Live vaccine trials against nocardiosis in yellowtail Seriola quinqueradiata

In an attempt to develop a vaccine against Nocardia seriolae, related species of live bacteria N. soli, N. fluminea, and N. uniformis were injected into yellowtail Seriola quinqueradiata. In addition, fish were challenged with a low virulence strain of N. seriolae to model the concept of use of a live vaccine. The fish injected with live N. soli and N. fluminea cells showed slight resistance against an artificial challenge with N. seriolae. On the other hand, the fish that survived the N. seriolae infection showed complete resistance to the N. seriolae challenge. These results suggest that protective immune responses against N. seriolae are induced in yellowtails.     

Mortality of yellowtail flounder, Limanda ferruginea (Storer), eggs: effects of temperature and hormone-induced ovulation

Yellowtail flounder, Limanda ferruginea (Storer, 1839), is a highly marketable fish species that used to be a significant groundfish in the northwest Atlantic region. The potential of yellowtail flounder for mass rearing has recently been investigated, but studies into aspects of mortality during early ontogeny are lacking. We investigated the impact of spawning inducement on mortality of eggs from fertilisation until hatch, and the effects of rearing temperature on mortality and hatching time in this multiple batch spawning flounder. An ash-free dry mass (AFDM) technique for determining mortality was employed. This technique reduced rearing tank disturbances and provided quantitative mortality estimates. We found (1) that spontaneously ovulated (SO) mean egg mass (AFDM 24.84±0.73 μg) was significantly higher than mean egg mass from induced ovulation (IO) (AFDM 20.09±0.54 μg); (2) that mortality of SO eggs was lower than IO eggs (p=0.028), (3) that SO eggs reached a relatively constant (maximum) mortality by about day 3 and IO eggs by about day 5; (4) that there was no significant difference in mortality between eggs incubated at 10 °C and those incubated at 4 °C (p=0.320); and (5) that time to 100% hatch took significantly longer for eggs incubated at 4 °C (256.0±73.5 h) than those incubated at 10 °C (77.9±11.6 h) (p=0.003). There was considerable variation in egg mortality among batches and females; nevertheless, general trends indicate that GnRHa-induction of ovulation tends to decrease egg quality. In addition, the AFDM technique was an easily employed and accurate method of determining mortality. We recommend that unless coordination of spawning is important to a particular rearing protocol, induced ovulation should not be employed.     

Yellowtail insulin-like growth factor 1: molecular cloning and response to various nutritional conditions

Insulin-like growth factor 1 (IGF1) plays an important role in fish growth. This study investigated the IGF1 response to various nutritional conditions in yellowtail. First, we cloned 1,075 bp of yellowtail IGF1 cDNA, which codes for a protein of 185 amino acids (aa). This is composed of 44 aa for the signal peptide; 68 aa for the mature peptide comprising the B, C, A, and D domains; and 73 aa for the E domain. The mature yellowtail IGF1 showed high identity to IGF1 of other teleosts. Insulin-like growth factor 1 mRNA expression in the liver and white muscle was measured to observe the IGF1 response to various nutritional conditions, because the liver has the highest IGF1 expression and white muscle comprises the largest fraction of the fish body. Only white muscle IGF1 mRNA expression decreased significantly by 3 wk of fasting and recovered by refeeding. In subsequent feeding ratio (1%, 2%, and 3%/BW/d) experiments, significant correlations to growth were observed in white muscle IGF1 mRNA expression at 2- and 6-wk points and in hepatic IGF1 mRNA expression at 4 wk point. These data suggest that IGF1 expression both in hepatic and white muscle is important for somatic growth in yellowtail. Furthermore, white muscle IGF1 mRNA expression showed better responses to somatic growth and nutrition status in our two experiments than hepatic IGF1 mRNA expression.     

Self-feeding behavior of yellowtail, Seriola quinqueradiata, in net cages: diel and seasonal patterns and influences of environmental factors

The self-feeding pattern of yellowtail, Seriola quinqueradiata, maintained in floating net cages was examined throughout a year, and the influence of seasonal changes of the photoperiod and water temperature on the feeding pattern was investigated. Two groups of 50 yellowtail (initial mean body weight about 50 and 80 g, respectively) were kept in two experimental net cages (2×3×4 m deep), with a self-feeding device installed on each. It was possible to maintain yellowtail in net cages using a self-feeder throughout almost a year. The number of daily food demands was greatly affected by the seasonal changes in water temperature. Yellowtail showed high self-feeding activity, above 18 °C; depressing water temperatures did not influence the activity when the water temperature was over 18 °C. However, the activity decreased when the water temperature fell below 18 °C. The feeding pattern in a 24-h period was greatly affected by the temporal changes in light intensity. Annual observations revealed that yellowtail showed generally crepuscular plus nocturnal feeding behavior and had two peaks of feeding activity a day. These peaks appeared at dawn and dusk; moreover, a clear association between the most active time of self-feeding and the period of the greatest change in light intensity was observed. The results suggest that a change in light intensity might stimulate the appetite of yellowtail or that there is a light level at which yellowtail prefer to eat.     

Food ration requirements of 0 yellowtail flounder Limanda ferruginea (Storer) juveniles

Several studies have shown that food ration can affect the growth of cultured fish. Determining the optimal food ration would help to achieve better growth and also provide direct economic benefits due to reduced food wastage, which would lead to commercial success. Therefore, we studied the effects of ration levels on growth performance of 0⁺ juvenile yellowtail flounder to determine the optimal food ration. Two experiments were conducted; the first experiment as a preliminary using ration levels of 1%, 2%, 4%, 6% body weight per day (% bw day⁻¹) held at 7.0 °C with a stocking density of 0.95 kg m⁻² (45% bottom coverage). Results of this preliminary experiment indicated that fish fed with 1% bw day⁻¹ had significantly lower growth (weight, length, body depth and specific growth rates (SGR)) than those fed with 2%, 4% and 6% ration. However, fish fed with rations of 1% and 2% showed significantly lower gross food conversion ratios (GFCR) than fish fed with 4% and 6% rations. Survival was not significantly affected by different ration levels. Based on these preliminary results, we used ration levels of 1%, 1.5%, 2% and 3% for the main experiment. Fish were held at 10 °C with a stocking density of 1.45 kg m⁻² (34% bottom coverage). Results indicated that fish fed with 1%, 1.5% and 2% bw day⁻¹ had significantly lower growth than fish fed with 3% bw day⁻¹. GFCR was significantly different for all four rations. It was lower for 1% than 1.5%, 2% and 3% rations. Survival was not significantly different between any treatments. We discuss our results with emphasis on growth and economics (i.e., feed wastage) and stress the need to balance both components in a commercial operation.     

Severe oxidative stress is thought to be a principal cause of jaundice of yellowtail Seriola quinqueradiata

The jaundiced yellowtail Seriola quinqueradiata had yellow discolored skin and hyperbilirubinemia. Histology revealed that hemosiderin and ceroid occurred in the spleen. In the vascular system, there were vasculitis and sinusoidal dilatation. Hepatic cells showed degenerative figures with pyknotic or karyolytic nuclei, basophilic cytoplasm, cytoplasmic vacuoles in various sizes, and ceroidosis. In the bile duct system, however, there was no noticeable change. Hemoglobin contents of jaundiced fish were similar to those of control fish. Plasma taurine levels of jaundiced fish were significantly lower than those of control (p0.05). Judging from 2-thiobarbituric acid reactive substance values, in vivo lipid peroxidation progressed in the liver of jaundiced fish. In the plasma of jaundiced fish, contents of unconjugated bilirubin were significantly higher than those of conjugated bilirubin. A large amount of substance Xs, breakdown products of bilirubin scavenging active oxygen species, existed in the plasma of jaundiced fish. Therefore, jaundiced fish may suffer from severe oxidative stress, which might be a principal cause of jaundice. Liver superoxide dismutase and glutathione peroxidase activities of jaundiced yellowtail were significantly lower than those of controls.     

Ontogenetic development of digestive enzymes in yellowtail kingfish Seriola lalandi larvae

The development of digestive enzymes was examined in laboratory-reared yellowtail kingfish larvae from hatching to 36 days after hatching (DAH). The specific activities of amylase, lipase, and alkaline phosphatase showed three distinct phases: a sharp increase in enzyme activity from hatching to the onset of exogenous feeding on 3 DAH, followed by a fluctuation and a general decline toward 18 DAH, and then a period of low activity from 18 to 36 DAH. The total activities of these three enzymes showed a gradual increase from hatching to 18 DAH, followed by a sharp increase toward 36 DAH. In contrast to other enzymes, the specific and total activities of trypsin reached the maximum on 15 DAH and 24 DAH, respectively, and then both activities declined to low levels toward 36 DAH. The dynamics of digestive enzymes corresponded to the anatomical development of the digestive system. The enzyme activities tend to be stable after the formation of gastric glands in the stomach on 15 DAH. The composition of digestive enzymes indicates that yellowtail kingfish is able to digest protein, lipid and carbohydrates at an early stage. However, due to the low level of amylase specific activity after 18 DAH, the carbohydrate component should remain at a low level in formulated diets for fish larvae.     

In vivo and in vitro effect of recombinant salmon growth hormone treatment on IGF-I and IGFBPs in yellowtail <Emphasis Type="Italic">Seriola quinqueradiata</Emphasis>

The role of growth hormone (GH) in regulating hepatic mRNA expression of insulin-like growth factor-I (IGF-I) and IGF binding proteins (IGFBPs) in yellowtail Seriola quinqueradiata was examined using in vivo and in vitro assays. Yellowtail hepatic IGF-I, IGFBP-1, IGFBP-2, IGFBP-3, and IGFBP-5 mRNAs were measured by real-time quantitative RT-PCR. Intraperitoneal injection of recombinant GH of chum salmon Oncorhynchus keta (rsGH) at a dose of 1 μg/g body weight resulted in a significant increases in hepatic IGF-I, IGFBP-3, and IGFBP-5 mRNA levels, whereas significant reductions in hepatic IGFBP-1 and IGFBP-2 mRNA levels were observed. For in vitro assays, liver slices were incubated with rsGH at different concentrations (doses: 0, 1, 10, 100, 500, and 1,000 ng/ml). Liver slices incubated with 100 ng/ml rsGH elicited a significant increase in IGF-I mRNA level. Similarly, a slight increase in IGFBP-3 and IGFBP-5 mRNA levels were also observed in liver explants incubated with rsGH. In contrast, a significant decline in IGFBP-1 mRNA levels was observed in liver slices incubated with 1,000 ng/ml rsGH. A slight decline in the level of IGFBP-2 mRNA was noted in liver explants with rsGH treatment. This study demonstrates the modulating effect of GH on the IGF system.     

Suppression of color degradation of yellowtail dark muscle during storage by simultaneous dietary supplementation of vitamins?C and E

The commercial value of yellowtail Seriola quinqueradiata meat is often reduced by color deterioration of the dark muscle during storage. The objective of this study was to investigate the effect of simultaneous dietary supplementation of vitamin C (VC) and vitamin E (VE) to cultured yellowtail on color degradation of the dark muscle during storage. Yellowtail were fed for 6 or 11 days on a diet containing 1% VC and 1% VE (vitamin group) or without these vitamins (control group). The amounts of VC and VE in the dark muscle of the vitamin group tended to be higher than those of the control group. During the storage of sliced meat at 23 and 4°C, the decreases in a* value (redness) of the dark muscle of the vitamin group was slower than those of the control group. The formation of metmyoglobin, which can occur along with myoglobin oxidation, in the dark muscle of the vitamin group was slower than that of the control group. These results suggest that simultaneous feeding of high amounts of VC and VE for a short period to yellowtail before harvesting could suppress the color deterioration of the dark muscle during storage.     

The timing of puberty in cultured female yellowtail flounder, Limanda ferruginea (Storer): Oogenesis and sex steroid production in vivo and in vitro

This study examines puberty in female yellowtail flounder, correlating macroscopic, histological and endocrine parameters at the gonadal level. Short-term ovarian tissue incubations were used to study estradiol-17β production in relation to oogenic stage. Examining 2 year classes of young flounder demonstrated that cultured females retain phenotypic plasticity in reproductive age with pubertal onset occurring in one-year, two-year and three-year-olds. Immature ovaries were steroidogenic and capable of responding to gonadotropic stimulation. Endocrine puberty in females was detected by a peak in estradiol-17β production during the cortical alveolar oocyte stage prior to any ovarian evidence of vitellogenesis. Puberty, once initiated, proceeded to ovulation within 8 to 12 months; vitellogenic oocyte development followed the group synchronous pattern. Estradiol-17β was clearly the dominant of the two measured plasma hormones during pubertal onset and throughout vitellogenesis. Plasma testosterone was consistently detectable at low levels by mid-to late-vitellogenesis. Ovaries showed the highest sensitivity to gonadotropic stimulation in vitro during late-vitellogenesis. Variable plasma levels in both estradiol-17β and testosterone occurred in preovulatory and ovulating females during the captive spawning period. Together the results show that yellowtail flounder can mature at a young age and small size when culture conditions permit. In addition, the early sensitivity to gonadotropin by the immature ovaries may be an important physiological determinant for the timing of puberty in this species.