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The determination of the optimum stocking density of hatchery-reared juveniles in relation to the carrying capacity of the nursery ground is important to the success of any stock enhancement programs. In order to estimate the surplus productivity of nursery grounds available to support hatchery-reared fish, a population growth model for juvenile Japanese flounder Paralichthys olivaceus was developed by extending an ecophysiology-based model on growth of juvenile fishes. Parameters of the model were adjusted using field-collected data from Ohno Bay in northeastern Japan. This model predicted the optimum stocking density, namely the maximum number of hatchery-reared juvenile Japanese flounder released into the bay without decreasing the growth of conspecific wild juveniles owing to food limitation. The optimum stocking density for juveniles of 89 mm total length was estimated to be 2000 individuals per 250,000 m 2 in Ohno Bay in 1989. Under the model, the most important factors affecting the optimum stocking density of Japanese flounder were the abundance of mysids and wild juveniles at the time of release. The effect of abundance of a competing flatfish species on growth of wild Japanese flounder was less important. [ABSTRACT FROM AUTHOR]

Highlights: [•] The relation of observational learning and schooling formation was investigated. [•] Fish of 20mm did not show schooling behaviour or observational learning. [•] Fish of 40mm showed schooling behaviour and observational learning. [•] The onset of observational learning coincided with that of schooling. [ABSTRACT FROM AUTHOR]

Abstract: Japanese flounder larvae and early juveniles were reared for 39 days after hatching (DAH) in order to determine pancreatic enzymes activities (trypsin and neutral lipase) using fluorogenic substrates during ontogeny and to compare enzymes activities of starved fish at different developmental stages. Larvae were fed L-type rotifers Brachionus plicatilis from 3 to 28 DAH, Artemia nauplii from 14 to 39 DAH and then gradually switched to pelleted feed from 25 DAH. Temperature was kept at 18 °C and photoperiod was 12L:12D. Three starvation trials were made: from 2 to 8 DAH (premetamorphic larvae), 23–29 DAH (metamorphic) and 31–39 DAH (metamorphic and postmetamorphic). Fluorometric determinations on individual larvae were made using specific substrates for trypsin and lipase activity. Trypsin and neutral lipase activities were found in 2 DAH larvae (0.18±0.09 U μg protein−1 and 25.12±7.36 nmoles 10−3 4MU μg protein−1 min−1, mean±SD, respectively), just at the time of first feeding. The evolution of both enzymes activities showed a profile marked by decreases during settlement (27–36 DAH). There is a significant increment (P <0.05) on specific trypsin activity at 15 DAH, corresponding with the beginning of Artemia feeding and a significant decrease of lipase activity at 6 DAH. In the first starvation experiment, larval mortality was 100% after 6 days. In the second trial a starvation-feeding experiment was done with two and four days starved larvae. Survival at day 6 was 50% and 25%, respectively. On the third trial post-metamorphic larvae were starved 5 days, survival at day 6 was 81%. As a general trend, enzymes activities get significantly lower (P <0.05) in starved fish. On two day starved–fed fish, lipase and trypsin activities levels took two days to reach control levels; on the other hand four days starved–fed fish showed significant differences two days after feeding again. During the last trial (31–39 DAH) lipase and trypsin activities went down, coinciding with juvenile settlement. These results show that trypsin and also neutral lipase activity measurements could be used for determining the critical periods during ontogeny and the nutritional status of larvae. [Copyright &y& Elsevier]

The effects of delayed metamorphosis on larval competence, and the postlarval survival and growth of Haliotis discus hannai (H. discus hannai) were examined. Competent larvae were induced to metamorphose at 5, 10, 15, and 19 days after fertilization by the addition of 1 μM γ-aminobutyric acid (GABA). Larvae in another group were maintained until individuals metamorphosed spontaneously. Metamorphosed individuals (postlarvae) were reared in the laboratory at 20 °C fed on a benthic diatom Cylindrotheca closterium or without any food in equivalent conditions. Metamorphosis, survival, and growth (determined from shell length) rates were measured. The percentage of individuals that had metamorphosed 2 days after the addition of GABA increased with the length of larval swimming period; larvae that were 5 and 19 days old when induced to metamorphose showed 19% (±5.6; S.E.) and 96% (±3.6) metamorphosis, respectively. The percentage of postlarvae that metamorphosed spontaneously increased after 17 days from fertilization and reached 96% (±3.0) at 24 days postfertilization. Survival rates of fed postlarvae depended on larval swimming period. For larvae that swam for ≤15 days, more than 80% of postlarvae survived throughout the 20-day experimental period. In contrast, the survival rate was only 57% (±2.9) for 19-day-old larvae. Postlarval growth rates did not differ significantly between larval swimming periods ≤15 days. However, the growth rate of postlarvae from 19-day-old larvae was significantly lower than that of ≤15-day-old larvae. In the starved treatments, survival rate was lower, and the final shell length of the dead animals was less, as larval period became longer. [Copyright &y& Elsevier]

Stocking hatchery fish into the wild should only be implemented at a level that can be sustained within the available food-based carrying capacity to avoid food resource competition with wild conspecifics. We constructed a simulation model to assess the growth of wild conspecifics under different magnitudes of hatchery fish releases, including no releases and actual release levels, and under different wild fish abundancies. Stocking of juvenile Japanese flounder Paralichthys olivaceus (approximately 70–110 mm TL) in Fukushima from 2003 to 2005 was analyzed. We monitored post-release changes in abundance of both cultured and wild juveniles and their prey (mysids). Our survey and modelled simulations revealed that the surplus prey production was sufficient to support the consumption of stocked and wild Japanese flounder (up to 150,000 individuals per site in 2003 and 2004), during which wild juveniles achieved nearly maximum growth rates. However, in 2005, during which a dominant year class of wild Japanese flounder occurred, the body size of wild juveniles was 49% smaller than the predicted size based on the maximum growth rates. This suggests that the food-based carrying capacity was reached in that year. Thus, it is important to evaluate the existing prey and wild juveniles levels and adjust the release accordingly every year. [ABSTRACT FROM AUTHOR]

Abstract: The jack mackerel, Trachurus japonicus, has a prolonged spawning season and widely spread spawning grounds. The population in the coastal waters of Japan seems to be composed of several cohorts spawned seasonally from different waters. To understand its population structure along the Tsushima Warm Current, we analysed hatchdates and growth histories of fish from Kunda Bay, the southern, central and northern East China Sea (ECS), the southern Sea of Japan, and Maizuru Bay. Seven cohorts were detected from fish collected between June 2005 and June 2006 in Kunda Bay. Comparing hatchdate distributions and growth trajectories of the seven cohorts with those of the other five regional samples, we did not find that cohorts collected in Kunda Bay originated in the southern ECS. Therefore, these coastal waters of Japan appear to be significant spawning grounds for juvenile jack mackerel. [Copyright &y& Elsevier]

Abstract: The purpose of this study is to document a genetic profiling approach to track a released hatchery strain of Japanese flounder Paralichthys olivaceus. Microsatellite DNA loci together with nucleotide sequences of the mitochondrial DNA (mtDNA) control region were used as genetic tags. In September 2001, flounder juveniles produced at a hatchery station (Japan Sea-Farming Association (JASFA) hatchery) were stocked into a natural sea area, and at around the same time and within the same area, a different hatchery strain was released by another hatchery station. In this study, we focus on tracking the released flounder produced at the JASFA hatchery, which were profiled with the two classes of molecular marker. A total of 1576 individuals including both wild and released flounder were entrapped in a set net during a 6-month period 1 year after the release (September 2002–February 2003). Of these, the 2001-year-class fish were sorted based on the total lengths (18–34 cm), and further screening to differentiate hatchery-raised fish was made by examining for hypermelanosis on the blind body side, which is a typical phenomenon which occurs in hatchery-raised flounder. Through these processes, 149 individuals were selected as a sample of hatchery fish released by either the JASFA hatchery or the other hatchery. Parental allocation based on the genetic profiles retrieved 35 of the 149 recaptures as survivors of the released JASFA fish. There was no significant difference in the family size distribution between the recaptured JASFA fish and that of juveniles estimated when the JASFA seedlings were released into the sea, suggesting that mortality of the released flounder had not arisen in favor of particular families under the natural environment. [Copyright &y& Elsevier]

Effective population size in captive populations is affected by several factors such as the number of contributing broodstock to the next generation, sex ratio of parents, and variations in family size, which can be accurately evaluated by examination of the pedigree structure in the populations of interest. Here we present an example of microsatellite-based pedigree tracing in a hatchery strain of Japanese flounder Paralichthys olivaceus to be stocked into natural sea areas. We also detail the potential effect of selective operations (size selection) on the pedigree structure. The hatchery strain we screened was founded by using 18 wild captives (6 males and 12 females) through the use of the mesocosm spawning method, and the pedigree of the offspring including 113 individual larvae collected within 24 h after hatching, 216 individuals of 1 month old, and 407 individuals of 4 month old was unambiguously identified. The contribution of candidate broodstock to the next generation was highly skewed as the contribution to almost all of the offspring was monopolized by a single male, and a half of the females did not produce any offspring. The contribution of one family to the released fish selected for larger size (total length) was significantly high, while those of other two families were low (P<0.008). [Copyright &y& Elsevier]

• Environmental DNA metabarcoding detects rare, endangered, and alien species in estuarine and coastal fish communities. • Latitudinal effect is found in estuarine and coastal fish communities across Japan. • Bank artificialisation negatively impacts estuarine and coastal fish community structures. • Ocean currents weakly affect estuarine and coastal fish community structures. • Environmental DNA metabarcoding is useful for large-scale assessing fish communities in estuaries and coasts. Estuarine and coastal ecosystems are important habitats for many fish species. Large-scale patterns of estuarine and coastal fish communities can be affected by geographical factors, such as water temperature and latitude, that limit or allow fish dispersal, and/or by environmental factors that select locally adapted species. Furthermore, increased human activity has also altered fish diversity by modifying estuarine/coastal environments. Here, we used environmental DNA metabarcoding to study a large-scale pattern of estuarine/coastal fish communities across 25 watersheds in the Japanese Archipelago, and we assessed the effects of geographical, environmental, and anthropogenic factors in structuring these communities. Of the 290 fish species detected, 47 species were red-listed by the Japanese Ministry of Environment, and nine were alien. Non-metric multidimensional scaling showed that latitude and water temperature affected both river-mouth and coastal-sea fish communities. Although other anthropogenic factors did not affect these communities, bank artificialisation rates influenced both. The relationship between each fish community and ocean currents was weak. The number of red-listed fish affected both fish community structures, whereas that of alien species did not. The numbers of red-listed fish in coastal points and of all fish species were negatively influenced to the human population density. These findings provide useful information that may be used to conserve estuarine and coastal ecosystems. [ABSTRACT FROM AUTHOR]