ゴギの産卵習性と仔稚魚

概要

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So-called 'gogi' (Fig. 1), Salvelinus leucomaenis (Pallas), is a non-migratory char attaining a length of 30 cm or less and occurs in the most upper reaches in the rivers in San-in and Sanyo districts of western Honshu. Because gogi-char has many pale spots on the head unlike other members of this genus and has slightly fewer gill rakers, this fish was described at first under the name of S. imbrius by Jordan and McGregor (1925). The present author, however, is considering it as one of the local races of S. leucomaenis, for the differences of meristic counts between gogi-char and S. leucomaenis from other districts are not so conspicuous (Table 2). Field o bservations on the spawning behavior and early life history of this char were made in Misaka-dani of Takatsu River system and Haza River of Sufu River system (Fig. 2A, B, C) both in Shimane Prefecture, mainly in 1962, 1971 and 1974. Spawning season of gogi-char extends from late October to middle November, when the water temperature goes down to 11～8°C. The spawning acts by a single female and several males are usually seen in the pool-side (Fig. 5 A, B, C), where the current is very slow or slight counter flow is noticed. In the male group, the hierarchy is recognized according to their body sizes and the largest one succeeds in mating with the female. This male usually darts and drives subordinate males away if they attempt to approach his partner,however, in early period of prespawning he is occasionally turned out from the spawning redd by counterattacks of a smaller male (Fig. 8 B). The female defends her spawning site from other females and digs the bottom. As the female repeats digging action (Fig. 9 A) at the same point at an interval of 1 to 3 minutes, a saucer-shaped depression is gradually formed, in which she takes a feeling or crouch posture (Fig. 10 A, B) after each digging action. These postures cause courtship by attended males (Fig. 10 C, D), especially by the largest one. The prespawning acts by a pair used to continue for half a day or more. The oviposition (Fig. 11 A, B, C) occurs in the center of finished depression and 20 to 100 eggs are shed into the crevices of gravel in it (Fig. 12 A, Table 3). Immeadiately after this, the mated male attacks his partner suddenly (Fig. 11 D). The female not only remains on her redd withstanding his attacks, but she makes violent counterattacks against him (Fig. 11 E). Thus, a series of fierce fight follows about 15 minutes. Soon after, the female starts undulating by swinging her body laterally (Fig. 12 B, C). After a quarter hour of this undulating, the males leave the redd and the female begins to dig the bottom at an area of about 20 cm upstream the eggs. The eggs in the depression are thus filled up gradually with the fine sand and gravel which are stirred up with postspawning digging actions of the female. Such postspawning acts generally continue for 2 to 5 hours. But sometimes, in several hours after the first oviposition the males appear again near the female who is making another depression slightly upstream site from the previous one and they repeat the same acts as they did in the first prespawning period. The next oviposition is usually seen in 20 to 30 hours after the first. Two or 3 days are necessary for 2 times of oviposition. The eyed eggs of gogi-char we r e obtained on December 15, 1962 from a spawning redd in Inkyo-no-tani, a tributary of Haza River (Fig. 2 C). These eggs were measured 5.1 to 5.4 mm in diameter and began to hatch on December 19. The newly hatched alevins (Fig, 13 A) preserving in 70 % ethanol were 13.8 to 14.5 mm in total length. About 20 days after hatching the alevins reached as long as 20.8 to 23.5 mm. Two-thirds of yolk substance were consumed, but parr marks were not visible as yet (Fig. 13 B) . It took 36 days for the present alevins to absorb the yolk sac completely in the water temperature of 16.5 to 12.5°C. The fry just after consumed their yolk were measured 24.1 to 27.4 mm and parr marks appeared in this stage (Fig. 13 C) . In the rivers, the fry start emergence from the gravel of spawning redd after middle or late in April. The fry after emergence (Fig. 13 D, E) are u s ually captured under the stones, pebbles and sunken withered branches in the shallow stream side where the current is very slow. As the fry grow, they gradually move into deeper waters. The stomach contents of 22 fry from Tossage-dani of Haza River (Fig. 2 C) ranging 21.7 to 63.9 mm were examined. They ate the larvae and a few of pupa of aquatic insects, especially of ephemerids (Table 4).山陰,山陽地方の河川の上流にすむゴギSatvelinus leucomaenis(Pallas)は,頭頂に白い斑紋をもつイワナで(Fig.1),全長30cmを越えるものは少ない.この魚は降海せず,雌雄ともに体側にパーマークを止めたまま,生涯を河川内ですごす.筆者は1971年と1974年に,島根県美濃郡匹見町紙祖字笹山地先の高津川水系の紙祖川に合流する三坂谷(Fig.2A,B;Fig.3A,B)で,ゴギの産卵習性を観察し,あわせてその稚魚などの習性について調査した.その他,1962年には島根県那賀郡金城町波佐字若生地先の周布川水系の波佐川(Fig.2A,C)で,産卵床からゴギの発眼卵を採集,脾化させて,仔・稚魚の形態と習性を観察した.この報告では,これらの観察と調査の結果について述べる.

著者

木村清朗
九州大学農学部水産学第二講座

ゴギの産卵習性と仔稚魚

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