Biological studies on the resource of Shirauo, Salangichthys microdon, in the Ishikari River system and its adjacent nearshore sea area
　The shirauo, Salangichthys microdon, is a fish with a one-year lifespan, a slender, semitransparent body (7-8 cm), and is an important marine resource. The fish inhabit brackish waters, lagoons, and estuaries in Japan. Adult shirauo are caught in gill nets in shallow water near or in the estuary of the Ishikari River on the west coast of Hokkaido in the spawning season between May and June, and young shirauo (3-6 cm) are caught in beach seines in an oxbow lake off the main stream between September and November. Because catches in these waters have been low in recent years, planning for stock enhancement is needed. To this end, I studied the biology of the shirauo, including the structure of the resource and their reproduction.
Between May and July of both 1994 and 1995, I collected many eggs attached to grains of sand in shallow water (<1.5 m depth) in the estuary, but only 2 eggs from the oxbow lake. This shows clearly that the main spawning ground is the estuary.
Young fish reached 11 km from the river mouth in the main stream, and the middle basin of the oxbow lake. They stayed in the main stream between June and August and in the oxbow lake between July and November. I collected more in the lake than in the river. Therefore, most shirauo hatch out in the estuary, enter the river, migrate to the oxbow lake, and later return to the sea. On the other hand, I collected many young fish in the estuary and nearshore waters between August and November. This suggests the existence of another group that does not migrate in summer and fall. The evidence suggests that shirauo pass the winter in the sea, because two males were captured by trawler offshore in February.
Hatchlings measure about 5 mm in body length between May and June, but grow rapidly, reaching 40-60 mm by November. Fish grew slightly faster in the oxbow lake than in the estuary. Growth slowed in winter, and females were larger than males after winter.
The Sr:Ca ratios of the otoliths of 98 individuals were measured. A life in fresh water gives a lower Sr:Ca ratio than a life in salt water. Profiles of the Sr:Ca ratios during the life history showed two different patterns. One was characterized by a high (slightly fluctuating) Sr:Ca ratio throughout the life history; the other showed an initial high Sr:Ca ratio followed by a lower value later in life. The former pattern suggested estuary-sea residents, and the latter, upstream migrants. These findings show that estuary-sea residents and upstream migrants occurred together in space and time.
The critical swimming speed (CSS) of shirauo was estimated as the endurance speed of fish swimming in a tunnel apparatus designed to measure stamina. The CSS is likely to determine the extent of migration upstream. Minimum, maximum, and mean CSSs of adults were 11.3, 21.0, and 16.8 cm/s. Because the flow velocity of the main stream of the Ishikari River is usually 0.5-1.0 m/s, this would make it difficult for juvenile shirauo to migrate upstream against the flow. But it is likely that shirauo migrate upstream in the salt wedge in the main stream, where the flow is very slow, because many juveniles were collected there between June and August 1996.
Juvenile shirauo moved into the oxbow lake through the Shibi Canal after reaching the mouth of the canal, 5.5 km from the river mouth, in July. To see how they moved into the oxbow lake through the canal, I recorded water current and daily change in fish distribution in the canal in July of 1995 to 1997. The vertical and horizontal distributions of juveniles were determined mainly by the direction of flow. The distributions suggested two immigration mechanisms. In one, juveniles stay around the canal mouth in the main stream during ebb tide, and are carried into the lake by the fast flood tide in the deeper layer in the center of the canal. In the other, juveniles stay near the bank of the canal to avoid the fast flow during the ebb tide, and concentrate in the center with the fast flow of the flood tide.
Numerical simulation by a box model was conducted to test these two immigration mechanisms. Three swimming behaviors were numerically simulated: (1) coming near the bank during ebb tide, (2) moving away from the bank during flood tide, and (3) staying around the mouth of the canal in the main stream during ebb tide. Passive movements of fish were dependent on water circulation. The simulation appropriately reproduced some characteristics seen in the field survey. The results suggested that staying around the mouth of the canal in the main stream during ebb tide is the most efficient behavior to move into the oxbow lake.
A video recording of the behavior of 50 adult shirauo in a 1-t aquarium was analyzed to reveal their spawning behavior. During mating, a female descended from her usual circular swimming in the upper layer to the bottom layer. Males assessed each other while crowded around the female and competed physically for the female, until a mating pair formed. The paired female led the male upward, and they spawned and ejaculated. Shirauo show a unique spawning behavior in which a pair shake their bodies and the male sticks his gonopore into the female’s while using his anal fin scales as a sucker. Spawning and ejaculation had a low success rate, maybe due to the female’s mate choice. Females repeatedly spawned a small number of eggs in repeated matings.
A pair of shirauo in a 200-L aquarium spawned about 2000 eggs in 4 batches from 22 May to 8 July in 1996. Batches were spawned at intervals of 8-11 days. Estimated batch sizes were 586 to 761. Each batch ended 29 spawning behaviors over about 7 hours, during which the whitish swelling in the female’s abdominal cavity which indicates mature eggs decreased in size and finally disappeared. With feeding, the whitish swelling gradually increased in size again until the following batch was spawned. This confirms that female shirauo can repeatedly produce mature eggs and spawn several times during a spawning season.
The sex ratio in the spawning ground during the spawning season was controlled by two factors: a decrease in the number of males from high to low during the spawning season, and an increase in females during full-moon and new-moon and a decrease at other times. The female gonadal size index (GSI) increased till late May and then decreased, and increased during full-moon and new-moon and decreased at other times. These results suggest that males come to the spawning ground earlier than females, but their numbers decrease through male-male competition; there is peak of spawning in late May; and females come to the spawning ground on the spring tide.
Two migration groups of shirauo inhabit the Ishikari River system and adjacent sea area: an amphidromous type, which the present study has shown for the first time, and an estuary-sea type, which is known from the Yura River in Kyoto prefecture and the Muromi River in Fukuoka prefecture. The amphidromous type may have an advantage: in using calm waters such as the oxbow lake, where food is abundant, they need less energy for swimming against the current, so they grow faster. However, I believe that the shirauo adopted an amphidromous habit not because of these advantages, but as a result of finding calm waters upstream. The formation of a salt wedge when fish hatch supports this scenario.
The oxbow lake is important for stock enhancement, because the catch of shirauo is large in other river systems with calm waters such as Takase River system (Lake Ogawara) in Aomori prefecture, Tone River system (Lake Kasumigaura) in Ibaraki prefecuture and Abashiri River system (Lake Abashiri) in Hokkaido prefecture.Thus, it is necessary to save the amphidromous type for stock enhancement in the Ishikari River, as shown by the positive correlation between the catch in the oxbow lake in fall and in the estuary in the next spring (except in the 1993 year class). On the other hand, a Ricker-type relationship was found between the catch in the estuary in spring and the catch in the oxbow lake in the next fall (except in 1993). This suggests that reproduction of shirauo is controlled mainly by its density, and secondarily by depletion during migration or by conditions during the run upstream. Because the present level of the shirauo resource is much lower than the peak on the Ricker curve, the adult resource must be raised to two to three times the present level for effective maintenance of the resource.
So it is important to control the catch of adults as well as young for stock enhancement. Stock might be enhanced in one or more of the following ways: (1) Don’t fish early in the spawning season, so the shirauo will spawn again. (2) Don’t fish late in the spawning season so as to avoid catching females. (3) Don’t fish during a spring tide, when many females come to spawn and their GSI is high. (4) Don’t fish when conditions allow amphidromous fish to run upstream. It will be necessary to find the most effective combination of these strategies by trial and error through fishery management.

Target strength of live arabesque greenling Pleurogrammus azonus suspended in the sea.

Target Strength (TS) of live arabesque greenling Pleurogrammus azonus, a kind of bladderless fish, were investigated at two frequencies (38kHz and 120kHz) to know how they relate to frequency or to fish size. Live greenlings (26.4～30.9cm,BL) were captured by angling at the Senposhi bank off northwestern Hokkaido. For measurement of TS using a quantitative echo sounder EK500 equipped on the research vessel, each sample fish was suspended below the bottom of the ship. Rayleigh probability density function (PDF), which were selected with AIC from two types of PDF, were fitted to distributions of back scattering amplitude at both frequencies to estimate averaged TS (TSavg). TSavg were estimated from -62.1 to -49.8dB at 38kHz, and from -57.9 to -47.2dB at 120kHz. TSavg at 120kHz were larger than them at 38kHz for 3.3dB (mean). Although TSavg increased with body length at both frequencies, regression model was fitted better at 120kHz than at 38kHz. These results yield two conclusions that arabesque greenling also has scattering characteristics of bladderless lives, and that 120kHz is more appropriate frequency than 38kHz for acoustic survey for this species.

Total length and age at maturity of pointhead flounder Hippoglossoides pinetorum in Ishikari Bay in western Hokkaido

This paper investigates total length and age at sexual maturity of pointhead flounder Hippoglossoides pinetorum in the Ishikari Bay, western Hokkaido. The samples were collected from May 1999 to February 2001 using beam trawl and bottom trawl gears of RV Oyashio. Monthly changes in gonad somatic indices showed that the main period of spawning were June and July. The point estimates of total length at 50% maturity were 217 mm for female and 170 mm for male. The maturity rates of female at each age 2, 3 and 4 years old were 21%, 70% and 97%, respectively. Whether the flounder reach sexual maturity depended upon the total length other than the age.

Age and growth of pointhead flounder Hippoglossoides pinetorum in Ishikari Bay, western Hokkaido

This paper investigates age and growth of pointhead flounder Hippoglossoides pinetorum in the Ishikari Bay, western Hokkaido. The samples were collected from May 1999 to May 2005 using beam trawl and bottom trawl gears of RV Oyashio. Ages were estimated from the number of opaque zones in otoliths, and a presumed birth date of 1 Aug. The von Bertalanffy growth formula fitted to total length TL(t) or body weight BW(t) at age t data were TL(t)=331(1-e -0.29(t-(-0.081))) for female, TL(t)=263(1-e -0.43(t-0.035)) for male, BW(t)=358(1-e -0.29(t-0.002)3 for female, BW(t)=151(1-e -0.46(t-0.122) 3 and for male. Using the conventional growth formula (Tominaga et al. 1996) causes young fish stock (Age 2 and 3) to be underestimated in contrast to use the result of this study in stock assessments.

Reproductive season of the north pacific giant octopus Octopus dofleini in coastal area around Bakkai, Wakkanai，northern Hokkaido in the Sea of Japan

Octopus dofleini were collected in Bakkai, Wakkanai, northern Hokkaido in the Sea of Japan from June 2003 to June 2004 to study the reproductive season. The ovary index (ovary weight / body weight × 100) and oviducal ball index (weight of oviduct and oviducal ball / body weight × 100) in females obviously increased from November 2003 to December 2003 and March 2003. They had spermatozoon in the oviducal ball. They were probably mature female before spawning. The spermatophoric sac index (weight of spematophoric sac, spermatophoric duct, accessory spermatophoric gl. and spermatophoric gl. / body weight × 100) in males obviously increased from October to December 2003 though the testis index (testis weight / body weight × 100) decreased. Almost of them had spermatophores in the spermatophoric sac or penis. They were probably mature and mated actively. This reproductive season was in thoese found around Rebun Island of northern Hokkaido.

Reproductive Cycle of the Japanese Littleneck Clam in the waters off Moheji, Tsugaru Strait

In order to determine the spawning season of the Japanese Littleneck Clam Ruditapes philippinarum found in the waters off Moheji in the Tsugaru Strait, we collected clams from May to November 2002 and observed the gametogenesis histologically. Almost all the clams had commenced gametogenesis in May. Spawning commenced in June, although the majority of spawning was observed between the end of July and the beginning of August. The condition factor peaked at the end of July. After the middle of September, gametogenesis had ceased and all clams appeared in a spent condition. These results indicate the spawning season of the Japanese Littleneck Clam in this area to be between the end of June and early September, and at its peak in late July / August.

Influence of stormy weather on triglyceride concentration in juveniles of Japanese scallop (Mizuhopecten yessoensis) under suspended culture.

Temporal change of triglyceride (TG) concentration in juveniles of Japanese scallop under suspended culture was examined off Oshamanbe from Aug. 25 to Oct. 21, 2004. Water temperature, salinity, chlorophyll-a, horizontal flow speed and vertical flow speed were monitored by continuous data recorders near the culture area at the same time. From Aug. 25 to Sep. 21, 4 times stormy weathers caused by low pressure passages ( include Typhoon no. 16, no. 18) were observed. The TG concentration decreased immediately after Typhoon no. 16 passage, and the low concentration continued. The stormy weather caused by low pressure passage was observed from Oct. 9 to 13. The TG concentration decreased immediately after the stormy weather, and increased at the 8 days after. It is showed that the TG concentration in juveniles of scallop decreased immediately when the juveniles experienced the stormy weather. These results suggested that the TG concentration becomes good index of physiological condition of the juveniles under suspended culture.

Growth and maturation of the perennial brown alga Sargassum confusum adhered on concrete plates

In order to develop a technique for algal bed formation, the growth and maturation of the perennial brown alga Sargassum confusum that adhered to concrete plates were examined. The concrete plates were placed at the ground level in the area with S. confusum vegetation at Minedomari, Atsuta, in July 2000. The plates were then relocated at a depth of 1 m and 3 m in December 2000, at 5 m in January 2001 in the same area, and at 1 m in January 2003 in the southern part of the area where Pacific herring spawn. The length of S. confusum on the plates was measured. The mean length of S. confusum on the plates set under severe wave conditions (depth: 3 m) was lesser than that under calm wave conditions (depth: 1 m and 5 m). The mean length of S. confusum on the plates was lesser than that observed in natural vegetation around the plates at the southern area. The maturation of S. confusum was observed on the plates at the age of three years. These results suggest that algal bed formation should be carried out for three successive years.