Reason behind selection of sites of administration of pituitary extracts?
Melatonin is an evolutionary highly conserved molecule that plays an important role in conveying the clock and calendar information to all living organisms, including man. The hormone is synthesized mainly by the pineal gland, and, to a lesser extent, by extrapineal tissues--such as the retina, Harderian gland, and gastrointestinal tract.

It is a standard slow acting hormone injection. Once the injection is in the animal, it just propagates throughout the animal's physiology and takes the normal melatonin chemical and physiological pathways that it does in vertebrates, it prompts the pituitary gland to produce a lot of different hormones including sex and serotonin. 

It doesn't act instantaneously if you overdose (humans have tried), OD negative effects occur after 1-2 days and thereafter. It doesn't react as violently as some other hormones such as Adrenaline, which is absorbed fast and dissipates fast and can give you a heart attack. Melatonin has a physiological effect that is more extended, for example steroids for inflammation.

The aim of intramuscular injection of the fish is that it is effective and safe, If you read the guides, they are concerned with the safety of the fish, it's nerves and vital organs... The tail is mostly muscle and the injection is IM. Once the hormone is injected, its absorbed by the animal over an extended period. There have been many experiments of pineal injection in rodents so refer to them to know the time frame of absorption.