One approach to the genetic improvement of aquatic organisms that has emerged as a discipline in its own right over the past two decades is transgenesis, the transfer of foreign genes into new hosts.

Transgenic fishes (or molluscs or crustaceans) can be defined as possessing within their chromosomal DNA, either directly or through inheritance, genetic constructs which have artificial origins. The key word for researchers, producers and even consumers here is within the chromosomal DNA: introduced constructs are incorporated into the target organism in such a way as to be expressed and passed along to subsequent generations.