1) I think that the understanding of symbols in the biblical narratives can be one of the most difficult questions of the interpretation. Often a majority „know“ that a symbol (or a metaphor) is used (for example: Mark's fig tree) but there are two, three, four views what the symbol mean and worth considering arguments on all sides (the same applies to the works of Virgil and other ancient authors).
One problem is that a symbol (or a metaphor) can have more than one meaning (the broken bread can be the body of the Lord but also the entirety of Christians can be the Lord's body) and it can have different meanings in the works of different authors (imho: Mark's and Matthew's trees). Another problem can be that a symbol with a known meaning is used in a different way (LXX-Isaiah's vineyard in Mark's parable of the tenants).
The use of fish in the feedings of the multitudes may be a symbol hard to understand. There could be the question whether Mark invented it or whether he knew Christian community meals with fish, and if the latter, whether he used nevertheless the fish as a symbol with a precise meaning or on the contrary not as a symbol.
2) From the point of view of the usual chronological order it seems that Paul used the word „fish“ in the way of the LXX as one of the basic groups animals (omitting the „Creeping things upon the land/earth“)
3) In GMark appeared three things
The first time the disciples are engaged with people is during the sending out of the twelve. The second time is their work at the feedings of the multitudes.
- the fish is the fish of the disciples and
- it's their job to set it before the people
I have no concrete idea about it, but there could be the possibility that in Mark's view the three things (disciples as fishers, as fishers of men and their job to set their bread and fish before the people) were related together and parts of the same overriding theme.