TRANSGENIC TECHNOLOGY IN FISHERIES/ AQUACURTURE

By:- Bijayalaxmi Nongmaithem / Ajit Kumar Ngangbam *

INTRODUCTION

The aquaculture industry is one of the fastest growing among animal food-producing sectors, increasing at an average rate of 8.8% per year since 1970. Global aquaculture produced 45.5 million metric tons of food fish excluding the aquatic plants valued at US$ 63.3 billion in 2000 with aquaculture contributing to about 43% of the total global fish production.

The contribution of aquaculture to global supplies of fish, crustaceans and molluscs continues to grow, increasing from 3.9% of total production by weight in 1970 to 32.4% in 2004. Aquaculture is growing more rapidly than all other animal food-producing sectors.

Worldwide, the sector has increased at an average compounded rate of 6.9% per year in terms of quantity since 1950, compared with only 1.2% for capture fisheries and 2.8% for terrestrial-farmed meat production systems.

Despite the predictions of a growing aquaculture industry, stagnant world capture fisheries and increased human population could lead to a global shortage of fish and fish products in the coming years. Use of biotechnology in aquaculture has the potential to alleviate these predicted fish shortages and price increases by enhancing production efficiency, minimizing costs and reducing disease.

Despite the increased production from aquaculture, there has been a decreasing trend in the average productivity of aquaculture farms primarily due to disease outbreaks. The presence of endemic pathogens, inadequate farm-management, environmental factors and poor water quality are the common causes of disease outbreaks in aquaculture farms.

Pathogen transfer due to international trade in live aquaculture animals and their products is also an important reason for major epizootics.

Disease outbreaks cause significant losses in aquaculture or fisheries production and trade and are affecting economic development of many countries. The recent estimates based on farm surveys in some Asian countries reported the annual losses due to disease in the region total more than US$ 3.0 billion.

Transgenic fish will be equally beneficial to aquaculture and is more effective than traditional breeding, techniques to develop new fish strains. In principle, the technology can be used among others to improve growth rate of the fish, control sexual maturation, sterility and sex differentiation, improve survival by increasing disease resistance against pathogen, adapt to extreme environment such as cold resistance and alter the biochemical characteristics of the flesh to enhance the nutritional qualities.

Successful attempts in the development of transgenic fish with increased resistance to diseases were reported in transgenic rainbow trout, Oncorhynchus mykiss with rainbow trout lysozyme, transgenic channel catfish, Ictalurus punctatus, transgenic medaka, Oryzias latipas and transgenic grass carp with human lactoferrin gene.

(Top) Non genetically engineered fish; (bottom) Genetically engineered fish/ Transgenic fish

TRANSGENIC ORGANISMS

The term "transgenic” refers to "an individual in which a transgene i.e an isolated gene sequence used to transform an organism has been integrated into its genome”.

Several different terms have been used for referring to organisms carrying an altered genetic composition due to the insertion of foreign genetic material through the use of genetic engineering or recombinant-DNA techniques. These include ''genetically modified/engineered organism'', ''transgenic/cisgenic organism'', ''recombinant-DNA organism'', ''living modified organism'', ''organism obtained through the use of biotechnology''.

The important practical applications of transgenesis in animals intended for food are listed below:

• Optimization of feed digestion to increase conversion efficiency and reduce environmental pollution.
• Increase of growth rate, milk production, litter size, etc.
• Improvement of food quality attributes such as tenderness, taste, texture and flavor.
• Resistance to diseases including human zoonoses thus potentially minimizing the use of antibiotics and other pharmaceuticals, enhancing animal performance, simplifying breeding and increasing the productivity of farm reared animals.
• Modification of carcass structure, milk or egg composition for nutritional or health benefits. An example to this approach is optimizing nutrient content, reducing the content of allergenic or toxic substances.

TRANSGENICS IN AQUACULTURE

A major problem facing in the aquaculture industry is outbreak of disease, as farmed fish are generally cultured at high densities and under stress, putting them at increased risk for bacterial infection. Fish are susceptible to numerous harmful bacterial infections.

Antibiotics can help provide disease resistance, but only a limited number were approved for use in aquaculture. Although there are effective vaccines available for some diseases, no effective methods are available for many common diseases. Also, the use of DNA vaccines is often labor-intensive and can cause high stress to the fish owing to excessive handling. A promising alternative involves the use of transgenic technology to produce strains of fish with increased disease resistance.

The first successful development of a genetically engineered or transgenic fish was reported in 1985, with human Growth Hormone gene microinjected into the fertilized eggs of goldfish (Carassius auratus). A DNA construct consisting of human growth hormone under control of the mouse metallothionein promoter was injected into the germinal disc of an early-stage goldfish C. auratus embryo.

This was followed by successful development of transgenic loach (Misgurnus anguillicaudatus) with human Growth Hormone gene. Many genetically engineered fish species have been developed since 1985, along with various methods for inserting foreign gene to fish such as microinjection, electroporation, infection with pantropic defective retroviral vectors, particle gun bombardment and sperm and testis-mediated gene transfer methods.

Till now, fish including Indian major carps, goldfish, Atlantic, coho, and chinook salmon, rainbow and cutthroat trout, tilapia, striped bass, mud loach, channel catfish, common carp, Japanese medaka, northern pike, red and silver sea bream, walleye and zebrafish have been genetically modified to produce select traits such as increased growth, increased feed conversion efficiency, cold tolerance, and disease resistance.

CONCLUSION

The rapidly increasing human population has resulted in demand for increasing amounts of food, not only food derived from agriculture, but also food from natural aquatic ecosystems and from aquaculture farms. The food derived from aquaculture includes cultured algae or seaweeds, finfish and shellfish.

Worldwide, the demand for fish continues to increase at a higher rate than wild fish population can support. Current fishing practices are proving increasingly unsustainable because of the diminishing global fish stocks due to over exploitation and outbreaks of disease, thus threatening biodiversity.

These concerns on depleting natural fish stocks, together with an ever increasing global market for food fish have led to a series of technological innovations such as transgenics and DNA vaccine technology to achieve increased production from aquaculture.

---

* Bijayalaxmi Nongmaithem (College of Fisheries, Tripura) and Ajit Kumar Ngangbam (PhD scholar, Department of Microbiology, UNESCO Centre for Marine Biotechnology, College of Fisheries, Mangalore) regularly contributes to e-pao.net . Ajit Kumar Ngangbam can be contacted at ajit_b2007(at)yahoo(dot)co(dot)in
This article was webcasted at e-pao.net on 18th June 2009.

---