Hi Alan.
RE: he strait is not well traveled because it is treacherous. What are the factors that make it so?
Just to clarify, I said, "The strait is quite treacherous and (the island) is not so easy to get to."
In good weather there are plenty of small and large ferries to take people out, but they are often closed down due to sea conditions.
Re; What are the factors that make it so?
Running up the east coast of Japan is the Kuroi current. That is very similar to the Gulf Stream in the US or the East Australian current.
North of Japan is the Sea of Okhotsk in Russia. There are many really large rivers that empty into this sea,
In the (dramatic)spring thaw a large amount of very cold fresh water rushes out.
This eventually collides with the Kuroi current pretty well where Kinkasan is. The produces an upwelling of nutrients and abundant sealife, similar to Newfoundland. It also produces very fast and unpredictable sea conditions.
In 2022 a tour boat sank in Hokkaido in this type of sea conditions. 22 lives were lost then.
Gary