Reproductive parameters of the chub mackerel <Emphasis Type="Italic">Scomber japonicus</Emphasis> estimated from human chorionic gonadotropin-induced final oocyte maturation and ovulation in captivity

ABSTRACT:   Final oocyte maturation and ovulation of captive chub mackerel Scomber japonicus with fully yolk-accumulated oocytes were induced by a single injection of human chorionic gonadotropin. Reproductive parameters, including spawning frequency and batch fecundity, which are required to estimate spawning biomass in pelagic fish by the daily egg production method, were analyzed. Germinal vesicle migration (GVM) occurred at 18–24 h post-injection, and the hydration and ovulation of oocytes were completed at 30 and 36 h post-injection, respectively. The results of the maturation process suggest that fish with GVM-stage ovaries captured in the daytime from the field are capable of spawning on the night following their capture. The oocytes used in the oocyte size-frequency distribution method for batch fecundity estimates should be at late GVM and more advanced stages. The results of sequential artificial insemination showed that the quality of ovulated eggs held in the ovarian lumen rapidly deteriorated as time progressed after ovulation. This indicates that the fertilization window for the ovulated eggs of chub mackerel lasts only a few hours, and spawning behavior should be performed within a few hours after ovulation in the wild population.     

Age validation,growth and annual reproductive cycle of chub mackerel <Emphasis Type="Italic">Scomber japonicus</Emphasis> off the waters of northern Kyushu and in the East China Sea

The age and growth of chub mackerel Scomber japonicus collected from the East China Sea and the northern waters off Kyushu between June 2000 and June 2001 were determined by observing the otolith surface after dipping it in xylene. The translucent and opaque zones on the otolith surface were identified, and the number of translucent zones was counted. Monthly changes in the frequency of fish with translucent zones on the otolith margin, and in the marginal increments, indicated that the translucent zones were formed between April and June. The seasonal pattern of annulus formation on the otolith became clear by observing the otoliths of fish with known ages, and the otolith formation in wild fish was consistent with that of fish with known ages. The mean gonadosomatic index of male and female fish was high from March to May, and spawning females were observed from mid-March to mid-May. The estimated ages were 1–5 years for males and 1–6 years for females. The von Bertalanffy growth parameters did not significantly differ between male and female. The model was obtained as FL _t=406×{1−exp[−0.372×(t+1.68)]     

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.     

Spectral sensitivity of juvenile chub mackerel (<Emphasis Type="Italic">Scomber japonicus</Emphasis>) in visible and ultraviolet light

Although chub mackerel (Scomber japonicus) is widely distributed all over the world, the relevance of its visual sensitivity to its ecology is not yet fully understood. We investigated spectral sensitivity in juvenile chub mackerel in the range of ultraviolet (UV) to visible light (369–652 nm) by electroretinogram (ERG) using light-emitting diodes (LEDs). Sensitivity peaked at a wavelength of approximately 482 nm in dark-adapted fish and 525 nm in light-adapted fish. A secondary sensitivity peak in the UV range at approximately 382 nm was found in both dark- and light-adapted fish. The UV sensitivity of chub mackerel may be attributable to UV transmissibility of the optical media and to the presence of a beta-band of visible light-sensitive visual pigments, and not to an alpha-band of UV visual pigments. This UV sensitivity may be useful for feeding or communication with other fishes.     

Effects of synthetic kisspeptin peptides and GnRH analogue on oocyte growth and circulating sex steroids in prepubertal female chub mackerel (Scomber japonicus)

In recent years, kisspeptin peptides, encoded by kiss genes have been used to manipulate reproductive processes in farmed animals, including fish. Our previous studies demonstrated that the chub mackerel brain expresses kiss1 and kiss2 and intramuscular injection of synthetic Kiss1 pentadecapeptide (Kiss1‐15) but not Kiss2 dodecapeptide (Kiss2‐12) accelerates spermatogenesis in prepubertal male chub mackerel (Scomber japonicus). In the present study, we evaluated their effects in prepubertal female chub mackerel. The gonadosomatic index (GSI) values of experimental fish did not show any significant changes. Condition factor (CF) values increased significantly in Kiss1‐15 treated fish, in comparison with control and GnRH analogue (GnRHa) injected fish. Histologically, only perinucleolar oocytes were found in all experimental fish. However, Kiss and GnRHa treated fish showed a significant increase in the perinucleolar oocyte diameter, in comparison with the control fish. Gene expression analyses revealed decreased expression of gnrh1 in the telencephalon‐preoptic region of the brain of Kiss2‐12 and GnRHa injected fish, in comparison with control fish. In contrast, GnRHa injected fish exhibited higher levels of fshβ in the pituitary, with no changes in the levels of lhβ among different treatments. Levels of circulating sex steroids, testosterone, and estradiol‐17β were significantly higher in Kiss1‐15 injected fish, in comparison with control fish. These results indicate that synthetic kisspeptin peptides and GnRHa can induce oocyte growth in prepubertal female chub mackerel.     

Determination of hormones inducing oocyte maturation in <Emphasis Type="Italic">Chalcalburnus tarichi</Emphasis> (Pallas, 1811)

Chalcalburnus tarichi is an endemic cyprinid species living in the Lake Van basin, in eastern Anatolia, Turkey. The present study was undertaken to determine which hormones induce oocyte maturation in C. tarichi. The levels of 17α,20β,21-trihydroxyprogesterone (20β-S), progesterone (P), 17α-hydroxyprogesterone (17α-HOP), 11-deoxycortisol (11-DOC), and 17α-hydroxy-20β-dihydroprogesterone (17,20β-P) were measured in fish caught from Lake Van and the Karasu River, and injected with human chorionic hormone (hCG) (1,000 and 1,500 IU/kg). Oocytes of fish caught from the lake were also incubated in vitro with different doses (50, 200, and 1,000 ng/ml) of 20β-S, 17α-HOP, 11-DOC, and 17,20β-P. 11-DOC was found to be the most effective hormone among those measured for inducing oocyte maturation in vivo and in vitro. 17,20β-P could not be determined in the plasma of any fish in vivo (P < 0.05). 1,000 IU/kg dose of hCG given by injection caused a statistically significant increase in all plasma hormone levels (P < 0.05). It was found that there was a significant decrease in the P level only at 1,500 IU/kg dose of hCG injected (P < 0.05), while the level of other hormones increased at this dose (P < 0.05). It was also determined that all the hormones were effective in germinal vesicle breakdown (GVBD) in in vitro oocyte culture (P < 0.05). However, 11-DOC was found to be the most effective hormone in GVBD at a dose of 200 ng/ml (70% GVBD). In conclusion, 11-DOC synthesized during final oocyte maturation in C. tarichi was found to be a potent inducer of GVBD, which shows that 11-DOC may be described as an oocyte maturation steroid in this species.     

Dim-light photoreceptor of chub mackerel <Emphasis Type="Italic">Scomber japonicus</Emphasis> and the photoresponse upon illumination with LEDs of different wavelengths

To study the absorption characteristics of rhodopsin, a dim-light photoreceptor, in chub mackerel (Scomber japonicus) and the relationship between light wavelengths on the photoresponse, the rod opsin gene was cloned into an expression vector, pMT4. Recombinant opsin was transiently expressed in COS-1 cells and reconstituted with 11-cis-retinal. Cells containing the regenerated rhodopsin were solubilized and subjected to UV/Vis spectroscopic analysis in the dark and upon illumination. Difference spectra from the lysates indicated an absorption maximum of mackerel rhodopsin around 500 nm. Four types of light-emitting diode (LED) modules with different wavelengths (red, peak 627 nm; cyan, 505 nm; blue, 442 nm; white, 447 + 560 nm) were constructed to examine their effects on the photoresponse in chub mackerel. Behavioral responses of the mackerels, including speed and frequencies acclimated in the dark and upon LED illumination, were analyzed using an underwater acoustic camera. Compared to an average speed of 22.25 ± 1.57 cm/s of mackerel movement in the dark, speed increased to 22.97 ± 0.29, 24.66 ± 1.06, 26.28 ± 2.28, and 25.19 ± 1.91 cm/s upon exposure to red, blue, cyan, and white LEDs, respectively. There were increases of 103.48 ± 1.58, 109.37 ± 5.29, 118.48 ± 10.82, and 109.43 ± 3.92 %, respectively, in the relative speed of the fishes upon illumination with red, blue, cyan, and white LEDs compared with that in the dark (set at 100 %). Similar rate of wavelength-dependent responses was observed in a frequency analysis. These results indicate that an LED emitting a peak wavelength close to an absorption maximum of rhodopsin is more effective at eliciting a response to light.     

Sound generated by a payao and comparison with auditory sensitivity of jack mackerel <Emphasis Type="Italic">Trachurus japonicus</Emphasis>

Sound generated by a payao, an anchored bamboo fish aggregating device, is believed to be attractive to fish; but until now, there is no available record of payao-generated sound. This study presents payao-generated sound recorded by a hydrophone at water depths of 5, 10 and 15 m from a fixed distance of 3 m relative to the payao, and compares the sound with the auditory sensitivity of jack mackerel Trachurus japonicus measured at discrete frequencies from 100 to 2000 Hz using the auditory brainstem response protocol. A consistent peak appeared in the sound spectrum at 49 Hz and showed an increasing sound pressure level with depth, which suggests that payao sound may come from the anchor rope. However, the contribution of the bamboo raft can not yet be discounted. The hearing threshold curve indicated that the most sensitive frequency range in jack mackerel is from 92.1 dB at 800 Hz to 111.0 dB at 200 Hz. These results show that the dominant frequency range of payao sound does not correspond with the high sensitivity frequency range of fish hearing.     

Immunoreactive changes in pituitary FSH and LH cells during seasonal reproductive and spawning cycles of female chub mackerel <Emphasis Type="Italic">Scomber japonicus</Emphasis>

The physiological functions of pituitary gonadotropins (GtHs) are well established in higher vertebrates, whereas those in teleosts are still poorly understood. To describe the role of GtHs during gonadal development of female chub mackerel Scomber japonicus, changes in follicle-stimulating hormone (FSH) and luteinizing hormone (LH) cells were investigated immunohistochemically during the seasonal reproductive and spawning cycles. FSH and LH cells were identified in the different cell types of the proximal pars distalis (PPD); FSH cells were located in the central PPD, whereas LH cells were localized along the border of the pars intermedia. To examine changes in FSH and LH cells, the percentage of FSH or LH cell-occupying area in the PPD was evaluated and represented as FSHβ-immunoreactive (ir) or LHβ-ir levels, respectively. FSHβ-ir levels increased significantly from immature to the completion of vitellogenesis, whereas LHβ-ir levels were maintained at high levels from early vitellogenesis to post-spawning. During the spawning cycle, which consisted of four stages from just after spawning to the next oocyte maturation, both FSHβ-ir and LHβ-ir levels showed no significant changes among different stages; however, LHβ-ir levels remained relatively high, and FSHβ-ir levels were constantly low. These results suggest that both FSH and LH may be involved in vitellogenesis and LH may act at final oocyte maturation in female chub mackerel, although the role of FSH during the spawning cycle is still unclear.     

Comparison of connective tissue structure and muscle toughness of spotted mackerel <Emphasis Type="Italic">Scomber australasicus</Emphasis> and Pacific mackerel <Emphasis Type="Italic">S. japonicus</Emphasis> during chilled and frozen storage

To characterize post-mortem changes in fish meat quality during chilled and frozen storage, muscle toughness and the connective tissue structure of muscle were compared between two mackerel species, spotted mackerel Scomber australasicus and Pacific mackerel S. japonicus. Measurement of the breaking strength of meat slices from both non-frozen fresh fish and frozen-thawed fish revealed that spotted mackerel meat was softer than Pacific mackerel meat. Under scanning electron microscopic observation, the connective tissue of non-frozen meat of spotted mackerel was thinner than that of Pacific mackerel. In addition, the collagen content in the spotted mackerel meat was lower than that of Pacific mackerel. These findings suggest that connective tissue structure might influence muscle toughness in mackerel.     

Transport and survival processes of eggs and larvae of jack mackerel <Emphasis Type="Italic">Trachurus japonicus</Emphasis> in the East China Sea

ABSTRACT:   Recent surveys showed substantial aggregation of larvae of jack mackerel in the southern East China Sea, indicating intensive spawning grounds near Taiwan. A numerical model was applied to investigate transport and survival processes of eggs and larvae of jack mackerel from the spawning area to the nurseries. The results show that: (i) the distributions of larvae simulated by the model agreed well with those obtained by field survey; (ii) the stock of jack mackerel in the Sea of Japan is composed of both groups from north of Taiwan and from the western coast of Kyushu. It takes more than two months for the former to reach the Sea of Japan, while it is within 40 days for the latter; and (iii) large proportions of the eggs and larvae spawned off the north of Taiwan are transported rapidly to the Pacific side of Kyushu by the Kuroshio Current, and the rest slowly to the east or north-east along the continental slope in the East China Sea. In contrast to the larval flux, survivors are more abundant in the northern East China Sea than in the Pacific Ocean, indicating that survival in the northern East China Sea would determine the jack mackerel stock in Japan.     

Fatigue analysis of the jack mackerel <Emphasis Type="Italic">Trachurus japonicus</Emphasis> by electrocardiographic monitoring during prolonged swimming

The effect of fatigue on swimming performance was examined by measuring the swimming endurance time and heart rate of the jack mackerel Trachurus japonicus [15.7 ± 0.8 cm fork length (FL), n = 15] during forced exercise in a flume tank at fixed swimming speeds of 4, 5 and 6 FL/s. Electrocardiographic (ECG) monitoring during the experimental process from control (0.8 FL/s) to exercise phase revealed a rapid cardiac response of T. japonicus to the elevation of swimming speed. The heart rate of T. japonicus significantly increased from the control level of 52.9 beats/min at a slow flow speed of 0.8 FL/s to 148.2 beats/min at 4 FL/s, 168.6 beats/min at 5 FL/s and 183.2 beats/min at 6 FL/s. During the fixed speed test, the heart rate of each individual fish was stabilized without any recognizable increase or decrease until the fish failed to swim because of fatigue. Fatigue analysis on endurance time demonstrated that prior swimming experience at prolonged speeds would impair the endurance performance during subsequent swimming exercise. Recovery time of the heart rate after the fish was fully exhausted by prolonged fast exercise increased with increasing swimming endurance time.     

Spawning induction of blue mackerel <Emphasis Type="Italic">Scomber australasicus</Emphasis> and eastern little tuna <Emphasis Type="Italic">Euthynnus affinis</Emphasis> by oral administration of a crude gonadotropin-releasing hormone analogue

Scombridae species, such as tunas and mackerels, often do not spawn in land-based fish tanks without hormone treatment. To induce spawning in various fishes, a gonadotropin-releasing hormone analogue (GnRHa) is often administered by pellet implantation. Noninvasive administration is desired to induce spawning in scombrids that are sensitive to handling stress. Spawning induction by oral administration has been reported in several fishes, yet this method has not been put into practice in the aquaculture industry since a considerable amount of GnRHa is needed. Utilization of peptide synthesizers is widespread, and antigen-grade GnRHa (AgGnRHa) produced by a custom-peptide supplier is approximately 100-fold cheaper than conventional reagent-grade GnRHa (RgGnRHa), although the purity of AgGnRHa is lower. Here, we confirmed that the spawning induction potency of AgGnRHa was similar to that of RgGnRHa by pellet implantation in blue mackerel Scomber australasicus. Oral administration of AgGnRHa [6.0 mg/kg body weight (BW) per day] showed an equivalent ability to induce spawning of the mackerel as pellet implantation (0.1 mg/kg BW). We could also induce spawning of eastern little tuna Euthynnus affinis by oral administration of the AgGnRHa. Further, the obtained eggs showed higher survival. Thus, the oral delivery of AgGnRHa could be a powerful tool to induce spawning in Scombridae.     

Microbial community in a multi-trophic aquaculture system of <Emphasis Type="Italic">Apostichopus japonicus</Emphasis>, <Emphasis Type="Italic">Styela clava</Emphasis> and microalgae

An integrated multi-trophic aquaculture (IMTA) system consisting of the ascidian Styela clava and the sea cucumber Apostichopus japonicus with microalgae was grown in a seawater mesocosm. Microbial populations in the water, sediment and shelters were monitored over time. The composition of the water microbial community in the IMTA system did not differ significantly from that of the traditional culture system without ascidians. Bacterial populations in the water, sediment and shelters were low in the traditional A. japonicus aquaculture system, and lower in the IMTA system. The number of Vibrios in the water, sediment and shelters in the IMTA system was significantly lower than in the traditional culture (p < 0.05), while the quantity of Bacillus was higher in the sediment and shelters. All of these results indicate that the IMTA system effectively inhibited the growth of harmful bacteria, an important positive function in the health of the culture. The bacterial diversity index in the water in the IMTA system was between those of the baited and non-baited traditional aquacultures and was slightly reduced in the shelters. Over time, the bacterial diversity index in the sediment gradually dropped, especially in summer and autumn, when it was significantly lower than in the traditional culture. In winter, the diversity index increased to some extent, approaching that of the traditional culture.     

Spawning induced by cubifrin in the Japanese common sea cucumber <Emphasis Type="Italic">Apostichopus japonicus</Emphasis>

The neuropeptide cubifrin-I and its derivative cubifrin-L have recently been shown to be potent substances that induce in vitro oocyte maturation and spawning in the Japanese common sea cucumber Apostichopus japonicus. We have examined the reproductive behavior of A. japonicus in vivo following the injection of cubifrin-L into the body cavity with the aim of determining the practical value of using this peptide to induce spawning in the hatchery setting. The responsiveness of ovarian fragments with oocytes >155 μm in diameter to cubifrin-L in vitro was well correlated with the spawning success induced in vivo by a cubifrin-L injection. Mature sea cucumbers injected with cubifrin-L displayed sequential reproductive behaviors, which comprised climbing up the side wall of the tank toward the water surface, waving of the head, and shedding of gametes. Gamete shedding started about 60 and 80 min after the injection of cubifrin-L in males and females, respectively, and was completed almost simultaneously in the two sexes about 2 h post-injection. Repeated injections of cubifrin-L at intervals of about 10 days successfully induced multiple spawnings in males and females. These results demonstrate that cubifrin-L can be used as an effective inducer of spawning in Japanese sea cucumber cultivation.     

Japanese nephropid lobster <Emphasis Type="Italic">Metanephrops japonicus</Emphasis> lacks zoeal stage

ABSTRACT:   Larvae of the Japanese nephropid lobster Metanephrops japonicus hatched in the laboratory were reared at 15°C, and the development and feeding were observed. All larvae hatched at the 'prezoea stage' with no natatory setae on the exopodite of the pereiopods. Without feeding, 50% of prezoea molted into the megalopa stage, having small buds as the exopodite, within 1 h and all molted within 22 h. The megalopa fed with Artemia nauplii, shrimp meat and pelleted food molted into the first juvenile stage with no exopodite after approximately 17 days. The average carapace lengths of prezoea, megalopa and the first juvenile stage were 3.2, 3.6 and 4.4 mm, respectively. The survival rate from hatching to the first juvenile stage was high (90–100%). This lobster may be the only known nephropid species with no zoeal stage.     

Spatial learning-induced <Emphasis Type="Italic">egr</Emphasis>-<Emphasis Type="Italic">1</Emphasis> expression in telencephalon of gold fish <Emphasis Type="Italic">Carassius auratus</Emphasis>

The immediate-early gene (egr-1) expression was used to examine the neuron’s response in telencephalon of goldfish during spatial learning in small space. Fishes were pre-exposed in the experimental apparatus and trained to pick food from the tray in a rectangular-shaped arena. The apparatus was divided into identical compartments comprising three gates to provide different spatial tasks. After the fish learned to pass through the gate one, two more gates were introduced one by one. Fish made more number of attempts and took longer time (P < 0.05) to pass through the first gate than the gate two or three. This active learning induces the expression of egr-1 in telencephalon as established by western blot analysis. Subsequently, the fish learn quickly to cross the similar type of second and third gate and make fewer errors with a corresponding decline in the level of egr-1 expression. As the fish learned to pass through all the three gates, third gate was replaced by modified gate three. Interestingly, the level of egr-1 expression increased again, when the fish exhibit a high exploratory behavior to cross the modified gate three. The present study shows that egr-1 expression is induced in the telencephalon of goldfish while intensively acquiring geometric spatial information to pass through the gates.