We could also probably argue the inverse of your question - that it became a symbol because of its usage in the Gospels, vs. being used in the Gospels because of its symbolism.
Note that at least four of Jesus' disciples (Peter, Andrew, James and John) came from a background of professional fishermen, and this was an extremely common profession in the region of Galilee and along the Mediterranean coast.
Jesus is also credited with at least three miracles (the feeding of the 5,000 - Matthew 14:13-21; Mark 6:31-44; Luke 9:10-17; John 6:5-15, the feeding of the 4,000 - Matthew 15:32-39; Mark 8:1-9, the temple tax - Matthew 17:24-27) that directly involved literal fish.
I saw one source that pinned the symbol to the timeframe of Nero's rule, which was from 54-68. Given that we don't have precise timeframes on the Gospels, this would already put the symbol within the earliest expected dates for them.
The underlying question, then, is "which came first?" In other words, where did the symbol come from? If the symbol influenced the Gospels, where did it originate? At the least, we'd assume that it must have been based on some earlier teaching or document. Or, did both develop simultaneously?