1. Technical Field
The present invention relates generally to methods of producing sterile fish, and more particularly, to method of inducing sterility in fish by altering developmental migration of gonadotropin-releasing hormone (GnRH) neurons.
2. Description of Related Art
In recent years the world has witnessed an alarming decline in commercial fisheries, the result of over fishing of wild fisheries stocks and indirectly, the failure of commercial aquaculture to reduce demand for fisheries products (i.e., by a sufficient increase in commercially farmed fish products). According to the Food and Agriculture Organization (FAO) of the United Nations, nearly 70% of the world's commercial marine fisheries species are now fully exploited, overexploited or depleted. Based on anticipated population growth, it is estimated that the world's demand for seafood will double by the year 2025. Therefore, a growing gap is developing between demand and supply of fisheries products, which results in a growing seafood deficit. Even the most favorable estimates project that in the year 2025 the global demand for seafood will be twice as much as the commercial fisheries will harvest.
The same trend exists in the United States marketplace. Per capita seafood consumption is on the rise, but United States seafood harvests are not increasing to meet the demand. Moreover, only 10% of the seafood consumed in the United States comes from domestic aquaculture and the United States ranks only tenth in the world in the value of its aquaculture production. As a result, the United States is overwhelmingly dependent on imported seafood, such as gilthead, seabream, with more than half of its supplies coming from overseas.
Worldwide, it is estimated that in order to close the increasing gap between demand and supplies of fisheries product, aquaculture will need to augment production five-fold during the next two and half decades. While there is a need to increase global and United States aquaculture production, it is clear that fish farming must develop as a sustainable industry without having an adverse impact on the environment.
In commercial fish species where sexual maturation occurs before the fish has reached market size, energy is spent on gonadal growth instead of muscle growth. Sterility increases the conversion of food energy to muscle (thereby resulting in larger fish fillets) and minimizes food energy diverted for development of the gonads. However current method of fish sterilization carries environmental issues, such as escape of cultured fish possess that may threaten the ecological imbalance or genetic contamination of wild populations. This threat will become even greater as transgenic fish are raised in commercial operations in the United States and abroad.
Aquaculture experts around the world agree that a mechanically simple, but effective, process that bypasses the traditional modes of inducing sterility would increase production efficiency, profitability and biosecurity in commercial aquaculture. Sterilizing transgenic or genetically-selected fish will minimize the possibility of potential aquacultured escapees propagating in the wild, especially in light of alarming reports of interbreeding between escaped animals and wild populations of the same species seem to be increasing in areas of intensive farming (Volpe et al., 2001; Carr et al., 1997).
Several methods currently exist that are used to induce sterility in fish. Chromosome set manipulation for the production of triploid sterile populations is used but it is a cumbersome procedure that must be individually developed for each species. Furthermore, generation of triploids does not always result in sterility. In induced-triploid rainbow trout (Arai, 2001) and Atlantic salmon (Donaldson and Benfey, 1987), males were seldom completely sterile. Because they maintain most of their endocrine competence, these salmonids also exhibit secondary sex characteristics, and as a consequence are susceptible to disease and exhibit no improvement in growth over diploids. Likewise, female triploid Atlantic salmon are commonly found to have a few normal oocytes within the ovarian matrix (Johnstone, 1993). Like CSM, inter-hybridization (or hybrid production) is a labor-intensive process that does not always result in sterility, as is clearly the case with the hybrid striped bass.
As a result of the above constraints, the production of sterile fish, although considered highly beneficial to commercial aquaculture, has not yet been developed for mass use in the industry. Thus, it would be advantageous to develop a method and system to induce permanent sterility in fish grown in commercial operations that overcome the problems of the previous complex and unsuccessful methods used for sterilization.