Choriogenin (Chg) and vitellogenin (Vtg) subtypes and estrogen receptor α1 (Erα1) are upregulated by estradiol-17β (E2) in the liver of female salmonids. The aim of this study was to examine the relationships among hepatic mRNA levels for chgs (chgHα; chgHβ; chgL), vtgs (vtgAs; vtgC), erα1 and serum E2 levels in female cutthroat trout (Oncorhynchus clarki) during a reproductive cycle. Levels of serum E2 and hepatic chg mRNAs, as well as hepatic vtg mRNAs, increased in correlation with the progress of vitellogenesis. In the ovulated fish, chg mRNA remained at high levels while serum E2 and vtg mRNA levels decreased. Hepatic erα1 mRNA levels exhibited a peak in August (at the beginning of vitellogenesis) before levels of E2, chg and vtg mRNAs start to increase. These results suggest that expression levels of chg, vtg and erα1 genes are potentially regulated through E2 stimulation by different mechanisms.
Previous studies suggest that the size of female Lucensosergia lucens at sexual maturity (SAM) was smaller in 2004 and 2012 cohorts compared to other years. However, to assess the spawning stock quantitatively, the number of females spawning per day (spawning fraction) must be clarified. Therefore, to evaluate SAM and spawning fraction, females were collected during spawning season from May to October in 2013-2015, and ovaries were observed histologically to confirm sexual maturity. The smallest mature female was 30.57 mm in body length (BL). The relationship between BL and proportion of maturity was analyzed using a generalized linear model. Mean BL (mm) at 50% maturity was 34.13 for the 2013 cohort and 29.48 for the 2014 cohort. The daily proportions of females before and after spawning varied from 2.4 to 26.5% in 2013, and from 6.0 to 22.2% in 2014. The spawning fractions were 0.214-0.347, and the ranges were smaller than the proportion of females both before and after spawning. This study showed that the SAMs of the 2013 and 2014 cohorts were smaller than those of the 1960-1990 cohorts, and the spawning fraction was comparable to estimates in the literature.
Our previous study elucidated that commercial Chlorella vulgaris induces the resting egg production of the euryhaline rotifer Brachionus plicatilis sensu stricto caused by selenium (Se) fortification. This study confirmed production efficiency and hatchability of resting eggs with the commercial Se-fortified C. vulgaris. The aforementioned parameters were compared to those with Tetraselmis tetrathele (control) which is an optimal diet for resting egg production and hatching. The rotifers pre-cultured with Nannochloropsis oculata, and the produced resting eggs were employed to test diet effects. The hatching rate of collected resting eggs was estimated and then hatchlings were used for next generation culture. For the resting egg production with two different diets, T. tetrathele induced significantly higher productivity in the first generation, while no differences were observed in the second generations between two diets T. tetrathele and C. vulgaris. The resting eggs with N. oculata showed 6.0% of hatching rate, whereas it notably increased with two tested diets. The highest hatching rate was observed with C. vulgaris feeding (74.0-81.0%) through the all tested generations. The obtained results exhibit the possibility of C. vulgaris feeding for the mass production of rotifer resting eggs.
To induce the maturation of Ulva mediterranea thalli, we examined the effect of sporulation inhibitor elimination and temperature and salinity conditions in culture. In the experiment on sporulation inhibitor elimination, the maturation ratio of dual-layer tissue disks was compared, for which 3-mm-wide disks were punched out from thalli, and single-layer tissue disks were combined into one layer containing two cell sub-layers, forming dual-layer tissue disks. The maturation ratio of the single-layer tissue disks (83.3%) was significantly higher (p < 0.05) than that of the dual-layer tissue disks (16.7%). In the experiment on culture temperature, the maturation ratios of the dual-layer and single-layer tissue disks were compared at 10°C, 15°C, 20°C, 25°C, and 30°C. The temperature range suitable for maturation was 20-25°C, and the optimum temperature was 25°C. Maturation of the single-layer tissue disks progressed rapidly and sharply, compared with the dual-layer tissue disks. In the experiment on culture salinity, the maturation ratio of single-layer tissue disks was compared using 100%, 80%, 60%, 40%, and 20% seawater and distilled water. Maturation occurred in 60%, 80%, and 100% seawater, and the optimum salinity was 100% seawater.
The reproductive traits of Barbatula oreas were examined in a population established in the Kaname River, Kanagawa prefecture. Males exhibited unclear seasonal patterns in their monthly mean gonadosomatic index (GSI), and discharged semina from their genital ducts throughout the year. Females exhibited obvious seasonal patterns in the monthly mean GSI, which was higher in spring (March-April) and lower in summer (July-August). Ovarian maturity was divided into three developmental phases (resting, vitellogenic and maturation), and the ovaries in the maturation phase, which had the characteristics of ovaries in the spawning season, were observed from March to July. Post-ovulatory follicles were found even in the ovaries of small fish (<100 mm SL), indicating spawning at age-1. Total fecundity was 1,049-10,671 per ovary, increasing exponentially with the increasing SL. It was considered that vitellogenesis occurred at a wide range of water temperatures and day lengths (7.5-20.8°C, 9 h 47 min-14 h 33 min), and spawning season began when there was a transition to a suitable spawning temperature (10.2-18.5°C). This population, which matured at age-1, seemed to have a higher intrinsic rate of natural increase compared to a native population.
The relationship between otolith and somatic size was examined for ayu Plecoglossus altivelis altivelis sampled in Lake Biwa. Combined allometric (otolith radius < 400 µm) and linear (otolith radius ≥ 400 µm) equations showed the closest fit relationship between otolith radius and standard length. Slower growing ayu tend to have larger otoliths than faster growing ayu at the same somatic size. This phenomenon has been called the growth effect and may be a cause of bias in the back-calculation of fish length from otoliths.
To determine the optimal time for releasing chum salmon into the Gakko River, otolith-marked juveniles reared to more than 1 g body mass were released in late February, mid-March, and late March of 2009 to 2011. Although factors that determine return rates are yet unknown, the return rates were highest for the mid-March (0.62%-1.02%) groups of all three brood years. Return rates for the late February and late March groups were approximately half that of the mid-March groups. These results suggest that releasing chum salmon in mid-March is optimal for increasing return rates.
To ascertain seasonal changes in food habits of the introduced catfish Ictalurus punctatus and prey availability, the stomach contents of 218 specimens (13.1-60.5 cm in standard length) collected in a reed area in Lake Kitaura, Japan, from May to August 2015, were examined. Stomach fullness index in immature and adult fish did not differ significantly between May-June and July-August. Ranking index of major food items (terrestrial plants, fragments of large fishes, chironomid larvae, Macrobrachium nipponense and Tridentiger brevispinis) in immature and adult fish largely changed from May-June to July-August, such changes being partly consistent to seasonal dynamics in prey abundance.
High mortality of age-0 hatchery-reared Pacific bluefin tuna (PBT) frequently occurs in sea cages during winter, particularly in fish farms around northern Kyushu in Japan. The hatchery-reared PBT are usually smaller than wild captured fish, suggesting that body size would be related to survival during winter. To reveal the relationship between survival and body size during winter, we examined survival and growth of age-0 PBT reared from fertilized eggs obtained in June (large-size group) and August (small-size group) in sea cages. The survival rate of age-0 PBT in the large-size group, after transfer from indoor tanks to sea cages till the next spring, was higher than the small-size group. High mortality occurred in PBT juveniles in the small-size group during winter, which showed a correlation with a decrease in water temperature. Moreover, the body size of large-size group was significantly larger than that of small-size group during the experimental period. These results suggest that body size is a critical factor affecting the survival of age-0 PBT during winter.