What is this spring attached to the reel below and where can I get one?
I have seen it on a number of Hawaii fishing videos on YouTube.
It is a reel tie down. When fishing from shore a rope is used to anchor the reel and pole to something solid. Some reel/pole setups have been lost to the ocean when left in a pole holder and a strike is taken.
I picked one up at Brian's Fishing on S. King St in Honolulu. Other stores also carry them. A few years ago I paid 7$ for a titanium one.