Horoka Tomamu Fish Survey 2015
The Horoka Tomamu Fish Survey was conducted by Dr Kasai Fumitaka, a fish biologist based at the Shiretoko National Park (World Heritage Site). After an initial exploratory meeting at Horoka Tomamu on 19 May, the actual survey was carried out on 1 June and 18 September 2015.
The border of the conservation area includes the Yonnosawa-gawa (literally 'stream of the fourth ford'), a small tributary that flows into the Horoka Tomamu River, one of the headwaters of the Mu, one of Hokkaido's 13 major ('class 1') rivers.
In the past we've noted many frogs and lizards, also salamanders, in or around the stream, but not fish. The location is high (around 500 metres) and the stream, while two to four metres wide, is quite shallow (20 cm and less).
There is also a weir, called Yubenosawa-seki, of about 1.5 metres high, eight kilometres downstream, that we thought might be an obstacle for fish ascending the river.
Visual spot checks (in a dry suit), underwater photos and capture and release of fish were undertaken in four locations on 1 June, and along the length of the stream from the first to the last (June) location on 18 September, mainly looking for evidence of spawning.
These far exceeded our expectations.
Four species were identified: masu salmon (Oncorhynchus masou), rainbow trout (Oncorhynchus mykiss), whitespotted char (Salvelinus leucomaenis) and stone loach (Barbatula toni). All of these are native, except for the rainbow trout which is introduced from the eastern Pacific.
Rough estimates of number are: rainbow trout 40%, masu salmon 35 , whitespotted char 20, and stone loach 5%.
The masu salmon, which is spawning in the stream, is probably from the ocean, which at Horoka Tomamu is 120 km away from the conservation area.
Two other possible species, Nozawa sculpin (Cottus nozawae) and Siberian lamprey (Lethenteron kessleri) were not found.
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Simon Holledge, 28 November 2015