Seasonal variations in major components of Crassostrea gigas from Seto Inland Sea

Fisheries Science - The proximate composition and nutritional content of oysters define their quality and commercial value. Cultivation site and harvest date may impact these characteristics. We...     

A numerical analysis of population dynamics of the sand lance (Ammodytes personatus) in the eastern Seto Inland Sea, Japan

The Japanese sand lance (Ammodytes personatus) population in the Seto Inland Sea shows large fluctuations every few years. Its recruitment to the fishery resources is much more dependent on the survival rate of 0-age fish than is the case for other fish species because 0-age fish directly recruit to next-year spawning adults. Since the 0-age fish population depends on its larval population, which is easily influenced by natural and human impacts, high larval mortality due to predation by adults and high dispersion of larvae by wind-driven currents from spawning grounds to other locations can have very important effects on stock fluctuations. A numerical model of population dynamics of the sand lance in the eastern Seto Inland Sea was developed that focuses on wind-induced transport and adult predation of the larvae. The model successfully simulated the two-year periodic fluctuation and was consistent with actual fluctuations in fishery catches. It suggests that the various coefficients given in the model are appropriate to explain the actual population dynamics of the sand lance. The model shows that adult predation on the larvae plays a very important role in the two-year periodic fluctuation, and the maximum predation rate is estimated to be ten individuals per day. Wind-induced transport modifies the impact of this predation. Variances in the two-year periodic fluctuation become smaller with weaker westerly wind velocity. The appropriate transport rates from Bisan Strait and the Harima-nada Sea are estimated to be 0.04 d^-1 and 0.01 d^-1, respectively.     

Winter longitudinal variation in the body size of larval fishes in the Seto Inland Sea,Japan

This study revealed the spatial variation in abundance and body size of larval fishes in the Seto Inland Sea, Japan, in January 2014 and 2015. Fish larvae were collected by a 1.3-m-diameter ring net towed at the surface and at 10-m depth at 21 stations. The most dominant species was the sandlance Ammodytes japonicus, constituting 82% of total larval fish caught. The body size of A. japonicus was greater [ca. 9 mm total length (TL) in 2014] in eastern areas than in western areas (ca. 5 mm TL in 2014). This trend was also observed in rockfishes (Sebastiscus marmoratus and Sebastes inermis species complex), suggesting a common phenomenon in this region. Because the water temperature was lower in eastern areas, it is likely that the longitudinal differences in larval body size are attributable to earlier spawning in eastern areas caused by different temperature conditions.     

Occurrence and distribution of larval and juvenile Japanese flounder Paralichthys olivaceus at sandy beaches in eastern Hiuchi-Nada, central Seto Inland Sea, Japan

ABSTRACT:   The occurrence and distribution of larval and juvenile Japanese flounder Paralichthys olivaceus and major demersal fish from May to September were examined at seven sandy beaches at the central Seto Inland Sea. The flounder appeared from late May to August and were mainly collected at a depth of 2–5 m. Settlement of larval flounder was seen from late May to late June. A total of 6412 individuals of 17 species representing 12 families were collected in the study area. The dominant fish species were Favonigobius gymnauchen (56.0%), Tarphops oligolepis (8.1%), Rudarius ercodes (7.8%), P. olivaceus (7.3%), Repomucenus spp. (7.0%) and Heteromycteris japonica (3.3%). The catches of flounder differed among beaches and months. The number of flounder in June at Ohama Beach, where the most flounder were collected, was 22.6 individuals per haul (400 m²). The density of the flounder correlated negatively to that of F. gymnauchen .     

Spatial and temporal variations of sediment quality in and around fish cage farms: A case study of aquaculture in the Seto Inland Sea, Japan

ABSTRACT: Fish cage farming generates large amounts of organic waste in the form of unconsumed feed and fecal matter, resulting in sediment deterioration and a threat to its own sustainability. Field studies analysed the scale of the impact of fish cage farming on sediment quality, variability of impact and spread of sediment enrichment in the vicinity of a fish farm. Two fish culture sites near the Fukuyama area of the Seto Inland Sea of Japan were chosen for the study. There was a significant difference between the quality of the sediment in aquaculture and non-aquaculture areas. The sediment underlying the fish cage farms was found to be extremely reducing, acidic and sulfidic. Sediment ignition loss did not vary markedly with the season. However, the redox potential and acid volatile sulfide content of aquaculture areas did show some seasonal variation. Sediment encircling a 165 m radius from the center of a farm in Yokota was judged as organically enriched by fish farming. A variation in sediment quality within the two sites and also within the stations was observed. Based on sediment quality, the stations could be categorized as less affected, highly affected or moderately affected.     

Re-examination of age and growth of daggertooth pike conger Muraenesox cinereus in the western Seto Inland Sea, Japan

Age determination and growth using otolith rings in Muraenesox cinereus was re-examined in the western Seto Inland Sea, Japan. Previous study in this area indicated that new rings were formed annually from March to May, and then from September to October once individuals had achieved four or five rings. In this study, monthly changes in marginal growth rate indicated that the first ring was formed before November in the year following hatching, and from then on another ring was formed annually in July or August. The birth month was determined to be August based on a peak in monthly change in the gonad somatic index. Estimated von Bertalanffy growth functions were L = 806.6{1 ? exp[? 0.33(t + 0.06)]} and L = 1264.0{1 ? exp[? 0.19(t + 0.15)]} for males and females, respectively. Lengths after 3 years of age in this study were 100 mm longer than those in a previous study for both sexes.     

Spawning grounds of red sea bream in the east Seto Inland Sea

We surveyed the distribution of red sea bream eggs between 2006 and 2010 in the east Seto Inland Sea to determine the location of the spawning grounds and the characteristics of preferred spawning habitats. We identified early-development stage red sea bream eggs using a monoclonal antibody assay. The early-development stage eggs were present in waters around Awaji Island (AW area) and around Ieshima Island, Shodo Island, and Bisan Seto (collectively the ISB area). The mean red sea bream egg density in the AW area was not significantly different from that in the ISB area. Based on the locations where we observed early-development stage eggs and the density of eggs, the ISB and AW areas appeared to be the primary spawning grounds in the east Seto Inland Sea during the survey period. A quotient analysis suggested that spawning fish had clear preferences for physical conditions. The areas suitable for spawning appear to be those with sandy substrate at a depth of 30–70 m. The temperature range suitable for spawning was between 16.5 and 21.5 °C.     

Fisheries resource management of the daggertooth pike conger,Muraenesox cinereus,using existing limited datasets in the western Seto Inland Sea,Japan

The daggertooth pike conger, Muraenesox cinereus (Forsskål), has become an important fish resource in the western Seto Inland Sea, Japan, since the 1990s. However, introducing sustainable fisheries resource management for this species is difficult in this region because stock assessments have not been performed, and official fisheries statistics for this stock were discontinued after 2007. This study used existing limited data sets to compile the first report for fisheries resource management for this M. cinereus stock. Yield‐per‐recruit analyses showed that increasing fishing pressure above current levels would provide only a minimal increase in expected catch levels. Hence, the current harvest level is considered to represent the upper limit of fishing pressure. Age composition in a given year could potentially be used to forecast landing abundance for the following 2 years. This study provides a basis for establishing effective fisheries resource management strategies for M. cinereus.     

Prey availability and daily growth rate of juvenile Japanese flounder Paralichthys olivaceus at a sandy beach in the central Seto Inland Sea,Japan

The aim of this study was to characterize a nursery ground (Ohama Beach, Hiuchi-nada) for the Japanese flounder Paralichthys olivaceus in the central Seto Inland Sea having insufficient availability of mysids, Nipponomysis ornata and Iiella oshimai. We examined daily growth rates of wild juveniles; this measure can be used as an index of quality of a nursery. Juveniles consumed not only mysids, but also Crangon uritai and fishes. Mysid availability was lower at the study beach than in the nursery in the Sea of Japan; however, mean growth rates (MGRs) from May to June varied from 0.75 to 1.84 mm day^?1 and equaled that in the nursery with sufficient availability of mysids in the Sea of Japan. Prey availability was high from May to June and decreased after July. Additionally, water temperatures after late July were higher than the optimal temperature. Multiple regression analysis suggested that the low availability of prey and high water temperatures reduced the MGR after July. The results indicate that the optimal season for releasing hatchery-reared juveniles is from May to June, a period characterized by high availability of prey.     

Relationships between the daily growth rate of Japanese anchovy Engraulis japonicus larvae and environmental factors in Osaka Bay,Seto Inland Sea,Japan

Fisheries Science - Seasonal changes in the growth rate of larval Japanese anchovy Engraulis japonicus collected by shirasu seine net boats in Osaka Bay in 2008 were estimated using otolith daily...     

Daily growth rate model of Japanese anchovy larvae Engraulis japonicus in Hiuchi-nada Sea, central Seto Inland Sea

In the present study, we developed a larval anchovy growth model in relation to sea temperature and food availability via food consumption and metabolic process terms, based on biological data from previous laboratory experiments and field surveys from 2003 to 2006 in Hiuchi-nada Sea, central part of Seto Inland Sea, Japan. To investigate when food shortage for larval anchovy and then recruitment failure occur in Hiuchi-nada Sea, anchovy food requirements were estimated by using the growth model, and we compared the food requirement with anchovy food availability. We applied an estimation method for growth model parameters, Hewett?CJohnson p and Q ₁₀, by minimizing the sum of squares of difference between mass-specific growth rates estimated by the models and those by otolith growth analysis. Parameter p was 0.86, slightly higher than typical values, and Q ₁₀ was 2.11, close to the value used for the biological model of larval northern anchovy. Food shortage for anchovy larvae did not occur in Hiuchi-nada Sea, although it was indicated that low food availability led to a low reproductive success rate. The newly developed growth model is considered optimal at present and useful to link environmental conditions and larval growth.     

Correlation of changes in seasonal distribution and catch of red sea bream Pagrus major with winter temperature in the eastern Seto Inland Sea,Japan (1972–2010)

Red sea bream Pagrus major is a commercially important fish in Japan. In eastern Seto Inland Sea (SIS), the catch has increased from 297 tons in 1972 to 2,039 tons in 2010. We examined the relationship, 1972–2010, between increase in catch and winter temperature, based on the catch in February and March and the lowest water temperature at 10 m depth. In 1972–1986, the lowest water temperatures in the inner SIS areas (Osaka Bay, Harima‐nada, and Bisan‐seto) were <8°C, which is physiologically unfavorable for red sea bream. However, in 1987–2010 temperatures were generally ≥8°C. In the inner areas, the catch during winter had been minimal until the early 1980s, presumably because most red sea breams moved to the Kuroshio‐influenced (warmer) Kii Channel area. However, the winter catch in the inner areas of SIS increased from the late 1980s with warm winters. In addition, the catch between April and June, the spawning season, increased in the inner areas from the 1990s, and the catch rate of the inner areas was more than twice higher in the 2000s than in the 1980s. The results suggest that expansion of the distribution area during winter due to warm winter and increase in egg production in the inner areas greatly contributes to the increasing in catch in the eastern SIS.     

Distribution and abundance of resting cysts of the toxic Alexandrium spp. (Dinophyceae) in sediments of the western Seto Inland Sea, Japan

Sediment samples were collected from 135 stations in the western part of the Seto Inland Sea (Iyo Nada, Suo Nada, Beppu Bay, Tokuyama Bay, Hiroshima Bay, Aki Nada, Hiuchi Nada and Bingo Nada) to determine the horizontal distribution and abundance of resting cysts of Alexandrium spp. ( A. tamarense  +  A. catenella ). Enumeration of the cysts was performed using the primuline-staining direct count method. Cysts of Alexandrium spp. were rarely found in Iyo Nada, Suo Nada and Beppu Bay, but were widely distributed in Tokuyama Bay, Hiroshima Bay, Aki Nada, Hiuchi Nada and Bingo Nada. Cyst concentrations ranged from not detected (ND) to 14, ND to 17, ND to 4, 93 to 8137, 8 to 4454, ND to 6, ND to 18 and 4–29 cysts/cm³ wet sediment in Iyo Nada, Suo Nada, Beppu Bay, Tokuyama Bay, Hiroshima Bay, Aki Nada, Hiuchi Nada and Bingo Nada, respectively. The majority of cysts occurred in Tokuyama Bay and Hiroshima Bay, where higher densities were observed in the inner bay and along the coastal margin. Relatively higher cyst concentrations were observed at stations with a higher mud content. The abundance of Alexandrium spp. cysts in western Seto Inland Sea is lower than in the eastern Seto Inland Sea, except for Tokuyama Bay and Hiroshima Bay. However, because sporadic blooms of Alexandrium have been observed, continuing monitoring is necessary to prevent paralytic shellfish poisoning outbreaks in this area.     

Food organisms and feeding habits of larval and juvenile Japanese flounder Paralichthys olivaceus at Ohama Beach in Hiuchi-Nada, the central Seto Inland Sea, Japan

ABSTRACT:   The abundance of food organisms and feeding habits of larval and juvenile Japanese flounder were examined during the period from May to August in 1999, 2000 and 2001 at the sandy Ohama beach, the central Seto Inland Sea. The food organisms collected with a sledge net consisted of 40 families from 18 orders, dominated by mysids, crangonids and gammarids. The mean densities of mysids, crangonid shrimp ( Crangon spp.), gammarids and fish were 2.74, 6.74, 2.91 and 0.15 individuals/m², respectively. The main prey of the flounder ( n  = 202; range of total length 9.80–75.95 mm) was mysids and small crangonid shrimp (<14 mm in body length). Prey fish availability was low, as the density of fish was low. The small crangonid shrimp was abundant, and the large crangonid shrimp, which could prey on larval flounder, was not abundant. The crangonid shrimp was important not as a predator for the flounder but as prey. The flounder preferred epifaunal mysids, Nipponomysis ornata and Anisomysis ijimai , to sand-burrowing mysids, Iiella oshimai , and avoided crangonid shrimp.     

Relationship between daily survival rates of larval Japanese anchovy (Engraulis japonicus) and concentrations of copepod nauplii in the Seto Inland Sea, Japan

We attempted to estimate the survival rates of larval cohorts (3–15 mm in body length) of the Japanese anchovy Engraulis japonicus population in Hiuchi‐nada, Seto Inland Sea, Japan, and examined the relationship to their food availability. The survival rates were directly calculated from the change in larval density during the survey. The estimated daily survival rate ranged between 0 and 89% d⁻¹, but increased with the increase in the mean concentration of small‐sized copepod nauplii (<100 μm in body length) sampled at 10 m depth. When the food concentration was higher than about 5 nauplii L⁻¹, the daily survival rate of larval anchovy reached an asymptote of approximately 89% day⁻¹. It might be possible to provide a framework for the forecast for larval abundance immediately prior to recruitment size (10–15 mm SL), based on larval abundance of the monitored size (3–8 mm SL) and the survival rate which is estimated from the concentration of small‐sized nauplii.     

Spawning season,spawning grounds,and egg production of red sea bream in Hiuchi-nada,Seto Inland Sea

The spawning season and grounds of red sea bream in Hiuchi-nada, the central part of the Seto Inland Sea, were described using a new method based on monoclonal antibodies for identifying Pagrus major eggs, and the daily egg production (standardized by the incubation time and survival rate) was estimated. At the peak of spawning (May), the ranges of sea temperature, salinity, and chlorophyll a where red sea bream eggs occurred were 14.8–17.4°C, 32.0–33.0, 0.5–4.4 μg/L, respectively. The main spawning grounds of the red sea bream were confirmed as being the areas near the Geiyo Islands, Misaki Peninsula, Saijyo, Niihama. The spatial spread of red sea bream eggs increased with the egg developmental stage. The ranges of daily egg production in 2005 and 2006 were 0.3–19.3 and 0.2–6.7 × 10⁹ eggs/day, respectively. In Hiuchi-nada, aquaculture farms are located close to the spawning grounds, and the potential spawning population from the aquaculture farms equaled or exceeded that of the estimated spawning population obtained by the egg production method. Red sea bream eggs in Hiuchi-nada might be produced by both wild and aquaculture-based spawning populations.     

The food habits of four species of triakid sharks, Triakis scyllium, Hemitriakis japanica, Mustelus griseus and Mustelus manazo, in the central Seto Inland Sea, Japan

ABSTRACT:   The food habits of 595 houndsharks of four species, Triakis scyllium ( n  = 179, 42–148 cm total in length), Hemitriakis japanica ( n  = 57, 42–102 cm), Mustelus griseus ( n  = 193, 39–100 cm), and Mustelus manazo ( n  = 166, 43–120 cm), found in the central Seto Inland Sea, Japan, from March 1997 to October 1999 and May 2000 to July 2002, were studied. T. scyllium changed their main food items from shrimps to echiuran worms then to cephalopods with their growth. Comparing food habits by the value of similarity (maximum = 1), the small-sized T. scyllium had a low value (0.17) compared to larger sharks. T. scyllium gradually increased the diversity of food until it reached 700 mm long in total length, however, after that it decreased. H. japanica appeared mainly in summer and autumn and ate cephalopods and fishes. M. griseus preyed on various crustaceans and decreased the diversity of food with growth. M. manazo preferred crustaceans and polychaetes. There was no certain tendency in the diversity of the food habit for M. manazo .     

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

ABSTRACT:     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.