My impression, as a native (AmE) speaker who has no special experience with people who research the life cycles of fish,* is also that rearing feels wrong or clumsy here. I understand rear in this kind of sense as a transitive verb: it denotes the action performed by the parent or a farmer, caring for young until they reach adulthood—not the action of growing up, performed by the young.
What dictionaries say
I checked a few dictionaries, and they don't list this sense for rear. The Oxford English Dictionary, the most complete English dictionary, devotes an entire page to rear as a verb, and does not list this intransitive sense. They suggest that only the transitive sense that you have in mind is standard. However, general-purpose dictionaries often cover specialized senses of words incorrectly. Note, though, that farmers are said to rear pigs, calves, etc.; the subject of rear doesn't have to be a parent of the animal who is reared.
The etymology might provide a clue. There are some intransitive senses of rear that come from the same origins, such as standing on the hind legs, usually said of four-legged animals. I don't find any reason to suggest that this kind of sense influenced fish researchers, though.
An educated guess
I can't give you a definitive answer, but I can offer you an educated guess, the nature of which might provide some clues about how English works in general—even if this particular guess is wrong.
First, there is a common way of using a transitive verb intransitively in English: the object of the verb used transitively simply becomes the subject of the verb used intransitively. For example:
Mom is cooking the chicken right now. → The chicken is cooking right now.
Jim broke the window. → The window broke.
Sometimes the intransitive senses are not found in dictionaries. Inventing an intransitive sense in the moment, as needed, is sometimes felt as natural use of grammar in English, depending on the verb and what you're trying to say. For example, see this and this. This intransitive way of using a transitive verb is sometimes loosely called the "middle voice" or, more precisely, "ergative" use of a verb. (Note that most people have never heard of the terms "middle voice" or "ergative"; they're mostly known to linguists.)
Second, when I searched Google Books, I found research around 1960 into ways of rearing salmon in "natural conditions". That is, the researchers were setting up artificial ponds to rear salmon commercially (still transitive use of the verb), but making the ponds as natural as possible. They often speak of salmon rearing: the job of getting the salmon to mature in artificial salmon farms. By the late 1970s, the term rearing seems to have been extended by researchers to include the maturing of salmon in a fully natural environment.
So, it appears that as rearing came to be used for commercial salmon rearing in natural-like but artificial ponds, scientists found it "natural" to treat rear as an ergative verb and speak of salmon rearing even in a fully natural environment, where the fish have neither humans nor their mothers to tend to them.
To get a really definitive answer, you'll probably need to hear from someone who studies the life cycles of fish (and who pays close attention to language).
* Wait a minute! I got my undergraduate degree at one of the leading schools for fisheries biology! I never heard rearing used intransitively, but my studies didn't involve me with that part of the school, so this is not strong evidence that intransitive rearing of fish is strange in that community.