Is the sentence think I catch a grammatically correct?

No, "think I catch a" is not grammatically correct. It's missing a verb and needs to be structured into a complete sentence.

Here are some possible ways to correct it:

* "I think I'll catch a..." (This is a more common and natural way to phrase it, adding the verb "will" and completing the sentence with a noun or noun phrase)

* "I think I caught a..." (This uses the past tense of "catch" and would be appropriate if referring to something you have already done.)

It's important to provide more context to give you the most accurate and helpful correction. What were you trying to say?