No.71 (Octobor,2006)
No.71 (Octobor,2006)
A－403 Katsuhisa Baba
Ecological study on spawning and early life stage of the brackish water bivalve Corbicula japonica in Lake Abashiri
The spawning probability of Corbicula japonica is expressed with temperature and salinity by the following equation:(1) 1/{1+exp[-(-16.12+0.57T?12.64S+0.59TS)]}, (T: Temperature, S: Salinity). The occurrences of the planktonic larvae can be well predicted by the equation in Lake Abashiri. C. japonica failed to spawn when the temperatures and the salinity did not exceed the threshold environments for the spawning; the threshold environments are expressed by the equation (1). The failure of the spawning was not an exceptional phenomenon in Lake Abashiri; the failure probably occurred in 10 out of 21 years for which data were available. The ovaries after the spawning season in the spawning-failure years were histologically characterized by massive atretic oocytes.
The vertical heterogeneities of abundances and size compositions of the planktonic larvae were observed in the water column. The most of larger (shell length: 170 to 200 μm) larvae were observed near the surface of the lake. On the other hand, the smaller (shell length: 140 to 170 μm) ones were mainly observed near the boundary between the well-oxygenated oligohaline upper stratum and anoxic polyhaline lower stratum. The salinity of the near the boundary was higher than shallower depths. I thought the heterogeneities probably related to the enhancement of the survivorships for the salinity-sensitive larvae. The larvae probably assemble to the higher salinity zone to gain sufficient salinity for the larval development in the early stages. The larvae migrate to the surface where the higher possibilities are expected to be transported to the marginal area of the lake, the habitat of C.japonica.
Few settled juvenile were observed on the muddy sediments. The favorable sits for the juveniles were sites where the content of silt-clay plus very fine sand was less than 36.9 %. Other restrictive sediment-environment factors, such as organic carbon, water content and total sulfide were highly correlated with the site-clay plus very fine sand; i. e. one factor can represent the favorable sediment conditions for the juvenile. Understanding the favorable conditions is very important for the management of fisheries grounds and it also facilitates the judgments about which sites are appropriate for the release of seed clams.
The juvenile growth of C.japonica in Lake Abashiri was extremely slow; it grew to the shell length of 0.7 mm at age 1 and 2.1 mm at age 2. I elucidated the proximal factor of the extremely slow growth by modeling between the environmental factors and the growth. The proximal factor was the less susceptible growth response to the environmental factors until the second winter. In many species of bivalve, populations from higher latitudes have a slower initial growth rate; but longevity and ultimate size in these populations are frequently greater than at lower latitudes. The extremely slow growth of C.japonica juvenile in Lake Abashiri is probably an extreme example of this phenomenon.
The spawning probability of Corbicula japonica is expressed with temperature and salinity by the following equation:(1) 1/{1+exp[-(-16.12+0.57T?12.64S+0.59TS)]}, (T: Temperature, S: Salinity). The occurrences of the planktonic larvae can be well predicted by the equation in Lake Abashiri. C. japonica failed to spawn when the temperatures and the salinity did not exceed the threshold environments for the spawning; the threshold environments are expressed by the equation (1). The failure of the spawning was not an exceptional phenomenon in Lake Abashiri; the failure probably occurred in 10 out of 21 years for which data were available. The ovaries after the spawning season in the spawning-failure years were histologically characterized by massive atretic oocytes.
The vertical heterogeneities of abundances and size compositions of the planktonic larvae were observed in the water column. The most of larger (shell length: 170 to 200 μm) larvae were observed near the surface of the lake. On the other hand, the smaller (shell length: 140 to 170 μm) ones were mainly observed near the boundary between the well-oxygenated oligohaline upper stratum and anoxic polyhaline lower stratum. The salinity of the near the boundary was higher than shallower depths. I thought the heterogeneities probably related to the enhancement of the survivorships for the salinity-sensitive larvae. The larvae probably assemble to the higher salinity zone to gain sufficient salinity for the larval development in the early stages. The larvae migrate to the surface where the higher possibilities are expected to be transported to the marginal area of the lake, the habitat of C.japonica.
Few settled juvenile were observed on the muddy sediments. The favorable sits for the juveniles were sites where the content of silt-clay plus very fine sand was less than 36.9 %. Other restrictive sediment-environment factors, such as organic carbon, water content and total sulfide were highly correlated with the site-clay plus very fine sand; i. e. one factor can represent the favorable sediment conditions for the juvenile. Understanding the favorable conditions is very important for the management of fisheries grounds and it also facilitates the judgments about which sites are appropriate for the release of seed clams.
The juvenile growth of C.japonica in Lake Abashiri was extremely slow; it grew to the shell length of 0.7 mm at age 1 and 2.1 mm at age 2. I elucidated the proximal factor of the extremely slow growth by modeling between the environmental factors and the growth. The proximal factor was the less susceptible growth response to the environmental factors until the second winter. In many species of bivalve, populations from higher latitudes have a slower initial growth rate; but longevity and ultimate size in these populations are frequently greater than at lower latitudes. The extremely slow growth of C.japonica juvenile in Lake Abashiri is probably an extreme example of this phenomenon.
A－404 Kazutaka SHIMODA, Kazuhiko ITAYA, Mizue MUROOKA and Noboru HOSHINO
Features and cohort analysis of brown sole Pleuronectes herzensteini population in northern Hokkaido, Japan
To reconsider the area of stock assessment of brown sole Pleuronectes herzensteini stock from the Sea of Okhotsk to Ishikari Bay, components of commercial landings, abundance index of juvenile fish and relationships of catch between districts were examined. Positive correlation was observed in the annual catch fluctuation between the present stock assessment area and the Tomamae district. Age structures and occurrence pattern of abundant year class of commercial landings show a geocline from the Sea of Okhotsk to Ishikari Bay. Positive correlation was observed between the abundance index of age one fish in the Ishikari Bay and that of the Sea of Okhotsk. Based on these results and previous mark-recapture studies, we concluded that the brown sole stock from the Sea of Okhotsk to Ishikari Bay is composed of one population. Results of cohort analysis indicate that population size of brown sole stock from the Sea of Okhotsk to Ishikari Bay are influenced by recruitment of dominant year class.
To reconsider the area of stock assessment of brown sole Pleuronectes herzensteini stock from the Sea of Okhotsk to Ishikari Bay, components of commercial landings, abundance index of juvenile fish and relationships of catch between districts were examined. Positive correlation was observed in the annual catch fluctuation between the present stock assessment area and the Tomamae district. Age structures and occurrence pattern of abundant year class of commercial landings show a geocline from the Sea of Okhotsk to Ishikari Bay. Positive correlation was observed between the abundance index of age one fish in the Ishikari Bay and that of the Sea of Okhotsk. Based on these results and previous mark-recapture studies, we concluded that the brown sole stock from the Sea of Okhotsk to Ishikari Bay is composed of one population. Results of cohort analysis indicate that population size of brown sole stock from the Sea of Okhotsk to Ishikari Bay are influenced by recruitment of dominant year class.
A－405 Kazutaka SHIMODA, Kazuhiko ITAYA and Mizue MUROOKA
Distinction of growth group based on otolith diameter length of brown sole Pleuronectes herzensteini in northern Hokkaido, Japan
Ⅰ. Growth group of commercially captured fish at age one to three
Growth group of brown sole Pleuronectes herzensteini population in northern Hokkaido Japan, were distinguished based on otolith diameter length. The Okhotsk Sea growth group was regard as 91.1% of the commercial landings at the Esashi fish market on the Sea of Okhotsk in autumn of fish age one to the winter of fish age three. All individuals of commercial landings at the Wakkanai fish market in the Soya Strait in the winter of age two were the Okhotsk Sea growth group. Over 86% of the commercial landings at Teshio to the Yoichi fish market in the Sea of Japan in the spring of age two were regarded as the Okhotsk Sea growth group. These results suggested that some of Okhotsk Sea growth group migrate to the Sea of Japan at age two in the winter and arrive in the Ishikari Bay at spawning season at age two. The body size of age two and three fish of the Japan Sea growth group captured by the survey in the Ishikari Bay and the coast of Obira, were smaller than that of commercial landings. We estimated that most of the Japan Sea growth group will be commercial landings after age three in the spring.
Ⅰ. Growth group of commercially captured fish at age one to three
Growth group of brown sole Pleuronectes herzensteini population in northern Hokkaido Japan, were distinguished based on otolith diameter length. The Okhotsk Sea growth group was regard as 91.1% of the commercial landings at the Esashi fish market on the Sea of Okhotsk in autumn of fish age one to the winter of fish age three. All individuals of commercial landings at the Wakkanai fish market in the Soya Strait in the winter of age two were the Okhotsk Sea growth group. Over 86% of the commercial landings at Teshio to the Yoichi fish market in the Sea of Japan in the spring of age two were regarded as the Okhotsk Sea growth group. These results suggested that some of Okhotsk Sea growth group migrate to the Sea of Japan at age two in the winter and arrive in the Ishikari Bay at spawning season at age two. The body size of age two and three fish of the Japan Sea growth group captured by the survey in the Ishikari Bay and the coast of Obira, were smaller than that of commercial landings. We estimated that most of the Japan Sea growth group will be commercial landings after age three in the spring.
A－406 Fujinori TSUDA, Hiroshi HOSHIKAWA, Yukio AGATSUMA and Kazuya TANIGUCHI
Gonad production of the sea urchin Strongylocentrotus nudus in relation to algal vegetation in shallow waters along the Sea of Japan, southwestern Hokkaido, Japan
Gonad production of the sea urchin Strongylocentrotus nudus in relation to the algal vegetation was clarified through gut content analysis in three sites (Yaoi, Rokujo and Biya) in Suttsu Bay along the Sea of Japan coast in southwestern Hokkaido from May to August 2001. The main food algae leading to gonad production were Sargassaceae and Desmarestia viridis (Yaoi), D. viridis and Colpomenia sinuosa (Rokujo) and Laminariales and D. viridis (Biya), coinciding with the algae with abundant standing crop that grew at each site, respectively. The gonad index (gonad wet weight×100 / body wet weight) was the highest in the Laminaria religiosa-dominated site with high algal standing crop. It is considered that gonad production is affected by the nutritive value of the food algae and ease of grazing and by drifts of detrital algae that originate in shallow waters.
Gonad production of the sea urchin Strongylocentrotus nudus in relation to the algal vegetation was clarified through gut content analysis in three sites (Yaoi, Rokujo and Biya) in Suttsu Bay along the Sea of Japan coast in southwestern Hokkaido from May to August 2001. The main food algae leading to gonad production were Sargassaceae and Desmarestia viridis (Yaoi), D. viridis and Colpomenia sinuosa (Rokujo) and Laminariales and D. viridis (Biya), coinciding with the algae with abundant standing crop that grew at each site, respectively. The gonad index (gonad wet weight×100 / body wet weight) was the highest in the Laminaria religiosa-dominated site with high algal standing crop. It is considered that gonad production is affected by the nutritive value of the food algae and ease of grazing and by drifts of detrital algae that originate in shallow waters.
A－407 Masaki AKINO, Tadaaki TAKEDA and Takuma IMAMURA
Studies on the quality characteristic of the scallop adductor muscle flakes
A popular name “scallop flakes” is new food that manufactured from muscular fiber of heat-treated scallop adductor muscles. As for this, a manufacturing method is easy and mass production is possible. There are forms, such as fresh, boiled and frozen, in the adductor muscle of the scallop used as materials. The texture and colors of scallop flakes are greatly different by raw materials. We firstly established an objective method that evaluated the texture and colors of scallop flakes. Scallop flakes manufactured from raw and boiled adductor muscles had good texture and color tone. On the other hand , scallop flakes manufactured from frozen adductor muscles didn't have sufficient texture. The texture was improved by defrosting frozen adductor muscles at low-temperature, but during defrosting a large quantity of sugar phosphates were generated in adductor muscle and the browning degree of scallop flakes was promoted. Therefore, we have thought preventive measures from another viewpoint are required.
A popular name “scallop flakes” is new food that manufactured from muscular fiber of heat-treated scallop adductor muscles. As for this, a manufacturing method is easy and mass production is possible. There are forms, such as fresh, boiled and frozen, in the adductor muscle of the scallop used as materials. The texture and colors of scallop flakes are greatly different by raw materials. We firstly established an objective method that evaluated the texture and colors of scallop flakes. Scallop flakes manufactured from raw and boiled adductor muscles had good texture and color tone. On the other hand , scallop flakes manufactured from frozen adductor muscles didn't have sufficient texture. The texture was improved by defrosting frozen adductor muscles at low-temperature, but during defrosting a large quantity of sugar phosphates were generated in adductor muscle and the browning degree of scallop flakes was promoted. Therefore, we have thought preventive measures from another viewpoint are required.