Is there a word for releasing a fish or an animal into a (natural or artificial) habitat?

Solution 1:

(Animal) repatriation might be the word you are looking for. Repatriation is properly defined by Meriam Webster as:

to restore or return to the country of origin, allegiance, or citizenship

The term repatriation is defined in this journal article, called: Relocation, Repatriation, and Translocation of Amphibians and Reptiles: Are They Conservation Strategies That Work? touches more on the subject. I have quoted their definition section below:

"For the purposes of this paper, we define the release of individuals of a species into an area formally or currently occupied by that species a repatriation, whereas releases of individuals into geographic areas not historically occupied by that species are termed translocations. Relocation involves moving an animal or population of animals away from an area where they are immediately threatened (e.g., by development) to an area where they would be less prone to habitat loss; ideally, relocated animals should be moved to habitats where they historically occurred, but this is not always the case."

The quote is from the definition section of the paper (2nd page of the pdf) in that link (it is publicly available). The paper has been cited over 500 times, according to Google Scholar For more information about the lead author

In the quote I have highlighted 3 terms, all of which are properly defined in the quote. So depending on the context one might choose one of those words.

Backup link, should the paper be removed from the earlier link

Solution 2:

'Release' and 'return' are the words used by the UK Government in regard to fish returned to rivers after being caught :

Report a catch return

how many fish you released

UK Gov - Fishing and Hunting

'Release' is the word used by the UK Government for freeing animals or wildlife into the environment :

Licence to release non-native wildlife and plants

UK Gov - Licence to Release

The two examples in the question are about different things. The fish in the example is taken home, not returned. The wolf cub is released back into its natural habitat.

But the heading question is about release and that is what is being answered here.

Relocation, repatriation and translocation are more complex and the paper indicated shows that it is not just about release and return but involves considerable efforts thereafter in care and husbandry.

Solution 3:

In the first instance, the OP is not releasing a goldfish into a bowl, they are keeping it in captivity. It matters not the size, material or what form the enclosure is, the captured animal is not free to roam as it pleases.

The second is an example of freeing or releasing an animal back into the wild (its natural habitat).