboratory Crosses. — Ripe males and females of the bluegill, green sunfish, and red-ear sunfish in the lakes and ponds named above were trapped and moved into aquaria in the laboratory. Males and females of mix milt and eggs. Two minutes were allowed for fer- tilization to take place, and then 200 to 300 fertilized eggs, fig. 3B, were poured into each of a number of clean petri dishes filled with aged tap water. As soon as the egg fluid, which had prevented the eggs from sticking, became diluted, the eggs adhered to the bot- toms of thepetri dishes. The fertilized eggs or zygotes were left undisturbed for approximately 15 minutes to allow them to become water hardened. The zygotes were then washed by lowering and raising the petri dishes four or five times in a large beaker filled with aged tap water. After the zygotes had been washed, enough water was added to each petri dish to cover the zygotes. This water was poured off and replaced approximately six times each day during the incubation period. After the embryos hatched, the larvae were placed in 10-gallon aquaria. The water in these aquaria was kept in constant gentle motion by releasing air from air stones placed close to the bottom. .\s soon as the larvae became free-swimming fry, they were placed in ponds containing no other fish and held for one or more growing seasons. Fig. 2.— Hardware cloth traps used to collect parent species from central Illinois lakes and ponds. These traps were used also to sample hybrid populations. /I, traps are shown here being assembled. B, a trap is being set in a small gravel pit pond. Fig- 3. — Procedures used in making laboratory crosses. A, eggs from a female red-ear sunfish are being stripped into a damp petri dish, where they are immediately fertilized with sperm from a male bluegill. B, after the fertilized eggs have been allowed to set for 2 minutes, they are placed in other petri dishes; they adhere to the bottoms of the dishes. Field Crosses. — Each of 16 small ponds (0.02 to 0.9 acre^ containing no fish was stocked with sexually mature fish: females of one species and males of an- other, table 2. At frequent intervals, the ponds were checked, and any nests that were found were examined to see if they contained eggs or fry. The ponds were also seined and, when young Fj hybrids were present, some of the hybrids were removed and used for stocking other ponds. Fj hybrid sunfishes produced by field crosses, like those produced by laboratory crosses, were held in ponds for one or more growing seasons. RESULTS OF HYBRIDIZATION In this paper, direct genetic isolation refers to spe- cies isolation resulting from incompatibility between the gametes of different species, while indirect genetic isolation refers to species isolation resulting from such differences as time of spawning, habitat requirements for spawning, and mating behavior patterns. Laboratory Crosses. — Several thousand free-swim- ming F] hybrids were produced in the laboratory from each of the six possible Pj crosses; these hybrids were proof that direct genetic isolation did not exist between the bluegill, green sunfish, and red-ear sun- fish. Incubation times of fertilized eggs from all Pj crosses were similar when incubation temperatures were similar. Eggs incubated at 70 degrees F. began hatching 47 hours after fertilization, and none hatched later than 6l hours after fertilization. The larvae be- one of these ponds became contaminated with female bluegills and the other two ponds dried up during the summer. The BxR cross has been made experimentally with success six times in Indiana (Krumholz 1950). The R X B cross was tried in four ponds. In three of the four ponds, no hybrids were produced. In the fourth, 11 young sunfish were found when the pond was Table 2. — Field crosses attempted by placing adult males of one species and females of another species in ponds con- taining no other fish. Ponds were stocked in May or early June and were censused by being drained, seined, or chemically treated the following August or September. The species used were bluegill (B), green sunfish (G), and red-ear sunfish (R). Pj Cross Size of Poml (Surface Acres) Number of Adults Used Fj Hybrid Generation Remarks (Male X Female) Males Females R X G 0.02 4 7 Yes Hybrids abundant R X G 0.04 8 3 Yes Hybrids abundant G X R 0.10 2 8 No G X R 0.02 4 6 No G X R 0.10* 8 11 No G X B 0.02 4 6 Yes Hybrids abundant G X B 0.90 30 35 Yes Hybrids abundant B X G 0.02 4 5 Contaminated B X G 0.03 7 10 Contaminated B X R 0.10 7 6 Contaminated B X R 0.02 4 6 Pond went dry BxR 0.20* 17 15 Pond went dry R X B 0.02 4 6 ■> 1 1 young found t R X B 0.10* 15 10 No R x B O.IO* 13 15 No R X B 0.80* 16 11 No *Size of pond estimated. tie 11 young sunfish found were too small to be identified as hybrids or as young of parent species. tthe came free-swimming fry during the fifth or sixth day after hatching. One group of green sunfish eggs fertilized with blue- gill sperm was accidentally incubated at temperatures ranging from 70 to 95 degrees F. Many of the BG Fj hybrids produced from this group of eggs had very short caudal peduncles. In a few of these fish, the posterior end of the caudal fin did not extend beyond the poste- rior edges of the soft dorsal and soft anal fins. Field Crosses. — Hybrids were produced in 4 and possibly 5 of the l6 ponds used in the experiment re- ported here, table 2. The R X G cross and the G x B cross, each cross tried in two ponds, resulted in the production of large numbers of F^ hybrids. The BxR cross was attempted without success in three ponds. These were all improper tests because drained. These young sunfish were so small (0.25-0.50 inch total length) that it was impossible to identify them as hybrids. An attempt to raise them in the lab- oratory failed. However, circumstantial evidence in- dicates that they were RB Fj hybrids. Vhen the pond was drained, the parent fish were sexed by dissection and only male red-ears and female bluegills were found. If this pond had been contaminated with male bluegills or female red-ears, it is probable that several thousand young would have been produced. The B X G cross was attempted in two ponds. Both ponds became contaminated with male green sunfish, and in each pond large numbers of young greens were produced. Consequently, these ponds gave no real tests of hybridization between bluegill males and green fe- males. The G X R cross was set up in three ponds. No Table 3. — Sex ratios of F\ hybrid sunfishes expressed as per cent males in individual samples. Parent species were bluegill (B), green sunfish (G), ana red-ear sunfish (R); in name of hybrid, male parent is given first. Number Sexed Number Sexed Total Number Per Cent Fi Hybrid by Dissection by Observation Sexed Males GR 252 252 48 RG 229 228 457 70 GB 44 44 70 BG 142 142 97 BR 45 65 110 97 RB 175 38 213 100 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 Fi HYBRID SUNFISH Sex was determined for varying numbers of indi- viduals of each of the six F, hybrid types. Most of the F, 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 F, 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 anal and 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 F] hybrid types, only the GR hybrids ex- hibited an approximate 50:50 sex ratio. As table 3 shows, the other five Fj 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 (193 3) reported that 81 per cent of 43 aquarium-reared BG Fj hybrids they examined were males. In the study reported here, 97 per cent of 142 BG Fj fish were males, table 3. Ricker (1948) sexed 428 BR Fj hybrids from field crosses and found that 98 per cent were males. In this study, 97 per cent of 110 BR Fj 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 Fj hybrid sunfish constructed, fanned, and guarded their nests with unusual vigor over a pro- longed period. The Fj hybrids RG and GB, each group with a ratio of 70 males to 30 females, produced large numbers of F2 hybrids, table 4. Neither the BG F] nor the BR F] hybrids, each group with a ratio of 97 males to 3 females, produced an F2 generation in any of our ponds. In Indiana, Ricker (1948) reported that BR Fj 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 Fj females were cap- tured and released unharmed. The possibility of in- compatibility of mating behavior between Fj males and females should not be overlooked as an explanation for the lack of production of an F2 generation in our Table 4.— Reproductive success of six Fj hybrid types wlien 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. l-"l Hybrid F2 Generation Remarks GR Yes Only a few F2 hybrids produced RG Yes F2 hybrids very abundant GB Yes F2 hybrids very abundant BG No BR No* RB No •Ricker (1948) reported K2 hybrids produced in two ponds con- taining I'] hybrids and no other fish. ponds. This incompatibility may be of a type that is expressed in some environments but not in others. The RB Fj hybrids were all males; so it was im- possible for these hybrids to produce an F2 generation. The GR Fj hybrids, with an approximate 50:50 sex ratio, produced a very small F2 generation in a gravel pit pond, fig. 4. Field observations revealed that their reproductive behavior was slightly abnormal. Males constructed nests and were extremely aggressive in guarding their territories. On two separate occasions a female was observed to approach a nest guarded by a male. The male faced the female and challenged her by flaring his opercular flaps. The female, which was greatly enlarged with eggs, swam slowly toward the male. The male left the nest and appeared to lead the female into it. The two fish started slowly swimming in small circles over the nest, with the female closer to the center. The male maintained a position slightly in advance of the female and, by constantly turning toward her, appeared to force her to swim within the limits of the nest. Before the spawning pair had time to make more than a few complete circles, nearby males Ki^. I. \ (i.l-acre gravel pit ponci that contained GR Fj hybrids. deserted their nests and rushed into the nest occupied by the spawning pair. A wild melee followed, during which males fought with each other and attempted to gain control of the female. The female hastily retreated toward deep water. A short time after the female had gone, the invading males returned to their own nests. When the nests in this pond were examined, approx- imately 9 out of 10 of those containing eggs had eggs scattered outside as well as inside the nests. All of the eggs which were outside the nests appeared to be covered with fungi, and most of the eggs in the nests were similarly infected. Noninfected eggs were col- lected from several nests and moved into the laboratory; none of these eggs hatched. The attacks on spawning pairs by males could limit the production of an Fj generation in one or more of the following ways: 1. Females could be prevented from spawning until the eggs which they contained were overripe and hence nonviable. 2. There might be lack of synchronization in the release of spermatozoa and eggs. Sunfish sperm- atozoa are capable of fertilizing eggs for a very short period of time, probably less than 60 seconds under natural conditions, after they are released into the water. 3. The presence of large numbers of fungus-in- fected eggs outside of the nest could lead to the pro- duction of so many fungus spores that a high incidence of fungus-infected eggs within the nest would result. It is interesting to note that there is a fairly close relationship between the success of certain Pj field crosses in producing an Fj generation and their Fj hybrids in producing an F2 generation. Both R x G and (' X B crosses produced large numbers of Fj hybrids each time these crosses were tried. The RG and GB hybrids produced large F2 generations. The production of a large F2 generation was not related to whether the Fj hybrids were produced from field crosses or lab- oratory crosses, as both crosses produced large F2 generations. GROWTH OF Fi HYBRID SUNFISH Growth was recorded for each of the six types of F} hybrids that were produced at various times during the summers of 1956-1959 and placed in ponds containing no other fish. Varying numbers of fry had been placed in the ponds, which differed in size, depth, shape of basin, and fertility. Population densities of Fj hybrids usually were considerably below those in ponds occu- pied by their parent species. Thus, no valid compari- sons in growth rates could be made between the vari- ous hybrids in these ponds, or between the hybrids and their parent species. The observed growth of one population of each hybrid sunfish type has been sum- marized below. RB Fl Hybrids. - Approximately 2,500 (1,000 per surface acre) 1-day-old free-swimming fry were placed in a 2.6-acre pond on August 14, 1956. ^^'hen this pond was drained on June 11, 1957, after 10 months (about 4 of which were warm enough for fish growth), it con- tained 463 hybrids ranging in length from 3.4 to 4.4 inches and averaging 4.0 inches. On that date, 102 of these hybrids were placed in a 0.05-acre pond. This pond was drained on July 2, 1958, after a little more than a complete growing season, and 92 hybrids were collected; they averaged 5.3 inches and ranged from 3.8 to 5.9 inches in total length. BR Fl Hybrids. - Approximately 1,100 (1,200 per surface acre) 1-day-old free-swimming fry were placed in a 0.9-acre pond on May 24, 1957. On October 12, 1957, after about 4.5 growing months, hardware cloth wire traps were set in this pond, and 40 hybrids were captured. These hybrids averaged 4.7 inches and ranged in length from 3.6 to 5.4 inches. They averaged 0.082 pound in weight and ranged from 0.035 to 0.100 pound. All 40 fish were returned to the pond. On July 28, 1958 (after 1 year and 2 months), 67 hybrids, which ranged in length from 5.4 to 6.9 inches and averaged 6.4 inches, were trapped and removed from this pond. The pond was treated with rotenone on September 23, 1958, and 43 additional hybrids were collected. After about two growing seasons, these hybrids averaged 7.4 inches and ranged in length from 6.2 to 8.3 inches. Average weight was 0.35 pound. GR F] Hybrids. — A 0.1 -acre pond was stocked on May 17, 1957, with approximately 1,000 (10,000 per surface acre) 1-day-old free-swimming hybrid fry. One hundred twenty-eight hybrids were trapped and removed from this pond on July 7, 1958, after 1 year and 2 months. At that time these hybrids ranged in length from 3.8 to 4.9 inches and averaged 4.4 inches. On August 15, 1958, 200 additional hybrids, which ranged in length from 3.9 to 5.1 inches and averaged 4.7 inches, were removed from this pond. On July 29, 1959, when the fish were about 2 years old, the population was again sampled by trapping. Fifteen hybrids were captured and returned to the pond. These fish averaged 7.7 inches and ranged in length from 7.3 to 8.2 inches. They averaged 0.41 pound in weight and ranged from 0.30 to 0.47 pound. RG F] Hybrids. — A 0.9-acre pond was stocked on June 11, 1956, with approximately 1,300 (1,400 per surface acre) free-swimming hybrid fry. On June 27, 1957, 79 hybrids, which averaged 5.0 inches and ranged in length from 4.1 to 5.8 inches, were captured in traps and then returned to the pond. During the period May 13-16, 1958, 327 hybrids, which averaged 6.8 inches and ranged in length from 5.2 to 8.1 inches, were trap- ped and removed from the pond. BG F] Hybrids. - Sixty-one (200 per surface acre) hybrid fry were placed in a 0.3-acre pond on July 3, 1956. Less than 3 years later, on May 8, 1959, this pond was treated with rotenone, and 10 Fj hybrids were collected, fig. 5; they averaged 8.6 inches and the hybrids were recaptured. After 2 growing seasons, these 44 hybrids averaged 6.9 inches and ranged in length from 5-2 to 7.5 inches. GROWTH OF PARENT SPECIES AND F] HYBRIDS Two experiments were conducted to directly compare growth rates of hybrids with those of their parent spe- Fig. 5. - Seven of 10 BG Fj hybrids collected from a 0.3-acre pond after approximately 5 years of growth. These hybrids averaged 8.6 inches in total length and 0.71 pound in body weiphi. ranged in length from 7.2 to 9.2 inches. In weight the fish averaged 0.74 pound and ranged from 0.50 to 0.86 pound. GB Fi Hybrids. - On August 26, 1957, ^22 (800 per surface acre) hybrids, which averaged 1.5 inches total length, were placed in a 0.4-acre pond. Wire traps were set in this pond on October 18, 1959, and 44 of cies. In the first experiment, a pond which contained no other fish was stocked with equal numbers of BG F| hybrids and green sunfish of approximately similar ages and sizes. In the second experiment, another pond was stocked in a similar manner with GR Fj hybrids, green sunfish, and red-ear sunfish. In both of these experiments the population densities of fish were con- 10 siderably below the carrying capacities of the ponds. Intraspecific and interspecific competitions were low, and the fish grew at rates approaching the optimum for their respective environments. Green Sunfish vs. BG F] Hybrids. — On June 3, 1958, a very small (0.02 surface acre) rearing pond was stocked with 1-day-old, free-swimming, laboratory- produced green sunfish fry. Another rearing pond of the same shape and size was stocked on four different dates (May 26, June 3, June 10, and June 18, 1958) with 1-day-old, free-swimming, laboratory-produced BG Fj hybrid fry. On July 10, 1958, both ponds were seined, and 171 green sunfish, which averaged in total length approximately 0.75 inch, and 171 BG Fj hybrids, which averaged in total length approximately 1.0 inch, were removed from the rearing ponds and placed in an 0.8- acre gravel pit pond, fig. 6, that contained no other fish. Ten months later (but only 4 months during which waters were warm enough for fish growth) the fish pop- ulation in the gravel pit pond was censused by trapping and rotenone poisoning. All of the fish recovered were individually measured, weighed, and sexed. Statistics on this population are shown in tables 5 and 6. The BG Fj hybrid and green sunfish males had in- creased in length at approximately the same rates. The hybrid males averaged 0.4 inch longer than the green sunfish males. However, as the hybrid sunfish aver- aged approximately one-fourth of an inch longer than the green sunfish when the pond was stocked, the length advantage of the hybrid males may not be significant. 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. Population density of the fish in this pond was low. At the time the experiment was terminated, the pond contained only 28 pounds of fish per acre. In the past this pond had supported considerably more than 100 pounds of sunfish per acre. Intraspecific and inter- specific competitions between the fish in this pond were very low, and the fish had grown at extremely rapid rates. The results of this experiment indicate that under low population densities there is not much difference between the growth potentials of BG Fj hybrids and green sunfish during the first year. Under high pop- ulation densities, the hybrid sunfish might grow faster than the parent types because of a superior ability to compete for food and space. msssEi I ' 1- 1 liybridi ol 11 Table 5.— The numbers of green sunfish and BG Fj hybrid sunfish placed in an 0.8-acre pond and the numbers and the percentages of the original stock recovered 10 months later. Category Green S unfish BG F] Hybrids Number Per Cent Number Per Cent Fish used in stocking (July 10, 1958) 171 100 171 100 Fish trapped and removed (April 29-May 8, 1959) 123 72 116 68 Fish recovered during rotenone census (May 8-11, 1959) 4 2 26 15 Total fish recovered (trapping and rotenone census) 127 74 142 83 Fish unaccounted for 44 26 29 1- Table 6.— Number and weights of green sunfish and BG Fj hybrids of various length classes recovered 10 months after release in an 0.8-acre pond. Total Green Sunfish BG Fj Hybrids Length, Number of Fish Weight Number of Fish Weight Inches Range, Grams Average Range, Grams Average Grams Pounds Grams Pounds MALES 2.8-3.2 3 7-8 7 0.02 3.3-3.7 2 7-14 10 0.02 6 8-13 10 0.02 3.8-4.2 5 16-24 20 0.04 5 14-26 20 0.04 4.3-4.7 13 24-38 31 0.07 6 23-36 28 0.06 4.8-5.2 33 37-49 43 0.09 10 37-65 58 0.13 5.3-5.7 12 47-56 51 0.11 79 52-81 63 0.14 5.8-6.2 29 79-92 80 0.18 Total 65 138 Average* 39 0.08 59 0.13 FEMALES 2.8-3.2 3.3-3.7 6 11-22 16 0.04 — 3.8-4.2 13 14-23 19 0.04 — 4.3-4.7 26 21-38 30 0.07 2 28-39 M 0.08 4.8-5.2 17 31-50 38 0.08 1 39 39 0.09 5.3-5.7 5.8-6.2 1 59 59 0.13 Total Average* 62 28 0.06 4 ::: 41 0.0<1 •The average total length of green sunfish males was 4.9 inches, females 4.4 inches; BG F, hybrid males 5.5 inches, females 5.0 inches. GR F] Hybrids vs. Green Sunfish and Red-Ear Sun- fish. -During the period August 1-15, 1958, 200 GR Fj hybrids averaging 4.6 inches total length, 200 green sunfish averaging 4.2 inches, and 200 red-ear sunfish averaging 3.5 inches were placed in a 1.1-acre pond that contained no other fish. During the spring of 1959 (April and May) some of these fish were taken by hook and line, some were trapped, and the rest removed by rotenone treatment of the pond. One hundred forty-nine GR Fj hybrids, 136 green sunfish, and 127 red-ear sun- fish were recovered. The numbers of fish recovered by each of the three methods of collection, the length- frequency distributions, and the range of weights are shown in tables 7, 8, and 9. The population densities in this pond were low. When the experiment was terminated, there were 26, 21, and 14 pounds per acre of GR Fj hybrids, green sunfish, and red-ear sunfish, respectively, in the pond; the fish ranged in length between 4.5 and 7.0 inches. This pond had in the past supported more than 150 pounds of sunfish per surface acre. Fish in this ex- periment grew at an exceedingly rapid rate; they in- creased in length about 2 inches (average) and nearly doubled their weight between August of 1958 and late May of 1959. The results of this experiment give addi- tional support to the concept that in uncrowded ponds there is little difference in the growth rates of hybrid sunfish and their parent species. 12 Table 7. — Numbers of red-ear sunfish, green sunfish, and hybrids of these species placed in a I.l-acre pond and num- bers and percentages of original-stock recovered approximately 9 months later. Category Red-Ear Sunfish Green Sunfish GR Fi Hybrids Number Per Cent Number Per Cent Number Per Cent Fish at stocking (August 1-15, 1958) 200 100.0 200 100.0 200 100.0 Fish removed by hook and line fishing (April 20-22, 1959) 3 1.5 24 12.0 23 11.5 Fish trapped and removed (May 19-22, 1959) 80 40.0 99 49.5 122 61.0 Fish recovered during rotenone census (May 22-25, 1959) 44 22.0 13 6.5 4 2.0 Total fish recovered 127 63.5 136 68.0 149 74.5 Fish unaccounted for 73 36.5 64 32.0 51 25.5 Table 8. — Length-frequency distribution of red-ear sunfish, green sunfish, and Fj hybrids of these species when placed in a 1.1-acre pond, August 1-15, 1958, and when recovered May 19-23, 1959- Numbers in parentheses show average lengths.* Red-Eai Sunfish Green Sunfish GR Fi Hybrids Total Length, Number at Number Number at Number Number at Number Inches Stocking, Recovered, Stocking, Recovered, Stocking, Recovered, Aug., 1958 May, 1959 Aug., 1958 May, 1959 Aug., 1958 May, 1959 (3.5 Inches) (5.4 Inches) (4.2 Inches) (6.2 Inches) (4.6 Inches) (6.4 Inches) 1.8-2.2 3 2.3-2.7 73 2.8-3.2 23 3.3-3.7 14 16 3.8-4.2 31 81 4 4.3-4.7 49 7 92 154 4.8-5.2 7 39 11 41 5.3-5.7 38 16 1 2 5.8-6.2 27 39 28 6.3-6.7 6 46 75 6.8-7.2 9 19 •The average length increment in the period was 1.9 inches for the red-ear sunfish, 2.0 inches for the green sunfish, and 1.8 inches for the GR Fj hybrids. Table 9.— Length-frequency distribution, range of body weights, and average body weights of red-ear sunfish, green sun- fish, and GR Fj hybrid sunfish that were recaptured May 19-23, 1959, from a 1.1-acre pond that had been stocked August 1-15, 1958, with 200 individuals of each kind. Red-Ear Sunfish Green Sunfish GR Fi Hybrids Total Length, Inches Number of Fish Weight Number of Fish Weight Number of Fish Weight Range, Grams Average Range, Grams Average Range, Grams Average Grams Pounds Grams Pounds Grams Pounds MALES 4.3-4.7 2 25-30 28 0.06 4.8-5.2 9 40-48 43 0.09 5.3-5.7 25 49-74 57 0.13 6 49-64 57 0.13 5.8-6.2 12 65-84 71 0.16 33 62-90 79 0.17 "i 83-95 87 0.19 6.3-6.7 5 88-100 93 0.20 44 72-110 97 0.21 46 80-110 98 0.22 6.8-7.2 9 111-150 126 0.28 16 110-132 119 0.26 FEMALES 4.3-4.7 5 28-36 33 0.07 4.8-5.2 30 34-48 44 0.10 5.3-5.7 13 42-60 53 0.12 11 47-67 51 0.12 2 72-86 79 0.17 5.8-6.2 15 60-83 73 0.16 5 65-99 T'-S 0.16 2^ 61-86 78 0.17 6.3-6.7 I 94 94 0.21 2 91-95 93 0.20 28 78-113 101 0.22 6.8-7.2 3 109-110 110 0.24 13 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 Pj crosses. From each of these crosses large numbers of Fj 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 P[ crosses was at- tempted two or more times. Only the G x B and R x G crosses produced significantly large numbers of Fj hybrids. Results of the other four Pj crosses were negative or inconclusive. 3. Of the six Fj 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 Fj and the GB Fj hybrids produced large num- bers of F2 hybrids; the GR Fj hybrids produced only a few Ft hybrids. The BG, BR, and RB hybrids failed to produce an F2 generation. 5. Varying numbers (200-10,000 per surface acre) of Fj 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 Fj hy- brids in these ponds could be made. 6. BG Fj 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 the experiment in this pond indicate that under low pop- ulation densities there is not much difference between the growth potentials of BG Fj 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 Fj 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 (6I 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 '■^chilling 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 AUerton Park allowed the use of certain ponds at the 4-H Memorial 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. N'isnow, assistant technical editor of the Illinois Natural History Survey. The photographs are the work of Dr. Bennett, Mr. William F. Clark, and Mr. Wilmer D. Zehr; Mr. Clark was formerly photographer and Mr. Zehr is presently photographer for the Natural History Survey. 14 LITERATURE CITED Hubbs, Carl L. 1920. Notes on hybrid sunfishes. Aquatic Life 5(9):101-3. 1955. Hybridization between fish species in nature. System. Zool. 4(l):l-20. Hubbs, Carl L., and Laura C. Hubbs 1931. Increased growth in hybrid sunfishes. Mich. Acad. Sci., Arts, and Letters Papers 13:291-301. 1932. Experimental verification of natural hybridization between distinct genera of sunfishes. Mich. Acad. Sci., Arts, and Letters Papers 15:427-37. 1933. The increased growth, predominant maleness, and apparent infertility of hybrid sunfishes. Mich. Acad. Sci., Arts, and Letters Papers 17:613-41. Krumholz, Louis A. 1950. Further observations on the use of hybrid sunfish in stocking small ponds. Am. Fish. Soc. Trans, for 1949, 79:112-24. Lagler, Karl F., and Charles Steinmetz, Jr. 1957. Characteristics and fertility of experimentally produced sunfish hybrids, Lepomis gihbosus and L. macrochirus. Copeia 1957(4):290-2. Luce, Wilbur M. 1937. Hybrid crosses in sunfishes. 111. Acad. Sci. Trans. 30(2):309-10. Moenkhaus, W. J. 1911. Cross fertilization among fishes. Ind. Acad. Sci. Proc. for 1910:353-93. Ricker, William 1948. Hybrid sunfish for stocking small ponds. Am. Fish. Soc. Trans, for 1945, 75:84-96. Thompson, David H. 1935. Hybridization and racial differentiation among Illinois fishes. 111. Nat. Hist. Surv. Bui. 20(5):492-4. (Appendix to Annotated list of the fishes of Illinois, by D. John O'Donnell.) 15 ' ■ •• w. ■<■••' :'■;■' ''■'.■{ '.M '-..■• i'-. ••■■,;■'■ in ■ ■ " ; ■ Vs,( ■ ' ■' •'- -.r. • ■ :• V" . „ •; ■ I . •■ •. -U-. . . . -t ' "AVI" ■-5':.',|.j