Biochemical composition and quality of captive-spawned cobia Rachycentron canadum eggs

Interest in the commercial production of cobia Rachycentron canadum continues to rise as additional insight is gained into the hardy and fast growing nature of this species. However, research regarding the biochemical composition of captive-spawned eggs and egg and larval quality remains scarce. Such data is essential as a common bottleneck to production is a steady supply of fingerlings for grow-out. This study quantified the biochemical composition and quality of cobia eggs produced over 2 spawning seasons by broodstock on a traditional ‘trash fish’ diet which is commonly fed to tank spawning cobia. Throughout the study, batch fecundity, proportion of floating eggs and percent hatch averaged > 1 million eggs,  0.8 and 70%, respectively. Batch fecundity was significantly higher during the second spawning season as a result of the increased size of the females which weighed 18/22 kg and 22/26 kg at the beginning of each season. A positive correlation was found between the proportion of floating eggs and hatch rate for both spawning seasons. No correlations were found between egg composition (total lipid (30.0 ± 1.1% dry wt), protein (25.4 ± 2.2% dry wt), carbohydrate (2.4 ± 0.3% dry wt), vitamin E (10.2 ± 0.6 μg/g wet wt) or dry weight (119.1 ± 5.5 μg/egg)) and egg quality (proportion of floating eggs, hatch rate, larval growth and larval survival). Further, no differences in egg composition were noted between seasons or over the course of each season. The fatty acid composition of cobia eggs varied between seasons possibly due to changes in the quality of frozen feed (fish, shrimp, squid) given to the broodstock. The only correlation between the fatty acid profile and egg quality was a decrease in the proportion of floating eggs as the total amount of n-3 highly unsaturated fatty acids increased. No relationship between egg quality and amino acid content was noted with the most prominent amino acids being glutamate, leucine, alanine, proline, lysine and aspartate nor were any differences detected between spawning seasons.     

Growth and survival of larval and juvenile cobia Rachycentron canadum in a recirculating raceway system

Cobia Rachycentron canadum is a fast-growing, pelagic marine species that has recently attracted aquaculturists in both the research and commercial sectors. The typical method of grow-out for this species is in outdoor systems where production is limited to locations and seasons conducive for adequate growth and survival. Expanding the culture of cobia to indoor recirculating aquaculture systems (RAS) would allow for the production of fingerlings throughout the year and extend production to cooler regions. Two rearing trials were conducted to examine the growth and survival of cobia from hatching through 4 (trial 1, T1) or 35 (trial 2, T2) g in RAS. Cobia larvae were reared in circular tanks placed in a raceway to control water temperature and quality. During early juvenile grow-out, fish were transferred without grading to a second raceway on 29 dph (T1) or over a period of grading from 29–43 dph (T2). Larval growth (1–22 dph) measured as standard length was similar for both trials ranging from  3.9 to 14.7 mm. However, larval growth measured as wet weight (0.033 g, T1; 0.026 g, T2) or dry weight (5.7 mg, T1; 3.9 mg, T2) was significantly greater on 22 dph during T1 as was the ratio between myotome height and standard length. These differences may have resulted from an increase in initial densities from 8.7 larvae l^− 1 (T1) to 14.7 larvae l^− 1 (T2) which apparently caused an increase in food competition and overall aggression. During juvenile grow-out, cobia reached 4.0 g on 43 dph in T1 and 35.4 g on 71 dph in T2 matching weights achieved during grow-out in outdoor ponds. Over the course of both trials, survival was similar to that reported in outdoor ponds. Mean survival (± S.D.) during the early rearing phase (hatching through 29 or 43 dph) averaged 13.2 ± 3.2 % and 10.4 ± 3.2 % corresponding to final densities of 0.9 ± 0.2 and 1.2 ± 0.4 fish/l for T1 and T2, respectively. During the first grow-out phase (29–43 dph), survival of fish moved into the open raceway was 64.5% in T1 and 88.7 % in T2. Survival of cobia during the second grow-out phase (43–71 dph) for T2 was 92.5%. The results of this study indicate that cobia can be successfully cultured in indoor systems from hatching through at least 35 g without negatively affecting growth or survival.     

Juvenile cobia (Rachycentron canadum) can utilize a wide range of protein and lipid levels without impacts on production characteristics

Two, 6 week feeding trials were conducted to evaluate the impacts of protein and lipid on weight gain, feed efficiency ratio values and biological indices of juvenile cobia (Rachycentron canadum). Utilizing a 2 × 3 factorial design, experimental diets containing two levels of crude protein (CP; 40 and 50%) and three levels of lipid (6, 12 and 18%), providing 14.4, 15.1 and 16.6 kJ available energy/g dry diet (calculated), respectively, were formulated for use in both feeding trials. In the first trial, cobia (initial weight 49.3 g per fish) was randomly assigned to one of the six experimental diets and fed to apparent satiation twice daily. At the end of the first trial, weight gain in cobia was not significantly impacted by protein levels with values ranging from 333% (50% CP) to 353% (40% CP). However, lipid significantly (P < 0.05) affected weight gain with fish fed the diet containing 18% total lipid returning the lowest growth of 293%. Feed efficiency ratio values were not significantly impacted by dietary protein or lipid levels and ranged from 0.46 (50% CP/18% lipid) to 0.51 (50% CP/6 and 12% lipid). Survival was significantly impacted by protein and lipid with fish fed the diets containing 50% CP and 18% lipid having lower (P < 0.05) survival rates of 90%.In the second trial, smaller fish were utilized (7.4 g average initial weight) under identical experimental conditions and dietary formulations. Weight gain was not significantly affected by protein or lipid levels and ranged from 1099% in fish fed the diet containing 40% CP/18% lipid to 1305% in fish fed the diet containing 50% CP/12% lipid. Feed efficiency ratio values, visceral somatic and hepatosomatic indices were significantly affected by protein and/or lipid. Muscle and liver lipid were impacted by dietary lipid (P = 0.0203 and 0.0012, respectively). Muscle protein was significantly impacted by dietary protein levels, while liver protein was affected by both main effects. Dietary protein and lipid had no impact on muscle ash.These data suggest that juvenile cobia can thrive on a wide range of protein and lipid levels, as well as a range of protein to energy ratios. Positive impacts of optimizing the protein component in terms of economic and environmental concerns, coupled with the ability to maintain the rapid growth rates this species are renowned for at lower dietary lipid levels, point towards beneficial consequences of further refinement of commercial cobia production feeds.     

Resistance of Cobia,Rachycentron canadum,Juveniles to Low Salinity,Low Temperature,and High Environmental Nitrite Concentrations

Abstract

Resistance of juvenile cobia, Rachycentron canadum, to low salinity, low temperature and high nitrite concentrations was examined under laboratory conditions. After acclimating juveniles to a salinity of 20 g/L (27.3°C), salinity was decreased by 2 g/L/day. The first fish died at a salinity of 8 g/L and 80% offish were dead within 24 hours of exposure to 2 g/L. Acclimation offish to 22.6°C (21 g/L salinity) followed by a temperature reduction of 0.53°C/day resulted in initial mortality at 12.9°C. The median-lethal temperature was 12.1°C and all fish were dead by the time the temperature reached 10.4°C. Fish exposed for 96 hours to nominal ? 32 mg/L nitrite-N survived. Results of this study indicate that cobia juveniles require a salinity and temperature of > 8.0 g/L and > 12.9°C, respectively, and that environmental nitrite should not be deleterious at concentrations normally found in aquaculture systems.     

军曹鱼幼鱼日粮锌最适添加量的研究

在以卵白蛋白为蛋白源的基础饲料（Zn含量为5．65mg／kg）中添加10、20、30、40、50mg／kg以ZnSO4为Zn源的Zn,使饲料中总Zn含量为15．97、25．66、33．90、45．85、55．88mg／kg。投喂军曹鱼幼鱼8周。试验结果表明,基础饲料组鱼出现生长不良、厌食、皮肤糜烂和白内障等症状,其成活率、增重率、血清碱性磷酸酶活性（AKP）显著低于添加Zn组（P〈0．05）,骨骼、肌肉、血清和肝脏Zn含量也表现出相同趋势。10mg／kg和20mg／kgZn添加量组鱼仍出现缺Zn症状,各项测定指标随Zn添加量的增加而增大。当饲料中Zn添加量达到30mg／kg时,各项指标趋向稳定,与添加量40mg／kg和50mg／kg组无显著差异。以增重率和骨骼Zn含量为主要评价指标,通过回归分析表明,以ZnSO4为Zn源时,军曹鱼幼鱼日粮Zn的最适添加量为42．86mg／kg。     

军曹鱼幼鱼维生素C需要量的研究

选用初体均重为25.85 g的军曹鱼幼鱼225尾随机分为5组,每组设3个重复,每个重复15尾.分别投喂添加5种不同剂量维生素C(添加量分别为0、37.5、75、150和300 mg/kg)的试验饲料,试验期为56 d,观察维生素c对军曹鱼幼鱼生长、组织维生素C积累量及非特异性免疫的影响.结果表明,随着饲料中维生素C添加量增加,军曹鱼幼鱼增重率和特定生长率呈现出上升后下降趋势,在75 mg/kg时达到最高值,显著高于0和300 mg/kg组(P<0.05).全鱼粗蛋白质含量随着饲料中维生素C添加量的升高亦呈现先上升后下降趋势,在75 mg/kg时达到最高值,全鱼干物质、粗蛋白质、粗脂肪、灰分含量在各组之间均无显著差异(P>0.05).血清维生素C积累量随着饲料中维生素C添加量的增加呈现先上升后下降趋势,在150 mg/kg时达到最高值且显著高于0 mg/kg组(P<0.05);脑和肝脏中维生素C含量在300 mg/kg时达到最高值.300 mg/kg组脑中维生素C含量显著高于0,37.5和75 mg/kg组(P<0.05),肝脏中维生素C含量显著高于0,37.5和150 mg/kg组(P<0.05).随着饲料中维生素C添加量的增加,溶菌酶活性呈现先上升后下降趋势,在75 mg/kg达到最高,显著高于0,37.5和300 mg/kg组(P<0.05);血清总抗氧化能力呈现上升趋势,75、150和300 mg/kg组显著高于0 mg/kg组(P<0.05);肾抗氧离子自由基在150 mg/kg组达到最高,但各组之间无显著差异(P>0.05).以鱼体增重率和血清溶菌酶含量为指标,经折线模型回归分析求得军曹鱼幼鱼饲料中维生素C的适宜添加量分别为70和80.72 mg/kg.     

Ecological and economic analysis for cobia Rachycentron canadum commercial cage culture in Taiwan

The structure of cost and return for cobia cage culture in Taiwan was studied by considering two major factors, geographical location and production scale. Although the geographical location had little influence on the input intensities, the production scale would have a significant impact on the cost structure. However, the performances of profitability were significantly affected by not only the individual main effects but also their interaction. As a result, the studied effects on the cost and return were quantitatively estimated by a series of statistical models, in turn quantitatively measured by a set of indices. A short-term strategy of the management suggested that the best choice regarding profitability is to locate the farming system with a large-scale operation in the coastal waters of Pindong. Additionally, the profitability could be further increased by selecting better quality fingerlings and feeds based on a set of the computed indices. A long-term strategy of the management strongly recommends that to increase the system size in cobia production would be much more profitable due to the positive economies of scale. Finally, a future study shall be interested in whether current speed has a limiting effect on the profitability.     

Lower Lethal Temperature for Juvenile Cobia Rachycentron Canadum

Cobia are a migratory marine species that have recently gained popularity as foodfish in the aquaculture market and for stocking. Unfortunately, when culturing these species, aquaculturalists may unknowingly expose these fish to temperature extremes not normally experienced in situ. We set out to test for the critical thermal minimum temperatures of juvenile cobia by exposing them to a simulated freeze, at a drop rate of 0.33°C per hour. We observed and documented behavioral effects due to low temperature exposure using criteria for loss of equilibrium and death. We determined that the median temperature for loss of equilibrium was 12.1 ± 0.40°C and the median lethal temperature was 9.7 ± 0.28°C for low temperature tolerance. We recommend that precautions be taken well before water temperatures reach 13°C in a freeze scenario.