I see both, e.g.,
Developmental rate of fish...
Development rate of fish...
are used to indicate how rapidly fish develop (grow and mature). Are both acceptable or do they mean different things?
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These two examples mean the same thing, but development rate is slightly clearer, and in many similar phrases there is a difference in meaning.
The word development in development rate is an attributive noun: a noun used to modify another noun, almost as if it were an adjective. (Some people call it a "noun adjunct".) When you use a noun attributively, it usually has a meaning equivalent to putting of in front of it. For example, soccer style means the same as style of soccer and development rate means the same as rate of development. (The order of the nouns is reversed from the noun of noun form, as you probably noticed.)
You'll notice that soccer can't be converted into an adjective, but development can. When a noun does have an adjectival form, like developmental, sometimes the noun noun and adjective noun ways of using it acquire different meanings. There's no rule for it. The two-word phrases hold meanings somewhat like individual words (much like phrasal verbs: pick up means something very different from pick).
Most often, the noun noun form is either some kind of established term, whose meaning can't be deduced from the two nouns, or a terse form of noun of noun. The adjective noun form often carries the expectation that the reader can deduce the meaning of the phrase from the two words separately, the way one normally can when an adjective modifies a noun. For example, a steam table is a special kind of table used for serving food and keeping it warm (you can look it up in a dictionary), but a steamy table is just a table with some steam around it or whatever makes sense in context (dictionaries won't have a special entry for that).
So, when you write the noun noun form, it often suggests that the two-word phrase is an established term or is short for noun of noun, and when you write the adjective noun form or noun of noun, it often suggests that the reader should infer the meaning in the ordinary way.
An example where it's decidedly wrong to use the adjectival form is the jargon term environment variable in computers. Sometimes people write it environmental variable, but this is confusing because it suggests that the variable has some sort of environmental quality about it that must be deduced from context, rather than the very specific meaning of a variable stored in a special region of the computer's memory known as "the environment".
An example where it's somewhat strange to use the noun of noun form is the term baseball player. Players of baseball sounds strange because everyone knows what a baseball player is. Using the noun of noun form is like explaining the meaning of the phrase to someone unfamiliar with it, but the meaning of baseball player is already clear to everyone (in America, at least). Since baseball has no adjectival form, if you want to say something in the form adjective noun where the adjective would mean baseball, you have no choice but to use baseball as an attributive noun.
The reason development rate of fish is probably clearer is because it definitely means rate of development of fish whereas developmental rate conceivably could have other meanings, like a kind of rate that itself develops. I don't think biology has such a term, but it's clearest to avoid luring the reader into even momentarily wondering "What about the rate is 'developmental'?"
None of the this is a rule, of course. The above are just common patterns, which influence people's expectations when encountering a phrase for the first time. There's no shortage of jargon terms and established phrases with the adjective noun or noun of noun form that have a meaning that can't be inferred from the words, like bottled water (which can't be replaced with bottle water: such a phrase would suggest that it has some special meaning different from bottled water). Another one is developmental biology (which, if it followed the normal pattern, would have been development biology). Maybe that's what led someone to write developmental rate of fish.
(Note that established phrases like high school and soft drink are not examples of this, since high and soft are not adjectival forms of nouns.)
Development is a noun and can be used per se meanwhile
Developmental is a adjective, so it is used to describe a noun.
So, Just to be clear, I would go with 2nd one, with rate being the noun
Developmental rate of fish
So going by the rule, I suppose Developmental is more apt as it is describing the noun rate, whereas Development is yet another noun.
*(Adjective + Noun = Makes sense to me
Noun + Noun = Doesn't make that sense as the first one does)
But both seems correct and boils down to one's preference, I suppose.
*My reasoning is that when two nouns are next to each other, one takes the form of an adjective, then why not be clear using adj + noun? (Not sure but mostly)