Annual changes in distribution depths of bighand thornyhead <Emphasis Type="Italic">Sebastolobus macrochir</Emphasis> off the Pacific coast of northern Honshu,Japan

ABSTRACT:   Body size dependent distribution patterns of bighand thornyhead Sebastolobus macrochir were examined using bottom trawl surveys in the Tohoku area, off the Pacific coast of northern Honshu, Japan. In northern and southern Tohoku, distribution depths of ≤10-cm standard length (SL) fish increased from 500–600 m in 2003 to 600–700 m in 2005 with an increase in density of 11–20-cm SL fish at depths of 500–600 m. In 2004–2005, small fish (≤10 cm SL) were distributed in deeper waters than large fish (11–20 cm SL). In ≤20-cm SL fish, the main distribution depths changed from 300–600 m in 1997–2002 to 500–600 m in 2003 in southern Tohoku, but not in northern Tohoku. Stepwise multiple regression analysis showed that negative correlations were found in biomass densities between a few demersal fish species and bighand thornyhead at depths of 300–400 and 400–500 m in southern Tohoku. These results suggest that distribution of bighand thornyhead was affected by intra- and interspecific competitions in the Tohoku area.     

Value-per-recruit analysis of bighand thornyhead <Emphasis Type="Italic">Sebastolobus macrochir</Emphasis> caught off the Pacific coast of northern Honshu,Japan

The yield-per-recruit (YPR) analysis has been used to assess growth overfishing of exploited fish stocks. Since the ex-vessel price per weight widely depends on the size of the fish, the result of the YPR does not imply the revenue. The value-per-recruit (VPR) analysis, which is an economic expansion of the YPR analysis, was conducted for bighand thornyhead Sebastolobus macrochir caught off the Pacific coast of northern Honshu, Japan. The results showed that, for the current age at first capture (t _c = 3), the current fishing mortality (F = 0.12) was close to the optimum level. However, for the current F, optimum t _c in the VPR model was 8 years and would lead to a 47% increase compared to the current revenue. The maximum YPR was obtained at t _c = 6 years, but the revenue at t _c = 6 only gave a 20% increase in the maximum revenue. The difference was caused by the size dependency of the ex-vessel price per weight. The VPR model would generate fisheries management plans with both economical and biological consideration and would help in making general consensus of fisheries management strategies.     

Growth changes in bighand thornyhead <Emphasis Type="Italic">Sebastolobus macrochir</Emphasis> off the Pacific coast of northern Honshu,Japan

ABSTRACT:     The biomass of bighand thornyhead Sebastolobus macrochir was increased by the high recruitment success of the 1999–2002 year classes off the Pacific coast of northern Honshu, Japan. In this study, the growth of bighand thornyhead was examined over a 9-year period from 1996 to 2004 in this area. The growth of the 1999 year class and the 2000–2002 year classes was reduced at 3 and 2 years old, respectively, while the 1999–2002 year classes were smaller than the 1993–1998 year classes. In 2-, 3- and 4-year-old fish, the relationship between abundance and mean standard length was expressed by negative linear regressions, while fish became smaller when abundance of the year class was larger. Mean bottom temperatures were stable at depths of 350–900 m; variations in water temperature were small in the main distribution area of bighand thornyhead. We discuss the factors affecting the growth of bighand thornyhead via changes in the demersal fish community and feeding habits.     

<Emphasis Type="Italic">Neoheterobothrium hirame</Emphasis> (Monogenea) alters the feeding behavior of juvenile olive flounder <Emphasis Type="Italic">Paralichthys olivaceus</Emphasis>

We monitored feeding behavior and survival of starved juvenile olive flounder experimentally infected with the gill monogenean Neoheterobothrium hirame. Infected flounder increased amount of the time spent in the water column by 117% when trying to capture live mysids, Neomysis sp. They also showed different feeding patterns from those of uninfected fish and made fewer attacks towards prey during one feeding attempt. Although the average numbers of mysids captured by individuals were similar between infected and uninfected fish, heavily infected fish tended to catch less prey. These results indicate that N. hirame reduces the feeding efficiency of the host for capturing live prey and possibly makes them more vulnerable to predation during feeding. We could not detect any obvious difference in survival rates between uninfected, lightly and heavily infected fish during 3 months of starvation. There was no evidence that starvation makes fish more susceptible to N. hirame. The present study provides first experimental evidence that N. hirame affects feeding behavior of juvenile olive flounder and supports the idea that this parasite indirectly reducing the host’s survival and may be responsible for the recent reduction of the flounder population in Japan.     

Repeatable immersion infection with <Emphasis Type="Italic">Photobacterium damselae</Emphasis> subsp. <Emphasis Type="Italic">piscicida</Emphasis> reproducing clinical signs and moderate mortality

Pseudotuberculosis is a bacterial septicaemia caused by Photobacterium damselae subsp. piscicida in several marine fish species. Yellowtail Seriola quinqueradiata is the most sensitive fish species to this disease. The internal organs of naturally infected yellowtail exhibit whitish spots, tubercle-like tissue structures, consisting of bacterial accumulations. There have been many trials for experimental infection, however adequate method of infection that reproduces moderate mortality and primary clinical signs has not yet established. Present investigation evaluated an immersion infection method by using logarithmic culture-phase bacteria resulting in higher mortality than that using stationary culture-phase bacteria. Typical white spots on the spleen and kidney were also observed constantly in dead fish. Transmission electron microscopy and fluorescent antibody microscopy showed bacterial clusters not only in the spleen and kidney but also in the blood channels in the secondary gill filaments. These results were confirmed repeatedly by plural experiments. The use of logarithmic-phase bacteria in immersion infection is an appropriate technique to reproduce moderate mortality and primary clinical signs, which will be a reliable infection method also for the challenge test of pseudotuberculosis vaccine.     

Estimation of terminal molting probability of snow crab <Emphasis Type="Italic">Chionoecetes opilio</Emphasis> using instar- and state-structured model in the waters off the Pacific coast of northern Japan

Terminal molting probability (TMP) of snow crab Chionoecetes opilio was estimated using instar- and state-structured model incorporating the terminal molt in the waters off the Pacific coast of northern Honshu, Japan. TMP is defined as the probability that terminal molt occurs within a year in an instar and is an important parameter for stock assessment. TMP was estimated from the model and the rate of terminal molted crabs (RTM) for instars, which means carapace widths over 30 mm. Carapace widths at RTM of 0.5 were 81 mm for males and 66 mm for females. TMP was estimated as 0.23 and 0.29 for these carapace widths, respectively. Carapace widths at TMP of 0.5 were 94 mm for males and 71 mm for females, and for these carapace widths 57% of male and 50% of female crabs were considered as old-shell which had survived 1 year or more from terminal molt. Changing the fishing or natural mortality by ±0.1, the maximum difference of TMP was less than 0.1 from default estimates.     

Induction of diploid gynogenesis in turbot <Emphasis Type="Italic">Scophthalmus maximus</Emphasis> with left-eyed flounder <Emphasis Type="Italic">Paralichthys olivaceus</Emphasis> sperm

Turbot Scophthalmus maximus exhibits sexually dimorphic growth, with females growing faster and reaching larger adult sizes than males. Thus, development of techniques for preferentially producing females is necessary to optimize production of these species. In this paper, gynogenetic diploids of turbot were induced by activating egg development with ultraviolet (UV)-irradiated left-eyed flounder Paralichthys olivaceus sperm combined with cold shock to prevent extrusion of the second polar body. The results of UV irradiation experiments showed that survival, motility, and duration of activity of P. olivaceus sperm generally decreased with increase in UV dose. The typical Hertwig’s effect was observed after fertilized turbot eggs with UV-irradiated P. olivaceus sperm and the optimal UV dose for gynogenetic haploid production was 36,000 erg mm⁻². At 15°C, appropriate timing of cold shock for retention of the second polar body in turbot eggs was at 6 min after fertilization. Results of different combinations of two shock temperatures (1 or 3°C) and four shock durations (15, 25, 35 or 45 min) at 6 min after fertilization demonstrated that shock of 25 min at 1°C gave the highest production of diploid gynogens (39.58% relative to its diploid control). The results of this study reveal that the use of UV-irradiated P. olivaceus sperm for activation of turbot eggs and cold shock for polar body retention is an effective method to produce gynogenetic offspring.     

Spawning frequency of the Tsushima Current subpopulation of chub mackerel <Emphasis Type="Italic">Scomber japonicus</Emphasis> off Kyushu,Japan

Female Japanese chub mackerel Scomber japonicus of the Tsushima Current subpopulation were collected during the spawning season from March to May 2001. A total of 137 adult females were caught between midnight and daybreak. A considerable number of fish displayed new postovulatory follicles (POF), whereas there was no evidence of germinal vesicle breakdown or hydrated oocytes in any of the fish collected. This suggests a daily spawning synchronicity toward midnight. To estimate the spawning frequency (S), the female reproductive state was classified into four criteria based on the degenerative stage of the POFs and the developmental stage of the oocytes. To stage the POFs according to age and determine the stage duration, ovaries from S. japonicus were induced to spawn in the laboratory and were sampled 0–72 h after ovulation at appropriate intervals. The average S, which is evaluated from four different indices, was 16.9%, corresponding to the average female chub mackerel spawned every 5.9 days (8.5 times) during the 50 days.     

Occurrence of toxic <Emphasis Type="Italic">Dinophysis acuminata</Emphasis> (Dinophyceae) in Uranouchi Inlet,Japan

In the Uranouchi Inlet, which has a commercial fishery of littleneck clam Ruditapes philippinarum, potentially toxigenic species of Dinophysis acuminata showed an annual occurrence except for the summer of 2000–2002. Relatively high abundances of D. acuminata were observed in April 2000 and November 2002. Some samples of D. acuminata cells, which were collected from the inlet in autumn 2002, contained pectenotoxin-2 (0.9–2.8 pg/cell) and okadaic acid (1.9–2.8 pg/cell). Cell length and hypotheca width of D. acuminata found in the Uranouchi Inlet were 33.7–45.0 and 21.0–31.6 μm, respectively. The length-width ratios ranged 1.21–1.71. Sequences of the 5.8S ribosomal RNA gene and flanking internal transcribed spacers, 1 and 2 of cells of D. acuminata from the inlet fell into a D. acuminata complex clade that included, D. acuminata-D. sacculus-D. acuta reported by others. In the inlet, occurrences of D. acuminata were detected within a broad water temperature range between 13.0 and 26.9°C, and those showing more than 25 cells/L were within 16.4–17.3°C for temperature and 27.6 and 34.1 for salinity. This is the first report that cells of Dinophysis acuminata from southern Japanese coastal waters contained okadaic acid and pectenotoxin-2.     

Weaning of greater amberjack (<Emphasis Type="Italic">Seriola dumerilii</Emphasis> Risso 1810) juveniles from moist to dry pellet

The weaning of greater amberjack (Seriola dumerilii Risso 1810) to a dry diet was studied in two consecutive experiments. The experimental fish were hatchery-produced and grown to their initial size on a soft pellet containing 50% trash fish and 50% commercial dry pellet. In the first experiment, three homogenous groups of greater amberjacks (n = 153, mean weight: 374 ± 43 g), initially adapted for a period of 30 days to a diet with a 40% moisture content, were fed on commercial pellets containing three different levels of dietary moisture (7, 20 and 40%, respectively) for 126 days. We found that fish needed a month to adapt to the 7%- and 20%-moisture diets and that moisture content of the diet had a significant effect on fish growth in terms of increasing feed consumption and lowering feed conversion ratio. The group fed on the 20%-moisture diet not only compensated for the initial adaptation period but presented the higher growth rate and best feed conversion ratio. In the second experiment, all groups of experiment 1 were fed the same commercial diet with a 7% moisture content for 72 days. The group that had been previously fed on the 20%-moisture diet showed the best growth performance. The implication of the above results for the weaning of greater amberjack to dry diets is discussed in terms of the digestion process and physiology.     

Circadian melatonin release <Emphasis Type="Italic">in vitro</Emphasis> by European sea bass pineal

Melatonin production by the fish pineal is under the control of photoperiod. In addition, in some fish species pineal melatonin production is self-sustained by an intrapineal circadian pacemaker, not in salmonids, though. The aim of this research was to test melatonin release in vitro by the pineal of sea bass held under LD 12:12 and DD conditions to determine the endogenous nature of melatonin rhythm. Circadian rhythmicity ( = 28.3 h) was found since rhythmic melatonin release persisted on DD.     

Effects of soy sauce <Emphasis Type="Italic">koji</Emphasis> and lactic acid bacteria on the fermentation of fish sauce from freshwater silver carp <Emphasis Type="Italic">Hypophthalmichthys molitrix</Emphasis>

The effects of soy sauce koji and the lactic acid bacterium, Tetragenococcus halophilus, were studied on the fermentation of fish sauce prepared from Chinese silver carp. The fish sauce prepared without koji and the lactic acid bacterium contained low levels of organic materials, total nitrogen, and organic acids. The use of koji was effective in increasing these qualitative parameters and further improved the amino acid score of the fish sauce. Addition of T. halophilus had an effect on lowering the pH value during the initial period of fermentation when the soy sauce koji was also supplemented. In contrast, T. halophilus-like bacteria were found to be predominant for all tanks fermented under the different starting conditions. Although it was not examined whether the T. halophilus-like bacteria observed after fermentation were the same as the starter-bacterium or not, it was suggested that T. halophilus plays an important role in the successful fermentation of silver carp fish sauce. Sensory evaluation conducted with Japanese and Chinese panelists also suggested the superiority of the use of koji for fermentation of silver carp fish sauce.     

Growing depth limit of <Emphasis Type="Italic">Zostera caulescens</Emphasis> in coastal waters along the Japan Sea coast of Honshu,Japan

There is a possibility that deep coastal marine macrophytes will be less critically affected by thermal stress associated with climate change and remain as refugia. Thus, information on them is expected to contribute to conservation of biodiversity in coastal areas affected by climate change. To document the deep-growing Zostera caulescens in relation to the light environment, field surveys were conducted in coastal waters of the central area along the Japan Sea coast of Honshu, Japan, and then the relationship between the light environment and seagrass depth limit was examined. In the coastal waters of Sado Island and Noto Peninsula, the presence of deep coastal communities of Z. caulescens was confirmed in the depth range of 20–25 m. Biomass and shoot density for Z. caulescens, ranging 34.1–51.3 g DW m^?2 and 83–112 shoots m^?2, were recorded at a depth of 20 m in Ryotsu Bay, Sado Island. For the genus Zostera including the deep-growing Z. caulescens, the relation between the extinction coefficient and seagrass depth limit was described by the fitted regression equation. The minimum light requirement of the deep-growing Z. caulescens was markedly lower than that of the shallow-growing Zostera marina.     

Growth and survival of grouper <Emphasis Type="Italic">Epinephelus</Emphasis><Emphasis Type="Italic">coioides</Emphasis> (Hamilton) larvae fed free-living nematode <Emphasis Type="Italic">Panagrellus redivivus</Emphasis> at first feeding

The free-living nematode, Panagrellus redivivus, was tested as live food for grouper Epinephelus coioides larvae during the first feeding stage. A series of experiments were conducted to determine the acceptability of the free-living nematodes in grouper larvae at first feeding, the optimum nematode density and the response of the larvae to nutritionally enriched nematode. All experiments were conducted in 200-L conical tanks filled with 150-L filtered seawater and stocked at 15 larvae L⁻¹. Duration of feeding experiments was up to day 21 (experiment 1) and 14 days (experiment 2 and 3). Brachionus plicatilis and Artemia (experiment 1) and Brachionus plicatilis alone (experiment 2 & 3) was used as the control treatment. Observations indicated that the grouper larvae readily fed on free-living nematodes as early as 3 days posthatching, the start of exogenous feeding. Optimum feeding density for the larvae was 75 nematodes ml⁻¹. The enrichment of cod liver oil or sunflower oil influenced the total lipids and n-3 highly unsaturated fatty acids of P. redivivus, which in turn influenced those of the grouper larvae, however, growth and survival of the larvae were not affected (P > 0.05). The results from this investigation showed that the nematode, P. redivivus, can be used as first live food for grouper larvae from the onset of exogenous feeding until they could feed on Artemia nauplii.     

Reproductive biology of blue marlin <Emphasis Type="Italic">Makaira nigricans</Emphasis> around Yonaguni Island,southwestern Japan

We studied the spawning seasonality and gonadal development of blue marlin Makaira nigricans using specimens captured around Yonaguni Island in southwestern Japan between February 2003 and February 2006. The mean (±SD) lower jaw–fork length of females (234 ± 24 cm) was greater than that of males (191 ± 12 cm). The smallest mature female and male were 183 and 160 cm, respectively. Most of the 717 females had immature ovaries. However, in March and from May to September, the ovaries of 26 females contained oocytes with yolk globules, hydrated oocytes, or postovulatory follicles. Most males had testes with a large amount of spermatozoa throughout the year. The occurrence of mature blue marlin at Yonaguni Island suggests that spawning occurs here. The mean condition factors (fatness of the fish) of both sexes decreased from March to June or July, presumably as they expended energy to reproduce. We discuss our results in the context of migration theory for blue marlin in the western North Pacific Ocean.