Eggs incubate in hatchery at Aurora lab 



Photo by Doug Yoder 



the biggest and best hybrid offspring. 

 Unlike many hybrids, this cross is 

 capable of reproducing. The cloning 

 also would alleviate the need for ac- 

 quiring or maintaining two different 

 parental stocks. 



To make triploids, Kerby does not 

 alter the white bass sperm, which 

 allows the male to contribute 24 

 chromosomes. But after fertilization, 

 the striped bass eggs are again heat- 

 shocked to retain the extra set of 

 chromosomes. Therefore triploids have 

 72 chromosomes and are, in theory, 

 sterile. 



This sterile hybrid has several ad- 

 vantages, Kerby says. If the hybrids 

 are accidentally or purposely set free in 

 the wild, they would not crossbreed 

 with natural stocks to alter their 

 genetics. "Some scientists speculate 

 that the sterile hybrids grow faster 

 because none of their energy is chan- 

 neled into reproduction," Kerby says. 



Next year, Kerby will work toward 

 creating tetraploids, or hybrids equip- 

 ped with 96 chromosomes. If these fish 

 are not sterile, the tetraploids could be 

 crossed with diploids to produce 

 triploids naturally. "Creating a brood 

 stock of tetraploids would be an easier, 

 more practical way to produce 

 triploids," Kerby says. 



All of this genetic manipulation is 

 naught if the hybrids don't thrive and 

 grow under culture conditions. But in 

 the case of the striped bass hybrids, 

 grow-out is not a problem. Kerby 

 stocked the fish in earthen ponds, in 

 freshwater flow-through circular pools 

 and in floating cages placed in brackish 

 estuarine waters. In all cases, Kerby 

 was able to produce marketable fish 

 (l'/2 pounds) in 14 to 18 months, and 

 the survival rate for hybrids stocked as 

 fingerlings exceeded 70 percent. 



At the experiment's completion, the 

 researchers harvested 5,291 pounds of 

 fish per acre from the ponds and 60 

 kilograms per square meter from the 

 cages. The pond harvest of hybrids 

 compares favorably with catfish 

 culture. For the lower-valued catfish, 

 4,900 pounds of fish per acre is con- 

 sidered commercially viable. 



To boost growth even more, Margie 



Gallagher, a UNC Sea Grant re- 

 searcher at East Carolina University, 

 is working with the protein/energy 

 ratios in fish feed. "In fish, the require- 

 ment for protein is complicated by the 

 fact that dietary protein can and is 

 used as an energy source," Gallagher 

 says. "Therefore in studies of protein 

 requirements of fish, it is necessary to 

 look not only at the effect of protein 

 levels on growth and metabolism, but 

 also the effects of protein/energy levels 

 on these processes." 



Gallagher's research with elvers, or 

 baby eels, showed that diets too high 

 in energy restrict the fish's feeding to 

 the level where the elvers don't meet 

 their protein requirements. But if the 

 energy level is too low, protein will be 

 wasted for use as energy instead of be- 

 ing used for growth. These findings 

 confirm earlier research on other 

 species: fish eat to meet energy needs 

 rather than their protein needs. 



Focusing on striped bass hybrids, 

 Gallagher is looking for that optimum 

 ratio of energy to protein needed to 

 spur growth. The proper ratio will not 

 only result in larger, faster-growing 

 hybrids, but it also will lead to a better 

 and more cost-effective feed for the 

 hybrid. 



As in other forms of aquaculture, 

 feed costs are a major part of the total 

 cost of production. Because striped 

 bass hybrids require a high protein diet 

 and protein is the most expensive in- 

 gredient in their feed, it's very impor- 

 tant to maximize its return. 



As Sea Grant scientists work out the 

 genetics and feed requirements of 

 striped bass hybrids, many believe the 

 fish is ready to leave the laboratory. 

 The striped bass is no longer a poten- 

 tial aquaculture candidate, scientists 

 say, but one on the verge of commer- 

 cial success. 



— Kathy Hart 



