The main reasons for genetic manipulation of species used in aquaculture are all directly connected to improved output/input ratios. They are:
a) to enhance growth and/or efficiency of food conversion,
b) to enhance commercially significant flesh characteristics,
c) to control reproductive activity and/or sexual phenotype,
d) to increase resistance of species to pathogens/parasites,
e) to increase tolerance to/of environmental variables such as temperature,
f) to modify behaviour, e.g. aggression, and
g) to control fertility and/or viability.
While all of these targets are desirable in aquaculture (though to a variable extent depending on the species being considered), work up to now has been focused primarily upon points a. And e.
Melamed et al. (2002) provide a useful commentary on some of these applications.