Whether they grow corn, or cattle – or fish, farmers are always looking for ways to get the most out of their crops. One recent trend in aquaculture is to manipulate the sex of fish populations in order improve profits. Raising either monosex populations, where all the fish are the same sex, or sterile populations, can provide substantial benefits to fish farmers.
Rainbow trout are the most widely raised food fish in Wisconsin, says Jeff Malison, director of the UW-Madison Aquaculture Program. However, over the past decade or so, fish farmers have also begun raising yellow perch, an especially popular fish for the restaurant trade. Raising monosex populations of either of these species offers several potential advantages.
In trout and perch, females are the preferred sex. Females grow bigger and faster than males do, resulting in larger filet yields and lower production costs. In addition, female fish are generally less aggressive than males, so an all-female population will be under less stress and will therefore be generally healthier and more resistant to disease than a mixed population. Finally, males of some species become sexually mature early and develop characteristics that lower their market value.
As part of an ongoing project, primarily funded by the University of Wisconsin Sea Grant Institute, Malison and his colleagues have developed methods to produce all-female populations of walleye and yellow perch. The process involves adding a form of the male hormone testosterone to the diet of young female fish. When these females reach maturity, they produce sperm, which is used to fertilize normal eggs. Since the sperm harbor female chromosomes, all the offspring from this pairing will be female.
Although producing all-female populations can provide benefits, Malison says there is one big drawback. When fish reach sexual maturity, they put a tremendous amount of energy into reproduction. In some species, the reproductive machinery can comprise up to a third of the fish”s body weight. This generally means a corresponding decrease in filet yield. One solution to this problem is to render the females sterile, so the energy never gets diverted to the reproductive process.
To come up with a method to produce sterile fish, Malison and his group have been delving into the world of polyploidy. Polyploid organisms have more chromosome sets than is usual for their species. Fish are normally diploid, that is, they have two chromosomes sets. Malison”s team set out to produce triploids, fish that have three chromosome sets and are typically unable to reproduce.
To induce polyploidy in perch and walleye, the researchers apply either heat shock or intense pressure to fish eggs soon after fertilization. This interferes with the division of the egg nucleus, so instead of retaining the usual two out of four parental chromosome sets, the egg retains three. The amount of heat or pressure, as well as the timing and length of exposure to these shocks, is all important. Too little can fail to produce triploids. Too much can kill many of the eggs.
According to Malison, this method of triploid induction works well for trout and walleye, but the shocks can reduce some of the usual resulting growth benefits in yellow perch. A potential alternative to this direct shock method is to create tetraploids, fish with four chromosome sets, and then mate them with the normal diploids. Assuming tetraploids will be fertile, pairing them with diploids should produce triploid fish.
The methods used to create tetraploids are similar to those used to produce triploids, except that the heat or pressure shocks are applied at a later developmental stage. Malison”s group has produced several hundred perch that have four chromosome sets. They are now waiting for them to mature, a long process, because the tetraploids seem slow-growing and weak. Malison says this is probably due to the shock they received rather than because of their altered genetic structure. If successful, the tetraploid/diploid pairing will result in triploid perch that never received heat or pressure shocks. The technique should eliminate the drawbacks produced by the direct shock method.