Osamu Tominaga

Influence of diet shift from formulated feed to live mysids on the carbon and nitrogen stable isotope ratio (δ 13C and δ 15N) in dorsal muscles of juvenile Japanese flounders, Paralichthys olivaceus

Abstract To estimate the accumulated food consumption of released fish, we investigated the temporal change in the stable isotope ratio of carbon ( δ 13 C) and nitrogen ( δ 15 N) in dorsal muscles when the diet of Japanese flounder juveniles was switched from a formulated feed to live mysids. The average δ 15 N of fish that fed on mysids ad libitum did not change during the experiment but the average δ 13 C gradually increased from −20.70‰ to −19.19‰ after the diet shift. The δ 13 C values at the start and at the end of the experiment were almost equivalent to the δ 13 C of the formulated feed and mysids, respectively. The relations between time after diet shift and the δ 13 C of fish fed mysids at 2.2% BW/day (2.2% group) and 4.4% BW/day (4.4% group) were as follows: δ 13 C=−17.25−2.03 exp(−0.04× t ) and δ 13 C=−17.18−2.09 exp(−0.05× t ), respectively. The change in δ 13 C of the 4.4% group was significantly greater than that of the 2.2% group (likelihood ratio test). Since the growth rate of Japanese flounder juveniles was extremely high and the δ 13 C of the control group that was fed the same formulated feed during the experiment did not change, it was suggested that the change in δ 13 C was mainly attributed to the growth rate of the fish. The combination of stomach content analysis with the stable isotope study makes it possible to estimate the growth rate in terms of body weight and consequently, cumulative food consumption of individual fish after release using the rate of change in δ 13 C.

Geographical dispersal and optimum release size of hatchery-reared Japanese flounder Paralichthys olivaceus released in Ishikari Bay, Hokkaido, Japan

Abstract The dispersal and optimum release size of Japanese flounder stocked in Ishikari Bay, the most northern area where fry was stocked, were examined by tagging and marking experiments and market surveys of commercial landings. In Ishikari Bay, 17,055 tagged flounder (mean TL = 13 cm) were released in September 1988, and 59,600 (mean TL = 8 cm) and 72,900 (mean TL = 6 cm) fin-clipped flounder were stocked between July and August 1989. Hatchery-reared flounder have an abnormal white and black pigmentation on the anocular side, which easily distinguishes them from wild flounder. The recovery rate of a released group of flounder with an average length of 8 cm (8.2%) was 2.4 times that of a group with an average length of 6 cm (3.2%). In the tagging experiment, a total of 330 (1.9%) recoveries were recorded between October 1988 and November 1991. The tagging experiment showed that the distribution of the stocked flounder extended southward to the Tsushima Current; no fish were recaptured to the north of the release site. The numbers of stocked flounder in the landings decreased with increasing distance from the release area and 75% of recaptured flounder were caught within Ishikari Bay. The tagging experiment also showed that 68% of the fish which recruited to the fishery at ≥25 cm TL were captured inside Ishikari Bay.

Field evaluation by RNA/DNA ratios on post-release nutritional status of released and wild Japanese flounder Paralichthys olivaceus juveniles

A large-scale juvenile Japanese flounder (Paralichthys olivaceus) release–recapture experiment was undertaken to find the optimal release season by evaluating the nutritional status of released fish at different seasons during which food abundance was significantly different. Forty thousand fish were released at depths of 1.5 m for early-release (May 29, 1997) and 2 m for late-release (July 2, 1997) (42.1±3.5 and 40.9±4.2 mm body length, respectively) in an experimental field, Wakasa Bay, the Sea of Japan. Samples were taken, after the releases, at Wada beach at intervals of 1, 2, 3, 6, 10, 16 and 30 days after release (DAR), including pre-surveys before each release. Released fish recaptured from the two different release groups totaled 764; 467 from the early-release group (ER) and 297 from the late-release group (LR). A total of 1956 wild flounder juveniles were simultaneously collected (1041 ER, 915 LR). ER fish were subject to higher food availability and were exposed to less pressure from predation by smaller wild juvenile flounder. RNA/DNA ratios in ER juveniles were significantly higher than those of LR fish during all samples. Especially, RNA/DNA ratios in ER juveniles were higher than in wild juveniles from 3 to 50 DAR. In the LR group, the nutritional status of juveniles was relatively low in shallower water. These findings corresponded well with feeding incidence examined by coworkers. Mass release of hatchery-reared juveniles apparently reduced RNA/DNA ratio of the wild juveniles right after releasing. The present study showed that earlier release of hatchery-reared juvenile Japanese flounder with higher RNA/DNA ratio could increase the possibilities of survival right after release in the nursery ground, and that RNA/DNA ratio appeared to be a good tool in evaluating nutritional status of released juveniles as well as wild juveniles in Japanese flounder.

Feeding ecology of dominant demersal fish species Favonigobius gymnauchen, Repomucenus spp. and Tarphops oligolepis at a sandy beach where larval Japanese flounder settle in the Seto Inland Sea, Japan

Feeding habits of Favonigobius gymnauchen, Repomucenus spp. and Tarphops oligolepis were examined during the period from May to August in 1999, 2000 and 2001 at a sandy beach in the central Seto Inland Sea, Japan. The three species of fish accounted for more than 60% of all fish collected in terms of number of individuals. All three species mainly consumed small crustaceans. However, the major prey of F. gymnauchen and Repomucenus spp. differed from that of T. oligolepis. F. gymnauchen and Repomucenus spp. mainly consumed mysids and small crangonid shrimp (<12 mm in body length), which predominated in the study area. T. oligolepis actively selected only epifaunal mysids Nipponomysis ornata and avoided crangonid shrimp and gammarids. The frequencies of occurrence of fish in the guts of the three dominant species were very low, and larval and juvenile Japanese flounder were not observed in any of the three fish species. The diet of juvenile Japanese flounder was similar to that of the three species. From these results, it appears that these three fish are competing species for the flounder.

Daily rations and food availability of Japanese flounder Paralichthys olivaceus, small flounder Tarphops oligolepis and sand goby Favonigobius gymnauchen at a sandy beach in the central Seto Inland Sea, Japan

To estimate the daily rations of Paralichthys olivaceus, Tarphops oligolepis and Favonigobius gymnauchen and to clarify diel variation in food availability to them, diel sampling was conducted over a 24-h period at a sandy beach in the Seto Inland Sea, Japan. The mean total length and body weight (BW) of P. olivaceus, T. oligolepis and F. gymnauchen were 32.6 mm (0.35 g), 23.7 mm (0.13 g) and 50.2 mm (1.03 g), respectively. The mean stomach contents index and percentages of stomachs with food were high during the day but low at night, indicating that these species are day-feeders. Paralichthys olivaceus and F. gymnauchen consumed mainly mysids and crangonids, whereas T. oligolepis fed chiefly on mysids. The estimated mean daily rations (95% confidence intervals) for P. olivaceus, T. oligolepis and F. gymnauchen made with the bootstrap method were 18.1 (9.3–31.3)%BW, 13.2 (7.9–24.3)% BW and 3.7 (1.7–7.4)% BW, respectively. Mysids were more abundant in the near-bottom layer during the day than at night, but the opposite was true for crangonids.

High-resolution mapping and time-series measurements of 222Rn concentrations and biogeochemical properties related to submarine groundwater discharge along the coast of Obama Bay, a semi-enclosed sea in Japan

High-resolution mapping along the coast and time-series measurements of the radon-222 (222Rn) concentrations in the shallow zone in a semi-enclosed sea, Obama Bay, Japan, were undertaken in 2013. The temporal and spatial variations in the 222Rn concentrations were analyzed in parallel with meteorological conditions, physical–biogeochemical characteristics, and the submarine groundwater discharge (SGD) flux measured with a seepage meter. These data indicate that the groundwater influences the water properties of the bay and that the groundwater supply pathways are not limited to the local SGD. The concentrations of 222Rn flowing into the bay from rivers was known to be relatively high because groundwater seeps from the river bed. High-222Rn water was almost always present around the river mouth, and northward advection of the water affected the distribution of 222Rn concentrations in the bay. The southward wind suppressed the advection of the high-222Rn water and largely controlled the temporal variations in 222Rn concentrations at a station located on the north side of the river mouth, whereas the local SGD affected the short-term changes in the 222Rn concentrations. The concentrations of 222Rn and chlorophyll-a, an indicator of phytoplankton biomass, show a significant positive correlation in the surface layer along the coastline in seasons when the nutrient supply was the main factor limiting primary productivity.

Reproductive cycle of yellow sea bream Dentex tumifrons in Wakasa Bay, the Sea of Japan off central Honshu

To determine the spawning season and reproductive cycle of the yellow sea bream, Dentex tumifrons, in Wakasa Bay, in the Sea of Japan off central Honshu, seasonal change in the gonadosomatic index (GSI) was examined and histological observations of both ovaries and testes were conducted. The female GSI gradually increased from May, maintained a high level between that vitellogenesis began from May and developed until September. Spent fish began to be observed in October and all the fish collected in December were spent. There was only one peak in the seasonal change in GSI of males and females in Wakasa Bay. Thus, yellow sea bream spawn only in autumn in Wakasa Bay. Differences in the reproductive cycle and number of spawning seasons were found between the two different latitudinal areas, Wakasa Bay and the East China Sea (low latitudinal area) where the spawning seasons were spring and autumn. An increase in water temperature is an important factor to initiate gonadal maturation (recrudescence) and is terminated by the short day-length in Wakasa Bay.

Isolation and characterization of 11 polymorphic microsatellite loci in the whitegirdled goby (Pterogobius zonoleucus) and cross-species amplification in the serpentine goby (P. elapoides)

Eleven polymorphic microsatellite markers were developed from a (CA)n‐enrichment library of the whitegirdled goby (Pterogobius zonoleucus). Polymorphism at these loci ranged from 2 to 12 alleles, and observed and expected heterozygosities from 0.05 to 0.90 and from 0.05 and 0.86, respectively. All loci conformed to Hardy–Weinberg equilibrium, with no significant linkage disequilibrium between all locus pairs. Cross‐species amplification tests were successful in P. elapoides, and most loci were polymorphic. These microsatellite markers will be useful in further population genetic studies of both species.

Relationships Between Submarine Groundwater Discharge and Coastal Fisheries as a Water-Food Nexus

We introduced the results of investigations conducted in Japan (Yamagata, Fukui, Hiroshima and Oita Prefectures) and discussed the contribution of SGD to the production of fishery resources and their prey (meiobenthic communities). Our recent surveys provided evidence of the contribution of SGD to the production of fishes and their prey. These results strongly support the hypothesis that the contribution of nutrients from SGD to the coastal fishery resources production is much higher than previously suggested. The negative ecological impacts of SGD on meiofaunal abundance were confirmed at the high seepage areas of Kamaiso where the small spatial scale presented strong heterogeneity in SGD environmental conditions. However, the meiofaunal abundance was not low at the low seepage area located slightly apart from the high seepage area. Therefore, the effect of SGD on meiofaunal communities could be limited. However, the environmental heterogeneity significantly enhanced the conservation of the meiofaunal diversity. Thus, SGD determined the property of benthic communities.

Molecular identification of macroalgal fragments in gut contents of the sea urchin Hemicentrotus pulcherrimus

In order to improve the efficiency of stock enhancement programs for the sea urchin Hemicentrotus pulcherrimus, information on food algal species, which affect growth and gonad production greatly, is necessary. Since it is difficult to identify species from the macroalgal fragments within the gut contents of the sea urchin by microscopic observation, we tried to apply a DNA barcoding method for gut contents analysis. We used a partial rbcL gene sequence for taxonomic section and newly designed primer sets, respectively, for brown algae and for red algae. Direct sequencing of the PCR products was carried out. Species identification was based on the phylogenetic relationship. We could objectively identify four species and two taxonomic groups (genus or family) in brown algae, and two species and four taxonomic groups in red algae from the gut contents. Sargassum hemiphyllum was the most abundant brown alga in the gut contents but was not dominant in the study site. The result showed the importance of identification to the species level. In addition, red algal epiphytes were detected with brown algal fragments. The DNA barcoding method will enable the researchers to verify the important role of epiphytes as a potential food source.