That river was "Akikawa" in Nagano pref, really?
You saw in the upstream "Akigawa(Hì)" of Tamagawa(½ì) name plates?http://www.kensetsu.metro.tokyo.jp/ikimono2/images/suikei_tamagawa.jpghttp://home.s08.itscom.net/sekix/img/rs1492065928.jpg
If so, see the Hì¦(gyokyo) part of the table and their official web.http://www.geocities.jp/f6kaneko/yuugyokenn.htmhttp://akigawagyokyo.or.jp/index.php?%E9%81%8A%E6%BC%81%E6%A1%88%E5%86...
¦ is shorten of Æ¦¯g(gyogyo kyodou kumiai) = Fisheries Co-operative Association.
Anyway, already answered outlines,
nearest fishing gear shop(ÞïX tsurigu-ten/Þï® tsurigu-ya) knows well.
If you can read Japanese well even if using translator with,
details are written in each prefecture's fisheries(
YÆ suisan-gyo) page.
(if without maps, it may take time to understand)
This is a Kanagawa pref as sample, V(yuugyo) means "play fishing" by amatures.http://www.pref.kanagawa.jp/cnt/f790/index.htmlhttp://www.pref.kanagawa.jp/cnt/f790/p504690.html
River/Lake fishing ticket(pass) is called "V(yuugyo-ken)" or "VØ(yuugyo-sho)" usual.https://goo.gl/f5yqUC
Also using another word "ÓD(kansatsu)" cases a lot. (depends on Gyokyo)https://goo.gl/CN7ry3
êú(=1ú ichinichi-ken)/ú(nikken) is 1 day ticket.(calendar date)
N(nen-ken) is valid for all days of ON-season ticket.
(invaild for OUT/OFF-season = ÖúÔ kinryo kikan)
¼z(hangaku-ken) is half price ticket for children or elder(depends on Gyokyo)
ðÖ/ðÖú(kaikin/kaikin-bi) is start date of ON-season.
If don't buy it beforehand, must pay x1.5 or x2 usual to patrol staff. (this is not a fine as penalty)
Maybe already you know this year 2018 is ½¬30N(Heisei sanjuu-nen).
If you want to know more about "River fishing", I can write more details well, is it necessary?
If so, pls tell me the specific season, area(or river name), kinds of target fishe you want to.
If you only need a method of searching,
it will come out someting info by searching simple words "River name and ÞÊ(chouka)" at Google.
ÞÊ is result of fishing, fr Þè(tsuri/fishing) + Ê/¬Ê(kekka/seika = results).
I do not know how much you are interesting in fishing now,
but I was able to introduce "Japan fishing show in Yokohama" if you asked me two weeks earlier.http://www.fishingshow.jp/english2018.php
This exhibition is the largest in the Asia, only once per year.
It will be held in Osaka this weekend, a bit smaller hall.http://www.fishing.or.jp/english/index.htmlhttp://www.fishing.or.jp/index.htmlhttps://youtu.be/P-lQ8BkUBXs
If you have time, I think that going to play is a lot of fun.
You may get excited that night and won't be able to sleep :)