Here's why:
* Prepositions show the relationship between a noun or pronoun and another word in the sentence. In this case, "in" shows the relationship between "fish" and "nets and traps."
* Conjunctions join words, phrases, or clauses. There's no joining of elements in this sentence.
* Linking verbs connect a subject to a predicate noun or adjective. There's no predicate noun or adjective in this sentence.
* Pronouns replace nouns. There are no pronouns in this sentence.