No.88(October,2015)
No.88(October,2015)
A-520 Noboru HOSHINO
Age determination of Japanese flounder, Paralichthys olivaceus caught from Ishikari Bay, using cross-sections of otoliths
The age of Japanese flounders (Paralichthys olivaceus) caught using commercial gill nets from Ishikari Bay in the Sea of Japan was determined using cross-sections of sagittal otoliths. It was possible to determine individual age by counting the fine growth increments formed on the otolith cross-section in the spawning season (July). The age of Japanese flounder landed to the port of Yoichi since 2005 ranged from 1 to 9 years, with the highest frequency being 2 years. This result suggests that the growth variation among individuals or year-classes is significantly large. In the class with total length >550 mm, the frequency of males was remarkably low, and females aged 3-9 constituted most of the class. Flounders caught in the feeding season (November to December) were younger than those caught in the spawning season (June to July), and individuals aged above 6 years were not found.
A-521 Noboru HOSHINO
Recent tendency of change in body size of sandfish, Arctoscopus japonicus in the Ishikari Bay
In this study, I examined the recent tendency of the growth of sandfish distributed in the Ishikari Bay, off Hokkaido. I found that in each year-class generated since 2009, the body length of sandfish collected by trawl surveys was extremely small. Consequently, the maturity rate of 1-year-old fish, amount of spawning, and progression of maturity for maternal fish have decreased. No significant changes were found in the egg size from 2009 to 2013. It was suggested that the recent tendency toward lower growth has prevented recovery of the stock size.
A-522 Mitsuru TORAO and Takashi YANAGIMOTO
Population structure of even-year pink salmon (Oncorhynchus gorbuscha) from the Nemuro Strait, Hokkaido determined with mitochondrial DNA analysis
Pink salmon Oncorhynchus gorbuscha populations in the Nemuro Strait, eastern Hokkaido were examined using mitochondrial (mt) DNA to evaluate their genetic structure. In 2014, even-year populations of pink salmon sampled from Rausu, Shunkari-kotan, Kunbetsu, Shibetsu, Nishibetsu, and Sakura (a tributary of the Tohoro River) rivers of the Nemuro Strait were examined in COI/ND5/D-Loop regions. We observed no significant genetic differences between these six rivers’ populations of the Nemuro Strait in these three regions of mtDNA. Also, we compared them with previously reported COI/ND5 regions of pink salmon from Rusya River in 2008 and 2009, flowing into the Sea of Okhotsk. The Rusya population in 2008 and the populations from rivers of the Nemuro Strait in 2014 did not differ, except vs. Sakura River in the ND5 region. All the populations from the Nemuro Strait in 2014, in contrast, significantly differed from Rusya in 2009 both for COI and ND5 regions. These results suggest the genetic structure of pink salmon populations in the Nemuro Strait region have similarities within the even-year linage. This similarity may reflect their frequent occurrence of straying in this region. On the other hand, the two-year life cycle of pink salmon has resulted in odd- and even-linage being genetically isolated also in eastern Hokkaido.
A-523 Makoto FUJIWARA, Hirofumi HAYANO and Yasuyuki MIYAKOSHI
Preliminary experiment of effects of high water temperature on maturation of adult chum salmon
To evaluate the effects of high water temperature on the final maturation of chum salmon (Oncorhynchus keta), 3 females and 3 males were kept in tanks at different temperatures (normal, 11°C; high, 20°C or 22°C), and cross-fertilization was conducted using the surviving mature fish. Females held at 20°C ovulated on the third day of holding, while all females held at 22°C died by the fifth day without ovulating. In the ovary of one of the dead females, morphological changes such as over-maturation (aggregations of yolk globule) were recognized. Eyed egg ratio and hatched fry ratio of eggs fertilized with males held at 20°C were lower than that of eggs fertilized with males held at 11°C. Also, survivals of eggs fertilized with females held at 20°C were lower until eyed-egg and the subsequent stages, indicating an influence upon egg quality that cannot be evaluated from the external appearance. These results suggest that water temperature higher than 20°C may affect the mortality of adult chum salmon, final maturation, and ovulation.
A-524 Hirokazu URABE
A-525 Hayato SANEYOSHI, Hiloshi KAWAMULA, Makoto FUJIWARA,
Yasuyuki MIYAKOSHI and Hirokazu URABE
Natural reproduction of chum salmon in Hashibetsu River, Hokkaido, Japan (Short paper)
Recent studies reported the presence of naturally-reproduced chum salmon, Oncorhynchus keta, in Hokkaido, but scientific knowledge about that in Japan is still quite limited. To obtain basic biological traits about naturally-reproduced chum salmon, we implemented a survey on natural spawning in fall 2010 and 2011 and resulting fry in spring 2011 and 2012 in Hashibetsu River, Northern Hokkaido, where enhancement has not been operated. Naturally spawning adults and fry were found in both the two study years, suggesting that the naturally-spawning population was maintained in Hashibetsu River.
A-526 Mahito MIYAMOTO, Mitsuru TORAO, Hayato SANEYOSHI and Kiyoshi KASUGAI
Coastal and riverine recaptures of pink salmon tagged in the coastal area of Nemuro Strait, Hokkaido (Short Paper)
To estimate the stock composition of pink salmon and to evaluate the effectiveness of temporary fishing closure to allow spawners to escape into rivers with a shortage of brood stock for the hatchery program, a total of 311 pink salmon captured by set-net fishery were tagged and released in the coastal area of the northern Nemuro Strait, eastern Hokkaido, in August 2011 and 2012. A total of 89 tagged pink salmon were recaptured, all of which were in areas north of the Notsuke Peninsula (within the range of approximately 40 km from release sites). Tagged fish were recaptured at adjacent coastal set-net fishery and in 9 rivers including 3 rivers where hatchery programs were not conducted. Our tagging study indicates that the autonomous fishery regulation in August should be effective to conserve pink salmon populations including naturally spawning fish in the northern region of Nemuro Strait.
A-527 Kiyoshi KASUGAI, Daisei ANDO, Yasuyuki MIYAKOSHI and Mitsuru TORAO
Changes in age composition of chum salmon caught on the Shibetsu coast and Shibetsu River (Short Paper)
Age compositions of chum salmon caught with a setnet on the Shibetsu coast and with a salmon weir in the Shibetsu River were investigated, eastern Hokkaido, northern Japan in 2011—2014. For both coastal and river catches, the proportions of age 5 were higher early in the season, although those of age 4 were higher late in the season; average ages decreased with the progress of the season. Average ages of river catches were higher than those of coastal catches in the same periods, then average ages of river catches changed 10—30 days later. Age compositions and average ages differed between the total coastal catches and the total river catches.
A-528 Hideo YOSHIDA and Yukio MIHARA
Catches of smooth lumpsucker, Aptocyclus ventricosus (Pallas) in Hokkaido (Note)
A-529 Mikimasa JOH
Age, body length, body weight, distribution, sex ratio, and annual change in body size at different ages of cresthead flounder collected in the sea-grass bed of the Notoro Lake (Note)