著者

上原 武則

出版者

長野女子短期大学出版会

雑誌

長野女子短期大学研究紀要

巻号頁・発行日

vol.5, pp.10-15, 1998-01-30

---

On Oct. 29, 1997, a specimen of salmonid hybrid supposed to be natural occurrence was caught by rod from the downstream of Inekoki-Dam, Azusagawa R., by Takayuki Hirai (a member of investigater with the Sôgô Kankyô Laboratory). The captor was immediately suspected that the present specimen was resembled to Tigertrout. Such a strange hybrid fish, however, isn't able to be classified into the wellknown salmonid species easily. Therefore, he forwarded it to the author for further examination and identification several days after its capture. The specimen, 13.2cm in total length and 26.4g in weight, is immature male having a pair of small testes (left piece is 16mm long, right one is 10), and has dark brown wormeaten spots on the dorsal and lateral surface, but not on the dorsal fin and adipose fin. These colouration and arrangement of spots are differ from those of Tiger-trout, especially Japanese char (iwana) and land-locked cherry-trout (yamame). However, morphological features including the distribution of vomerine teeth, counts of gill rakers, vertebrae and others are intermediate between those of iwana and yamame. On this papers, the author attempted to ascertain the strain of this hybrid fish and discuss about the presumable occurrence in the wild.

著者

上原 武則

出版者

長野女子短期大学出版会

雑誌

長野女子短期大学研究紀要

巻号頁・発行日

vol.4, pp.8-19, 1996-12-10

---

The intergeneric hybrids which have been produced by the artificial breeding between Salmo trutta and Salvelinus fontinalis are usualy called "Tiger trout" or "Zebra" in Europe and America, because the colour markings on their body surface resemble the stripes of these animals. These hybrids produce rarely under natural conditions in the river, where the both species intermingle. (James.H.Allan, 1977) In Japan, however, this natural hybridization has scarcely been reported in the past. On 24 September 1978, a single specimen presumed to be one of "Tiger trout" hybrids was caught from the upper streams of Taishoike Dam, where Salmo trutta and Salvelinus fontinalis suspected its parental species were crowding together. Since 1975, about 300 specimens of Salmonid fishes have been caught and examined during our studies of the Taishoike areas. Of all these specimens collected during these 20 years, 102 were Salvelinus fontinalis and 47 were Salmo trutta. However, only one specimen presumed "Tiger trout" hybrid was caught. Therefore, this hybrid constitutes about 0.7 per cent of the total population of parental species: Salmo trutta and Salvelinus fontinalis. The author attempted to ascertain whether or not such a specimen is really a hybrid from these two species, and to analize the factors for natural hybridization in Taishoike Dan. The present specimen was male of 3+ years age, 166mm in body length (195mm in total length), 38mm in body breadth, 52.5g in weight, and its milt weighed only 0.4g in the early spawning season. It had golden brown colour with broad, dark brown vermiculations on the lateral surface, dark brown spots on the dorsal and adipose fins. It doesn't seem to look like either of its parental species. Some measurements of the numerical characters of this hybrid are as follows: 110 scales on lateral line, 15 gill rakers on 1st arch and 57 vertebrae. These measurements showed that this hybrid specimen is between Salmo trutta and Salvelinus fontinalis. The characteristics of the present hybrid are similar to those shown in the photograph and the description about the "Tiger trout" (Salmo trutta ♀×Salvelinus fontinalis♂) by Suzuki; Fukuda (1973), except a little difference. From these facts, it is evident that this natural hybridization occured from the intercrossing of Salmo trutta♀ and Salvelinus fontinalis ♂. Since 1925, the eggs and fry of Salmo trutta and Salvelinus fontinalis have been imported into the Taishoike areas from North America and Inawashiroko Lake and others. Especially, the local fishermen continued to stock this water area with Salvelinus fontinalis untill 1971. Since then, both species have increased in number. Although the opportunity for natural hybridization may be fairly frequent, the hybrid produced by such intergeneric crossing as Salmo trutta × Salvelinus fontinalis is rarely to be found in the natural environment due to low fertility of parental species, and low survival rate of F_1-hybrifds. Moreover, these F_1-hybrids "Tiger trout" produce neither fertile eggs and milt, and so they are all sterile. (Suzuki;Fukuda, 1971・1973) Because of these facts, this hybridization may exert very little influence upon the normal reproduction of Salmonid fishes in these natural water areas.