Present status and conservation of the markless forms of stream-resident masu salmon Oncorhynchus masou (the so-called 'iwame') in Japanese mountain streams

As a markless form of stream-resident masu salmon (Oncorhynchus masou), the so-called ‘iwame,’ which can be distinguished from the sympatric normal form of the salmon by their plain body pattern, has been previously reported among seven local populations (A–G) in Japanese mountain streams. In the present study, all populations but B (where any information is entirely missing) were surveyed in a snorkeling survey (detection rate: 34.7%) in 2005 to elucidate the recent conditions of the markless forms. The ratios of the markless forms were 13.0% (9/69), 0.0% (0/364), 5.6% (68/1,205), 0.8% (1/132), 0.0% (0/1,181) and 64.6% (155/240) in A, C, D, E, F and G, respectively. Markless forms might indeed be already extinct in C and F where the stocking of non-native hatchery individuals has been carried out extensively. The same survey was repeated in D in 2009: the markless ratio had significantly decreased to 2.4% (19/783), although the reason for this was unknown. In D, the ratio of the markless form was higher in the mainstream as opposed to the tributaries in both 2005 and 2009, which might imply that the habitat of the markless form differs from that of the normal form.