So here's what the fishing rod actually does:
ROD - Larger fish variety per rod. Better rod, unlocks more fish (plus the weight of fish)
BAIT & HOOK - How much faster you can get a fish on the line (more fish in less time)
REEL & LINE - Makes it "easier" to catch fish, basically success rate.
If you cant get the fish you wanna catch, you might need a better rod for you to get into that new set of fish.
Here's also info on the rarer(?) fish to get (in photos)



