this is the source providing sufficient information about the life cycle of salmon. From the article we could see that the life cycle involves varies stages :Incubation, Emergence, Freshwater Rearing, Estuary Rearing, Estuary Ocean Transition, Ocean Residence ... The two of them as you can notice are called "Freshwater Rearing and Estuary Rearing". Now what gets stuck in my mind is their name - Rearing. According to dictionaries, rearing means "Bring up and care for until they are fully grown" so to be able to say that the fish is reared in the freshwater or estuaries, there has to be a factor - mother fish - to care for the young, right? However, when we look through the life cycle we cannot see any other fish playing a role in the growth process of salmon. It reaches maturity on its own, from egg to the maturity. In conclusion, the question is that, how is the name of the process, rearing, relevant to the process itself? Why is that stage named as "Rearing" if a fish gets matured without any help ?
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.
Dr. Anopheles is breeding mosquitos in his laboratory. → Mosquitos are breeding in Dr. Anopheles' laboratory.
Sometimes the intransitive senses are not found in dictionaries. Inventing an intransitive sense in the moment, as needed, is sometimes felt to be a 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 published around 1960 into ways of setting up artificial ponds or tanks, used for rearing salmon commercially, to be as similar to natural conditions as possible (still transitive use of the verb). Salmon farmers speak of salmon rearing: the job of getting the salmon to mature in controlled or artificial conditions. 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. Unlike maturation, the word rearing would therefore suggest that the natural places where the salmon mature are similar to the artificial ponds where they're raised commercially! The intransitive extension of rearing might also reflect the interest that biologists usually take in salmon populations: preserving, maintaining, stabilizing, or increasing them—not so different from salmon farmers. I doubt that you'd find rearing used intransitively (as much) in regard to the natural habitat of a species that people don't like—say, mosquitos.
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.