SOME CURRENT NORTH AMERICAN WORK IN HYBRIDIZATION AND SELECTION OF CULTURED FISHES

by

K.E. SNEED
Bureau of Sport Fisheries and Wildlife
Stuttgart, Arkansas, U.S.A.

1 INTRODUCTION

Fish genetics in terms of ordinary Mendelism is when fish of two variable types are mated or crossed and the filial generations either mated together, backcrossed to a parent type or outcrossed to a third type. This approach to fish genetics is easily understood and usually gives rise to results in a short period of time. A similar definition can be applied to hybridization. That is, two different species, genera or families can be crossed and the first filial generation then crossed, backcrossed or outcrossed to give the hybrid of desired qualities.

The simplicity of these definitions and the practices involved are relatively easy to understand and can be applied by the average fishery biologist or trained fish-culturist.

However, such simple terminology and easily understood principles do not apply to genetic selection where large populations rather than individuals are involved. According to Butler (1961), the total elimination of a recessive gene in a fish population of 100 000 in which one percent of the population carries the double-recessive gene would require hundreds of generations, if all the double recessives were completely harvested from each generation (90 generations to reduce the number to 0.1 of one percent) and infinitely longer for total elimination. Hence, let us keep our thinking for this seminar confined to fish culture and management through selection and hybridization of individuals, not populations.

It is biologically easy and mathemetically provable that both undesirable and desirable characteristics can be introduced quickly through fish culture into a standing population. The elimination of an undesirable trait in fishes is very difficult. Hence, the ordinary fishery biologist carries a burden of which, apparently, he is unaware. If he acts wisely, many generations will be grateful. If he acts unwisely, and promiscuously introduces unwanted characters, whether it be hybrids or “selected” varieties, tens of generations would suffer by it.

Already, it is too late to turn back in some instances - 15 or more hybrid catfishes may have escaped into the Cahaba River drainage of Alabama; trout adapted to the hatchery environment have been released to the wild in several states (fortunately the survival is nil); salmon that do well under cultivated conditions have been sent to sea; and pike, tilapias, sunfishes and bass hybrids have been released promiscuously (and I emphasize the word) to further cloud the biological aspects of natural populations. What the results will be only a very wise man can determine. At any rate, let us be responsible biologists, as surely as those who tamper with the environment are responsible for their actions.

The importance of North American work in hybridization and selective breeding of fishes probably lies not in previous efforts in this field, but in what is being done now and planned for the future.

The practice of fish culture itself is a notable selector of fish. Fish in their natural habitat are adapted to their environment as a result of physiological and morphological mechanisms acquired through millions of years of evolutionary influences. These adaptations developed slowly, except in times of catastrophic change or sudden mutation, but the impact of the fish cultural environment has been and is severe and rapid. Artificial selection may operate somewhat independent of the environment, but only briefly. Often we think that the hatchery environment is relatively safe and stable; it is not necessarily true, especially in the pond culture of warm-water species.

The species now being cultivated around the world are newly introduced to fish cultural facilities, where environmental conditions are quite different from those in their natural habitat. Pond, lake, raceway or aquarium conditions impose different, and usually severe stresses upon the fish. Only those with adaptive attributes and a viable genetic make-up survive and thrive. The intolerant individuals or populations either die or are discarded by the fish-culturist.

2 FISH GENETICS LABORATORY

The Bureau of Sport Fisheries and Wildlife recently established the Fish Genetics Laboratory at Beulah, Wyoming, to study the genetics of cold-water fish. Construction of the planned facilities for selective breeding of fish is not completed. However, the completion of one building replacing an old hatchery destroyed by flood in 1965 has permitted a limited research programme during 1967.

The studies initiated have been dictated largely by expediency. From the many possibilities for profitable research on fish breeding the laboratory staff selected a diversified few to pursue with its small staff and interim facilities. Preliminary evaluation of these will be possible within a few years.

2.1 Rainbow trout breeding

2.1.1 Procedures and definitions

The current breeding programme can be divided into three general areas: (i) identification of variants exhibiting simple Mendelian inheritance, (ii) development and maintenance of random bred and inbred stocks, strains, or lines, and (iii) selective breeding for specific characteristics such as rapid growth and early maturity. The first is simply a matter of testing the potential Mendelian character by a series of crosses, backcrosses, intercrosses and incrosses. The second area involves selection of healthy individuals without regard to any special phenotypic character, from either unrelated or closely related stocks. The selective breeding phase requires most of the effort in the breeding programme. The fish are selected primarily on the basis of performance in respect to the character under selection. However, other factors such as viability, especially in the early phases of evaluation, may decide if the lot is retained.

Two-way selection is practiced, mainly for control purposes. However, it is possible that fish possessing a character developed by downward selection could have intrinsic value, particularly for use in research laboratories.

The terms individual (often called mass), sibling, half-sibling and progeny testing are used to identify methods of selection. These terms simply indicate criteria used to evaluate the breeding worth of the mass lot. Individual selection refers to selection based solely on the individual's own performance or phenotypic value. In other methods, the broodstock is chosen on the basis of their relatives' performance, e.g., the selection of potential breeders for DDT susceptibility on the basis of the response of their brothers and sisters to DDT exposure. Sibling, half-sibling, or progeny selection are forms of family selection, but the term “family” is also used where families (usually sibling lots) are retained or rejected as units in accordance with their mean phenotypic value. Here, the selected individual's own performances contribute to the selection. The method of selection is determined by the character of interest, availability of rearing space, and relationship of individuals in the group under selection. All these methods and a combination of methods are used when feasible.

Both factorial (often called diallel) and single matings are made for most purposes. In a given amount of space, the single mating approach provides greater potential diversification among the progeny since more parents are represented. However, the factorial design (each of a group of males mated to each of a group of females) is a useful tool for evaluating parental contribution as well as lots. A factorial mating produces a number of half-siblings for simultaneous evaluation. For example, a complete 4 × 4 factorial (4 pairs of parents) produces 16 sibling lots, and the fish in each lot are half-siblings to fish in 6 other lots.

A dry diet, open-formula granules and pellets, is fed in accordance with the premise that the feeding rate is inversely proportional to the cube root of the average weight of the fish (hatchery feeding charts reveal this relationship). The weight of daily rations for a given lot of fish equals the total weight of the fish load times a feeding factor or constant divided by the cube root of the average weight of the fish. Thus the formula is .

For example, employing a feeding constant of 0.1 (K), 10 kilograms of fish (W_t) averaging 8 grams in weight (Wa) are fed 0.50 kilogram per day; 10 kilograms of 64-gram fish are fed 0.25 kilogram. Using a slide rule for the calculations, this method provides a rapid means for determining the amount of feed without the aid of charts. A relatively high feeding factor was selected to prevent masking of genetic growth potential that might occur as a result of underfeeding. The use of this factor yields feeding levels that are essentially the same as those recommended by Deuel et al in 1952.

2.1.2 Inheritance of albinism

Albinism in rainbow trout appears to be a simple autosomal recessive. Several male and female albino and wild-type (normally coloured) fish were mated in all combinations in December 1966. Albino × albino crosses produced all albino progeny; wild × albino and wild × wild resulted in all normally coloured fish. Progeny sibling lots derived from each type of cross are being reared to maturity for further test crosses. Recently, precocious F₁ male progeny from wild × albino crosses were mated with F₁ albino females. The backcrosses yielded albino and wild-type embryos in numbers closely approximating the 1:1 ratio expected in classic Mendelian inheritance.

2.1.3 Breeding for genetic diversity

This work involves the breeding and maintenance of random-bred stocks to preserve heterozygosity. Unrelated fish (individuals not known to be related) from 1964 or 1965 year class New Zealand, Sand Creek, De Smet, Donaldson, and Arlee albino stocks were mated during the 1967 breeding season to produce the random bred stocks for 1967. Similarly, broodfish from the above and 1966 Wytheville and Manchester stocks are being mated to yield current year class stocks. These year class stocks are being reared and maintained for future random breeding.

2.1.4 Breeding for genetic uniformity

The purpose of this work is to develop several different strains or lines characterized by genetic uniformity within lines and genetic diversity between lines. Sibling lots of New Zealand, Sand Creek, Donaldson, De Smet, and Arlee albino stocks were produced during the 1967 breeding season. Mating activities for 1968 season have provided sibling lots of New Zealand and Sand Creek rainbows that are unrelated to 1967 lots. This year's matings also produced sibling lots of Manchester and Wytheville stocks. A few lots of each stock and year class are being reared to maturity for brother-sister inbreeding.

2.1.5 Selective breeding for year of maturity

During the 1967 spawning season, trout that had matured in two years from Sand Creek, New Zealand, and Donaldson stocks were mated. Also, trout that matured in three years from De Smet, Sand Creek, and New Zealand stocks were crossed with males of uncertain age at maturity from the same stocks. Approximately 30 lots are being reared to maturity for selection purposes.

Selective breeding for the 1968 season was initiated with matings of two-year old maturing fish from Wytheville and Manchester stocks. Fish selected from 1965 Sand Creek, New Zealand, and Donaldson stocks that did not mature last year are being held for possible matings as three-year old (or older) maturing phenotypes.

2.1.6 Selective breeding for growth

Sibling lots were produced by random matings of Sand Creek, New Zealand, Donaldson, and De Smet stocks. The growth of 200 lots was measured for a period of 13 weeks following swim-up. The average weight attained by the best growing lot was about eight times greater than that of the poorest. Seventy lots were retained for further evaluation. Subsequent selection reduced the number of lots to 24 by the end of the year. The eightfold maximum difference in average weight has remained relatively constant since initial selection. Most of the remaining lots will be reared for another year.

Production of 1968 sibling lots has been underway since September. Broodfish from several stocks reared under similar conditions were individually selected on the basis of attained size and are being mated. Evaluation of the early growth of approximately 100 lots has been initiated.

The substantial variation observed between the average weights of sibling lots and in the individual sizes of broodfish is very encouraging. It suggests a considerable amount of genetic diversity which, of course, must be present for successful selective breeding.

2.1.7 Selective breeding for tolerance of crowded rearing facilities

Randomly selected groups of fish from stocks of 1965 Donaldson and 1966 Manchester, New Zealand, Sand Creek and 1967 Manchester rainbows are being reared in heavily loaded circular tanks. The average growth of fish in these tanks is reduced significantly and some mortality occurs. The fish will be maintained under these conditions as long as space is available. Surviving individuals exhibiting extremes of growth will be selected for breeding. The 1966 Manchester stock was the only group containing mature fish before the end of the year. About 50 matings of selected individuals have been made.

2.1.8 Selective breeding for formalin tolerance

Randomly selected broodfish from 1964 and 1965 stocks yielded a number of 1967 sibling lots for evaluation. Sample fish from 170 lots were exposed at 14 weeks after swim-up to formalin concentrations of 175 and 525 microlitres/litre for 6 hours. Resistant lots suffered less than 50 percent mortality at the high concentration while the low concentration killed more than 50 percent of the fish in susceptible test lots. Siblings or survivors from the more resistant and susceptible lots are being retained for future breeding.

Breeders for 1968 are being selected and mated in accordance with their own or their progeny's response to formalin. Also, mating of unselected broodstock is producing additional sibling lots for evaluation. By the end of the year a total of about 200 sibling lots was on hand, scheduled for testing in the spring.

2.1.9 Selective breeding for DDT tolerance

The 170 sibling lots of the 1967 year class evaluated for formalin tolerance were also tested for their response to DDT. Randomly selected test fish were exposed to DDT concentrations of 13.3 and 40 microlitres/litre for 12 hours. The difference in tolerance of susceptible and resistant lots was similar to that noted for formalin. Generally, DDT-resistant fish were from lots that exhibited relatively poor formalin resistance. Selected lots are being reared for future breeding.

Here, as in the case with formalin, groups of the 1968 breeders are being selected on the basis of individual or progeny performance. Sibling lots produced by these parents and additional lots from unselected parents will be evaluated in the spring.

3 WARM-WATER FISH CULTURAL LABORATORIES

3.1 Catfish hybridization

For some time, sportsmen and biologists have recognized the importance of hybrid fishes for sport and food (Smith, 1961). In many instances, hybrid animals possess desirable characteristics such as rapid growth, tolerance to unfavourable environmental conditions, resistance to disease, and other attributes not always apparent in either parent species. At the same time, a hybrid animal may not possess undesirable characteristics found in one or both of its parents. Catfishes are increasing in importance in warm-water areas of the world. Since hybrid catfish appeared to be of value in fish husbandry and management in the United States, research was begun to develop methods to produce and culture the hybrid catfishes and to make preliminary observations on their growth and desirability as compared to the parent species.

The catfish hybridization work was begun about five years ago by Harry K. Dupree, O.L.Green and Kermit Sneed at the Southeastern Fish Cultural Laboratory, Marion, Alabama. Most of the work in this field, and it is rather extensive, is being continued at Marion by Dupree and Green.

During April to mid-June, the normal spawning season for catfishes in south central United States, gravid channel catfish, Ictalurus punctatus; white catfish, Ictalurus catus; blue catfish, Ictalurus furcatus; flathead catfish, Pylodictus olivaris; black bullhead catfish, Ictalurus melas; yellow bullhead catfish, Ictalurus natalis; and brown bullhead catfish, Ictalurus nebulosus, were paired with sexually mature males of the same species and approximately equal size. Females were injected with fish pituitary or human chorionic gonadotropin to induce spawning (Sneed and Clemens, 1959 and Clemens and Sneed, 1962). After spawning had commenced the female fish was netted, subdued with hand pressure around the caudal peduncle, and then held in a normal upright position, tail slightly below the head. By a gentle stroking of the abdomen, the eggs were stripped into a water-filled, plastic-film-lined basket. At the same time, small portions of excised, macerated testes from another species of catfish were dabbed into the water above the extruded eggs to bring about fertilization. After the desired number of fertilized eggs were obtained, the egg mass was left undisturbed for approximately 10 minutes to water-harden. The water-hardened egg mass was then detached from the plastic film, placed in a woven-wire enclosure and positioned in a mechanical hatching device. Care of the eggs during embryonic development, hatching and yolk-sac period, and initial feeding stages of hybrid fry were similar to published techniques for channel catfish.

These techniques were used to produce 24 different hybrid types from 7 common species of freshwater catfish. Based on our observations to date, we predict that the remaining hybrid catfishes can be produced except possibly those involving the female flathead catfish. All our attempts to produce hybrids from eggs of flathead catfish and sperm from any of the 6 species of catfish resulted in infertile eggs or eggs which disintegrated a few days after apparently being fertilized. However, the sperm from flathead catfish was used successfully to fertilize the eggs of the other catfishes. We have no adequate explanation at this time.

3.2 Growth comparison of catfish fingerlings

A series of 3 experiments was conducted in aquaria over a 3-year period starting in June 1965. Test fish for each experiment were hatched over a 5-day period and when stocked averaged 0.5 to 1.5 g each. In Experiment I all categories of fish were offered equal amounts of feed; those in Experiment II were offered equal rates of feed, and those in Experiment III were offered ad libitum amounts of feed. Fish were re-weighed at 3-week intervals and revised feed allowances calculated. At the termination of the 9-week experiments, weight gain and feed conversions were calculated.

Results of three experiments that compared six species of catfish with their hybrids on the basis of weight gain, growth potential and feed conversion indicate tentatively that some of the hybrids have possibilities as sport or food fishes (Table 1). Also by utilizing selection and genetic studies, we speculate that growth rate and feed efficiency can be further increased. However, all factors considered, it appears even without selection and genetic studies that the obviously fast growth rate and efficient feed utilization of the white catfish × channel catfish hybrid make it a desirable fish for intensive culture. We have also observed in our research ponds that such hybrid fish weighing up to several pounds are more robust than the channel catfish and lack the large bull-head of the white catfish. In addition, it appears from some of our preliminary work that this hybrid has a greater tolerance to low oxygen than either the channel catfish or white catfish species. This in itself would make this fish desirable in (intensive) cultures since low levels of dissolved oxygen associated with the large feed allowances represent the first limiting factor in production and the greatest threat of mortality.

We have also observed that some catfish species and their hybrids perform differently in aquaria than in ponds. Possibly some of this difference is due to natural behaviour, i.e. sensitivity to unnatural light and sounds or limitation of space inherent in aquarium facilities. A notable example is the blue catfish which competes actively with channel catfish in managed ponds and in natural lakes and rivers but performs poorly in aquarium experiments (Table I). Others of these hybrid groups which perform poorly in aquarium studies may perform well in managed ponds. Thus, before any hybrid can be discounted for use in sport-food cultures, preliminary tests should be conducted in ponds incorporating variables such as stocking and feeding rates and rates of water exchange.

Further studies are in progress on the spawning and culture of the hybrid catfishes. The blue catfish × channel catfish hybrids have reproduced for two seasons, and it is believed that many of the other hybrids will also reproduce. We also have work in progress that includes taxonomic studies, serum and tissue protein comparisons, and oxygen requirements. Whether or not hybrid catfish take a significant place in warm-water fish husbandry remains a subject of continuing research.

4 REFERENCES

Butler, L., 1961 The genetics of selection in fish populations. Can.Fish Cult., 29:21–4

Clements, S.P. and K.E. Sneed, 1962 Bioassay and use of pituitary materials to spawn warm-water fishes. Res.Rep.U.S.Fish Wildl.Serv., (61):30 p.

Deuel, C.R., et al., 1952 The New York State fish hatchery feeding chart. Fish.Res.Bull., N.Y., Conserv.Dep., 3:61 p.

Smith, S.B., 1961 Selectivity and hybridization in management of fish stocks. Can.Fish Cult., 29:25–30

Sneed, K.E. and H.P.Clemens, 1959 The use of human chorionic gonadotropin to spawn warm-water fishes. Progr. Fish-Cult., 21(3):117–20

Table I

Percent weight gain and feed conversion of 6 catfish species and 15 of their hybrids

1. All hybrid categories are first generation except where otherwise noted.
2. Feed conversion equals weight of feed offered (dry weight)/weight gain (wet weight).
3. Results averaged from triplicate aquaria of fish offered equal amounts of feed over a 6 week period. Feed allowances for all aquaria, 4 percent body weight per day, adjusted at the end of each 2-week period based on the average weight of the heaviest triplicated category of fish.
4. Results averaged from duplicate aquaria of fish offered feed at the rate of 3 percent body weight per day over a 9-week period. Feed allowances for each pair of aquaria recalculated each third week based on the average weight of fish in the paired aquaria.
5. Results averaged from duplicate aquaria of fish fed ad libitum amounts of feed over a 9-week period.