Yasufumi Fujimoto

Circadian Changes in Serum Concentrations of Steroids in Japanese Char Salvelinus leucomaenis at the Stage of Final Maturation

Abstract Circadian changes in serum concentrations of testosterone (T), 11-ketotestosterone (11KT), estradiol-17β (E2), 17α,20β-dihydroxy-4-pregnen-3-one (DHP), 17α-hydroxyprogesterone (OHP), cortisol (F) and progesterone (P) were investigated in the spermiated/ovulated Japanese char Salvelinus leucomaenis for over three days using newly developed time-resolved fluoroimmunoassays. Testosterone and DHP in both sex and 11KT in male showed significantly (P<0.05) higher serum levels just before/after onset of darkness (15:00 or 18:00), and the levels during night and daytime were significantly (P<0.05) lower than those of the peak levels. Serum F levels in both sex during dark phase were significantly (P<0.05) higher than those levels during daytime. A surge of serum OHP concentrations in both sexes was observed at the time of twilight (03:00). The peak time of serum T, 11KT and DHP levels were approximately 6 hours prior to those of serum F and OHP levels. Serum E2 in female and P in both sex fluctuated intensely during sampling period, and did not show remarkable changes. These results strongly suggest the existence of circadian-like diel changes in serum T, DHP, F and OHP levels in both sex and 11KT in male, and no variations in serum E2 in female and P in both sex in spermiated/ovulated Japanese char under the stage of final maturation. Furthermore, relationship between circadian rhythms of steroid hormones and spawning behaviors are discussed in the present study.

Influence of modern irrigation, drainage system and water management on spawning migration of mud loach, Misgurnus anguillicaudatus C.

We investigated the influence of human activities on the spawning migration of the mud loach, Misgurnus anguillicaudatus. Mud loaches migrate from rivers to paddy fields for spawning. In this study, the number of adult loaches in paddy fields showed a peak in late May and rapidly decreased in July. In early July, water is discharged from paddy fields in order to allow the fields to dry for a few weeks, and water is again supplied later. The spawning season of the loaches probably spans from mid May to August; this was concluded by observing physiological indicators such as the gonad somatic index, oocyte maturity and serum levels of testosterone, estradiol-17β and 17α, 20β-dihydroxy-4-pregnen-3-one. When the loaches mature, they migrated to paddy fields if they could swim upward through the drainage ditches. However, the drainage ditches of modern paddy fields were reconstructed to deepen the water levels. The artificial drought of the rice culture management led to a discharge of water and loaches from the paddy fields during their spawning season. These results indicate that modern rice cultivation causes obstruction of the migration route and loss of spawning grounds; consequently, loaches use limited space and period of the paddy field for their spawning.

Ontogenetic changes in the trophic position of a freshwater Unionidae mussel

Freshwater mussels have often been used as indicators of a trophic baseline (i.e., primary consumers) in the food web. To assess the utility of a large filter-feeding mussel, Cristaria plicata, as an isotopic indicator, we compared the carbon (delta C-13) and nitrogen (delta N-15) stable isotope ratios of the mussel with those of its potential food sources, particulate organic matter (POM) and sediment organic matter (SOM). The delta C-13 values of large mussels (shell length > 140 mm, -31 parts per thousand to -29.2 parts per thousand) were similar to those of POM (-30.2 parts per thousand) rather than those of SOM (-27.6 parts per thousand), indicating that mussel carbon was derived mainly from POM (mainly phytoplankton). In contrast, the mussels exhibited 6.3 parts per thousand to 9.0 parts per thousand higher delta N-15 values than did POM. Assuming a previously reported trophic enrichment factor (+3.4 parts per thousand), the trophic level estimate of the mussel ranged from 2.9 to 3.6, indicating that they functioned as secondary rather than primary consumers. Our results also revealed positive correlations between shell lengths and delta N-15 values, suggesting that the mussels changed their trophic position from primary consumer (shell length 140 mm) with growth. A significant inter-annual difference was found in the y-intercepts of the regression lines between shell length and delta N-15, indicating that mussels can reflect inter-annual changes in the isotopic baseline (i.e., primary producers).

Application of a high-resolution acoustic video camera to fish classification: An experimental study

In this study, we try for fish classification using high resolution acoustic video camera at 3.0 MHz center frequency (ARIS). Newly observation method with wide in vertical and narrow in horizontal beam (WVNHB) was proposed for getting high quality acoustic image of swimming fish, and tested an image processing algorithm basing on Normalized Cross Correlation (NCC) for fish classification. This high-frequency acoustic video camera was applied to fish classification for the first time. Using the proposed observation method, we successfully obtained high quality acoustic images of fish and performed an image processing algorithm on the images.

Habitat Use and Shift of Two Sympatric Freshwater Sculpins (Cottus pollux and Cottus hangiongensis) during the Spawning and Non-Spawning Seasons

ABSTRACT The spawning seasons and distributions of sympatric sculpins, namely, the small egg-type (SE) of Cottus pollux and C. hangiongensis, were investigated in a coastal river of northeastern Japan. The spawning seasons for both species last for four months, and the ovulation rate peaks in January and April for C. pollux and C. hangiongensis, respectively. The males of both sculpins nested in cavities under rocks in stony areas. C. pollux was distributed in reed marshes in summer and in stony areas during the spawning season, while C. hangiongensis was distributed in stony areas throughout the year. There was a difference in habitat use between the males and females of C. pollux during the spawning season; the males were distributed in stony areas for a longer period than the females. These results indicate that mature C. pollux shifts its habitat from marshes to stony areas for spawning.

Effect of natural light conditions on the use of cover in concrete block structures by Japanese dace Tribolodon hakonensis

The effects of light intensity on the use of cover by Japanese dace, Tribolodon hakonensis, under natural and experimental conditions were examined. Concrete blocks were placed to provide cover in a study pool and the numbers of dace in open water and under cover were recorded. During mid-afternoon, the number of dace in open water decreased while the number under the block cover increased. An experimental tank was also prepared, one part of which was exposed to direct sunlight (5000–50 000 lx) and the other part was screened to allow moderate light intensity (shade rate: 27% or 93%). The dace were able to select either part. In higher daylight intensity, the dace were induced to stay longer in the shaded part. These results show that the dace use cover as a shelter with lower light intensity as compared to direct sunlight. This study demonstrates that the preparation of lower light intensity shelters by providing additional cover is an important preservation technique in the freshwater ecosystem.

New method of fish classification by using high-resolution acoustic video camera-ARIS and local invariant feature descriptor

A new observation method of fish classification by using acoustic video camera (ARIS) at 3.0 MHz center frequency was developed. Using the method, high-quality acoustic image was obtained. After getting the acoustic image of fish by ARIS, we utilized a method called Scale-Invariant Feature Transform (SIFT) [1] into fish classification instead of the method that we used before, which is called Normalized Cross Correlation (NCC) [2], and then we compared and contrasted position matching rate of the two methods of NCC and SIFT, and improved the SIFT algorithm to increase the accuracy.

Zoobenthos are minor dietary components of small omnivorous fishes in a shallow eutrophic lake

We examined whether small omnivorous fishes (smaller than ~100mm long) integrate littoral, pelagic and benthic pathways in a shallow, eutrophic lake (Lake Izunuma, Japan). The surface of the lake was covered by a dense vegetation of floating-leaved macrophytes, and small species dominated the icthyofauna. We determined the δ13C and δ15N ratios of five dominant species of small omnivorous fishes. Using a stable isotope analysis in the R mixing model, we determined the possible contribution of three potential food sources (epiphytic algae, zooplankton and zoobenthos (larval chironomids)) to omnivorous fish tissue compositions. Four omnivorous fishes (Gnathopogon elongatus elongatus, Pseudorasbora parva, Biwia zezera and Tridentiger obscurus) subsisted largely on epiphytic algae and zooplankton, whereas zoobenthos contributed little to their diets. Acheilognathus rhombeus subsisted mostly on epiphytic algae. Thus, in this shallow, eutrophic lake, omnivorous fishes incorporated both littoral and pelagic production into the food web, but rarely benthic production. The dominant benthic chironomid larvae often burrow several centimetres into the sediment, and the low dietary contribution of zoobenthos to small fishes may be due to inefficiency at foraging on buried benthos associated with fish body size.

Phylogeography of Endangered Bitterling Acheilognathus melanogaster Endemic to Eastern Japan

The bitterling Acheilognathus melanogaster is a critically endangered primary freshwater fish endemic to the Pacific side of eastern Japan. To elucidate A. melanogaster genetic structure, we investigated phylogeography in nine populations, using gene sequences of mitochondrial Cytochrome b (Cytb), as well as nuclear Rhodopsin (Rho) and glycosyltransferase (Glyt). We found four Cytb-based geographical clusters unevenly divided between the northern and southern regions, with smaller groups in the south. Of the nuclear genes, Glyt did not show geographical differentiation, whereas Rho formed two clusters: one widely occurring and another restricted to central regions. Genetic diversity was generally higher in southern than in northern populations. Our results suggest that conservation of southern local populations is particularly important in maintaining the genetic diversity of this endangered fish.