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Transgenic Chinook salmon from the New Zealand King Salmon Company. The top 3 fish are transgenics: 11 months old with an average weight of 850g, while the bottom fish is a non-transgenic sibling of the same age, weighing 280g
Transgenic Chinook salmon from the New Zealand King Salmon Company. The top 3 fish are transgenics: 11 months old with an average weight of 850g, while the bottom fish is a non-transgenic sibling of the same age, weighing 280g
Courtesy of Seumas Walker

Introduction

Genetic biotechnologies in aquaculture focus primarily on increasing growth rate. They can also focus on improving disease resistance and increasing environmental tolerance. There are several biotechnologies that can be readily applied to farmed aquatic species.


Selective breeding

Selective breeding, i.e. traditional animal breeding, started with the common carp several thousand years ago. However, it has only recently been applied to a handful of other species of food fish such as catfish, trout and tilapia. As a result, many farmed aquatic species are still very similar to their wild relatives. Selective breeding programmes have however yielded significant and consistent gains; for instance a 5-20% improvement in growth rate per generation in farmed species such as Atlantic salmon, catfish and tilapia.


Manipulation

Chromosome-set manipulation is a technique that can be used to produce so-called 'triploid' organisms that have three sets of chromosomes instead of the usual two. Triploids generally can not reproduce and so it was initially thought that the energy that was not channelled into reproduction would go instead to increasing growth rate, but this has not in fact proven to be the case. The advantage of triploids seems to be rather in their functional sterility (although that does not reach 100%). For example, triploid oysters do not produce gonads and are therefore marketable at times of the year when mature oysters have an off-taste because of gamete production ('gametes' are sex cells -- the ovum, or egg in the female and the sperm in the male).

Monosex culture
In aquaculture, one sex is often more desirable for the market than the other. For example, female sturgeon produce caviar, male tilapia grow faster than females, female trout and salmon generally grow faster than males. The production of single sex groups of fish takes advantage of this sexual dimorphism and can be accomplished by manipulation of the developing gametes and embryo. The manipulation can be in the form of DNA denaturisation in the gametes followed by chromosome-set manipulation or alternatively, by hormonal sex-reversal and subsequent breeding.

Administering appropriate hormones can change the phenotypic (physical) sex of many aquatic species. For example, genetically male tilapia can be turned into 'physical' females through estrogen treatments. These genetic males, that are physically female, are then mated with normal males to produce a group of all-male tilapia that grow faster and have less unwanted matings than a group of mixed-sex tilapia. (In mixed sex culture of tilapia, mating at a small size often leads to overcrowding and stunting). Some of the all-male offspring of this process have two male chromosomes and these fish can be used in turn as broodstock for subsequent generations. A big advantage of this technique is that all-male populations can be produced in those subsequent generations without the use of any hormones.

Hybridization

Hybridization is another simple genetic technology that has become easier with the development of artificial breeding techniques, such as the use of pituitary gland extract and other hormones to initiate gamete development and induce spawning (the release of fish eggs). An increased understanding of environmental cues that influence reproduction, such as day length, temperature or water current has also played an important part in improving breeding programmes. Fish farmers can now overcome many of the natural reproductive isolating mechanisms that species develop in the wild. Hybridization can also be used to produce single sex groups of fish when the sex-determining mechanisms in the parental lines are different (for example, hybridization of Nile tilapia and the blue tilapia).

Technological improvements

These improvements in reproductive technologies have also assisted aquaculturists greatly in their efforts to domesticate aquatic species. In addition, by making it possible to remove the natural constraints and timing of breeding, farmers are able to mate many more species at the times that are most beneficial, and thus help to ensure a steady and consistent supply of fish to the market.


Growth of transgenic technology

Genetic engineering is a vague term that has come to be nearly synonymous with gene transfers i.e. the production of transgenic fish or genetically modified organisms (GMOs). This technology is progressing rapidly and it is now possible to move genes between distantly related species; for instance, a gene that produces an anti-freeze protein has been transferred from the winter flounder to strawberries. Gene transfer in fish has usually involved genes that produce growth hormone and has been shown to increase growth rate dramatically in carp, catfish, salmon, tilapia, mudloach and trout. The same anti-freeze gene that was put into plants was put into salmon in the hopes of extending the farming range of the fish. The gene did not produce enough of the protein to extend the salmon's range into colder waters, but it did allow the salmon to continue growing during cold months when non-transgenic salmon would not grow. Transgenic technology is currently in the research and development stage; to our knowledge there are no transgenic aquatic plants or animals available to the consumer.

 
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