Often we get asked how fish are stripped of there eggs and mass produced etc....for those that have not seen this sort of thing this is how eggs are often fertilized in a mass production scenario.
When it comes to hybrids works the same just with two different species.
Keep in mind, we would never do this in our fish room but this is mostly done for food production for people and these farms are important ...it is also done in the ornamental trade though as well.
Not all fish have to be killed to be stripped, but apparently these african Clarias do...most North American fish (trouts, salmon etc.)can be stripped(that is lightly squeezing eggs & sperm out of males and females) without killing the fish.
Also talking to someone that has experimented with large synos they strip without killing apparently.
The normal way is that the injected hormones are made in the pituariry (I might have misspelled this word) and in fact, quite often a pituariry extract is given. The hormones are released into the blood, and reach in a few seconds or minutes the gonads
For injection the spot will not matter that much - as the hormanes are distributed all over the body anyway.