Some interesting online stories about the Matagi.
Matagi bear hunting seems to be a dying culture.
Probably partly due to people just don’t hunt as much as they used to, environmental groups objecting to the practice, and for the Matagi, as shown in the video posted by Todoroki-san, in past times it was a group effort, modern rifles make that less necessary, and traditions fade away when communities are no longer as isolated as they were only two generations ago.
https://japanesemythology.wordpress.com/ancient-matagi-bear-hunting-tradition-of-japan-dying-out/
The Moon Bear as a Symbol of Yama - Its Significance in the Folklore of Upland Hunting in Japan
https://nirc.nanzan-u.ac.jp/nfile/783
As Paul Gaskell wrote in his blog post the terrain and climate of the upland areas of Japan made it not possible for the agricultural rice culture of the lowlands. Where the Matagi existed for centuries, had their own beliefs, superstitions, and language.
Some of the mystery of the origins of the word Tenkara may also be due to the fact that since the upland culture was isolated from the urban centers there was little interest in those communities, so it was little documented.
Paul wrote that perhaps the name Tenkara originated from this group that hunted bears in the spring and fall, and fished during the summer months.
He also wrote that the Matagi " used the words tegara, tenkara, tengura, tenkarako, or tenkako to describe flying insects."
Interestingly on page 6 in the above document it states they used a word - ana gari (穴狩り) which meant Den-Hunting, or literally Hole-Hunting. With the way spoken Japanese changes it’s pronunciation due to Rendaku ( 連濁) leads me to wonder if the gari (hunting) suffix also fits with the words tegara, tengura. There is also the word - maki-gari 巻狩り, which as you might guess from being familiar with Kebari maki , or winding the fly, is a word that means - surround hunting, used when a group of hunters surrounded the bear.
Probably misguided speculation on my part to wonder if Tengara may somehow evolved from words that meant - Insect Hunting or Fish Hunting. Languages do funny meandering over time.
However, foot note #35 on page 18, discusses the use of taboos, & substitute words (for example, they never used the word for bear during a bear hunt), among fishing communities, maybe a substitute word for fishing was used while trying to catch fish, 
and that fishing was a group activity. Much as in Japan today Tenkara often seems to be a group activity.
Anyway, I find it intriguing to wonder how their beliefs about Yama no Kami , God of the Mountains, and his ownership of all creatures that lived in the woods, was applied to the Matagi’s taking of fish from the streams. Or if they turned to fishing if they had violated some bear hunting taboo, and had to forgo hunting for a time. Matagi were also forbidden to hunt if their wife was pregnant, Perhaps they turned to fishing during those times.
We are familiar with Genryu (源流) Headwaters. On page 14 is another interesting word that may apply to the same areas. Areas that were avoided by all except the upland peoples. Okuyama (奥山) Remote Mountains. Believed to be dangerous, wild and the place of spirits.
The fastest way to find reference to fishing is to use Edit / Find function. Mostly there isn’t a lot, but it’s a fun peak at the historical Matagi culture.
