Diving behavior of immature, feeding Pacific bluefin tuna (Thunnus thynnus orientalis) in relation to season and area: the East China Sea and the Kuroshio–Oyashio transition region

Twenty‐four archival tags were recovered from Pacific bluefin tuna previously released off Tsushima Island in the East China Sea. By analysis of the time‐series data of the pressure and the ambient and internal temperature from the 24 tags, we examined the relationship between the tuna's pattern of diving and the thermocline depth. In the East China Sea, diving and feeding events occurred throughout almost the entire day in both winter and summer, suggesting that the purpose of diving is for feeding. In summer, the feeding frequency was greater than that in winter, which corresponds to the fact that growth is more rapid in summer than in winter. During summer in the Kuroshio–Oyashio transition region, on the other hand, feeding events were much more frequent than those in the East China Sea, in spite of a lower diving frequency. The mean horizontal distance traveled was also significantly higher and it seems that in this area they may move horizontally to feed on prey accumulated at the surface. We conclude that, in addition to the ambient temperature structure, the vertical and horizontal distribution of prey species plays an important role in the feeding behavior of Pacific bluefin tuna. One bluefin tuna migrated to the Oyashio frontal area, where both the horizontal and the vertical thermal gradients are much steeper. The fish spent most of the time on the warmer side of the front and often traveled horizontally to the colder side during the day, perhaps to feed. This implies that there is a thermal barrier effect, in this case from the Oyashio front, on their behavior. The frequency of feeding events was low, although all the monitored fish dived every dawn and dusk, irrespective of the seasons or location. It is possible that these twice‐daily diving patterns occurred in response to the change in ambient light at sunrise and sunset.     

Bluefin tuna (Thunnus thynnus) distribution in relation to sea surface temperature fronts in the Gulf of Maine (1994–96)

Fishery‐linked aerial surveys for bluefin tuna (Thunnus thynnus) were conducted in the Gulf of Maine (GOM) from July through October, 1994–96. Each year, from 507 to 890 surface schools were detected and their locations examined in relation to oceanographic conditions. Correlations between bluefin tuna presence and environmental variables were explored for sea surface temperature (SST), distance to a SST front, frontal density (relative density of all SST fronts seen in a given 1 km area for 2 weeks prior to each tuna sighting), and bottom depth and slope. Mean SST associated with bluefin schools was 18.1°C (±2.8). Schools were located at a mean distance of 19.7 km (±19.6) from SST fronts, and in water masses with an average frontal density of 28.2 m km^?2 (±35.7). Mean bottom depth of detected schools was 139.0 m (±70.3), and mean bottom slope was 0.7% rise (±0.7). A binomial generalized linear model fit to these variables indicated that bluefin are seen closer to fronts than locations in which no tuna were seen. Using simple and partial Mantel tests, we investigated the spatial correlation between bluefin tuna presence and the environmental variables, controlling for spatial autocorrelation. For each day that schools were sighted, we performed 24 Mantel tests, on a combination of response and predictor variables. The spatial relationship between bluefin tuna and SST fronts was inconsistent. Our analysis identified significant spatial structure in the bluefin school locations that had no significant correlation with any of the measured environmental features, suggesting that other untested features, such as prey density, may be important predictors of bluefin distribution in the GOM.     

东太平洋金枪鱼延绳钓兼捕鲨鱼种类及其渔获量分析

2003年7月-11月份期间,对东太平洋公海海域(03°S~17°S,96°W~146°W)的超低温金枪鱼延绳钓兼捕鲨鱼进行调查。发现共兼捕到9种鲨鱼,分别是尖吻鲭鲨、长鳍鲭鲨、鳄鲨、人眼长尾鲨、路氏双鲨、长鳍真鲨、镰状真鲨、大青鲨和黑异鳞鲨,隶属于3目6科。共钓获鲨鱼24 711 kg,其中大青鲨是优势种类,占鲨鱼渔获尾数的50.4%,占鲨鱼渔获总重量的76.5%,平均个体重量达到64.9 kg。调查获得9种大洋性鲨鱼的渔获率,其中以大青鲨的最高,达到0.964尾数/千钩,鳄鲨次之为0.497尾数/千钩,其它鲨鱼的CPUE均在0.200尾数/千钩以下,此外估计了东太平洋海域金枪鱼延绳钓兼捕鲨鱼总渔获量,约在4.0~5.9万吨。     

金枪鱼酶解液脱色工艺及其对腥味物质和金属元素的影响

为了研究活性炭对金枪鱼酶解液脱色的最佳条件,本文利用活性炭对金枪鱼暗色肉酶解液进行处理,研究活性炭添加量、pH值、温度(T)和脱色时间(t)对脱色率和蛋白质损失率的影响,采用正交试验对脱色工艺进行优化,同时探究了活性炭脱色工艺对酶解液腥味物质和金属元素含量的影响.结果表明,金枪鱼暗色肉酶解液最佳脱色条件为活性炭添加量3％、pH值3.0、温度70℃、时间1h.该条件下脱色率为89.77％±0.77％,蛋白质损失率为13.47％±0.72％;脱色后酶解液挥发性物质含量降低了85.39％,其中三甲胺、己醛、壬醛等特征腥味物质显著降低;活性炭对Pb的吸附作用较明显,其吸附率为93.2％,而对其他金属元素的吸附率不大.研究表明,经过活性炭处理过的酶解液颜色浅,腥味低,具有一定的应用前景,可为酶解液的进一步利用提供依据.     

A Comparison of the Effects of Two Prey Enrichment Media on Growth and Survival of Pacific Bluefin Tuna,Thunnus orientalis,Larvae

Three experiments were conducted to investigate the growth, survival, and standardized cohort biomass of Pacific bluefin tuna, Thunnus orientalis, larvae fed nutritionally enhanced prey during the first week of feeding using two commonly used, commercially available enrichment media, AlgaMac Enrich and Marine Glos. T. orientalis larvae exhibited exponential growth in standard length and dry weight. The daily specific growth rates in length and weight are the first reported for T. orientalis larvae and the averages ranged from 3.8 to 4.1% and 27.5%, respectively, for larvae in the AlgaMac treatment and from 4.1 to 6.1% and 31.5%, respectively, in the Marine Glos treatment. Average daily growth rates in length ranged from 0.16 to 0.23 mm/d for larvae in the AlgaMac treatment and from 0.17 to 0.27 mm/d for those in the Marine Glos treatment. Daily growth rates in length were similar to those reported for other tuna larvae reared in the laboratory but slower than most published estimates for larval tunas in situ at similar water temperatures. Mean prey number per gut was positively associated with mean prey level in the tank. Both enrichment media appear to be good sources of nutritional improvement of planktonic prey for T. orientalis larvae.     

Analysis of sinking death using video images of the swimming performance of Pacific Bluefin tuna (Thunnus orientalis) larvae

In this study, we aimed to clarify the mechanism of sinking death during the larval stage of Pacific Bluefin tuna Thunnus orientalis by investigating the effects of swimming performance on sinking death, using a behavioral approach. Swimming performance was examined 3–9 days after hatching (DAH) under day and night light conditions in cuboid experimental tanks. Swimming behavior variables such as swimming speed and swimming angle were measured under both light conditions. Larvae in the daytime experiment and larvae with inflated swim bladders at night were distributed on the surface layer of the water column. In contrast, larvae with uninflated swim bladders at night were frequently observed swimming vertically or sinking to the bottom of the tank. Larvae with inflated swim bladders at night were always distributed beneath the surface until the next morning (survival rates were 100 %). However, larvae with uninflated swim bladders at night swam upward repeatedly and later sank to the bottom of the tank (survival rates were 60 % and 38 % at 5 and 9 DAH, respectively). Larvae with uninflated swim bladders were not always able to maintain their swimming depth by swimming until the next morning. Additionally, their swimming speed and vertical swimming frequency (ratio) depended on the illumination and swim bladder conditions. Our findings show that larvae with uninflated swim bladders at night were associated with a higher risk of sinking death. The swimming energy capacity of Pacific Bluefin tuna larvae, which indicates the total amount of the energy that enables individuals to swim throughout the night without feeding, was found to be linked to sinking death.     

金枪鱼围网沉降特性

根据沿围网下纲缚扎的10只温度深度计(TDR-2050型)自动记录的23网次有效数据,对中国中西太平洋金枪鱼围网的下纲沉降速度、沉降深度及其与时间的关系进行了研究。结果表明:(1)网具未达到理论沉降深度,网具中部平均沉降深度为160 m,为最大拉紧高度(311.1 m)的51.4%;(2)网具中部沉降深度与时间的关系为h=–0.000 2t2+0.3663 t;(3)网具各部位的沉降速度不同,取鱼部最慢,网具中部次之,后网头最快;(4)下纲各部位沉降速度均随着深度的增加而减小,沉降速度波动幅度随深度增加按大—小—大的规则变化;(5)网具中部沉降速度与时间的关系为:v=3×10–7t2–7×10–4 t+0.418 9。本研究结果可为改善金枪鱼围网沉降性能提供基础数据,并为海上生产提供指导性参考。     

Effect of water temperature on the feeding activity and the resultant mercury levels in the muscle of cultured bluefin tuna Thunnus orientalis (Temminck and Schlegel)

Tuna muscle often contains high levels of mercury, and fish samples with mercury concentrations ten times higher than the specified safety standards have been reported. Here, we report on the relationship between water temperature and the concentration of mercury in the tail muscle tissue of cultured bluefin tuna Thunnus orientalis. The fish used in this study were cultured at Fisheries Laboratory of Kinki University (Amami Experimental Station, Kagoshima, Japan). One hundred fish weighing 26.2–89.4 kg were selected for analysis between February 2007 and January 2008. Water temperature during rearing ranged from 21 to 29 °C. The total mercury levels were measured using the reduction vaporizing atomic absorption method after acid digestion. Body weight increased approximately 1.5 times that observed in a previous study, despite feeding activity either being the same or less than that observed previously. The average mercury concentration in white muscle was 0.353 mg kg^?1, remaining almost constant and independent of body growth. Unlike previous studies, seasonality was not observed in this study. Based on these findings, water temperatures within a certain range were considered to stabilize feeding activity and increase feeding efficiency. Consequently, water temperature is considered to have a moderating effect on seasonal fluctuations in muscle mercury concentrations in cultured bluefin tuna.     

Biotransformation of tuna waste by co-fermentation into an aquafeed ingredient

Dried skipjack tuna ( Katsuwonus pelamis) waste (red meat, gills, viscera, fins, etc.) were mixed with 25% wheat flour and inoculated with a starter culture of Lactobacillus plantarum National Collection of Industrial Microorganisms (NCIM) 2912 (10⁸–10⁹ cells mL⁻¹) and Bacillus licheniformis MTCC 6824 (10⁷–10⁸ cells mL⁻¹). Changes in the nutritional quality (crude protein, crude fat, crude ash, crude fibre and nitrogen-free extract and aminoacids) were monitored during a fermentation period of 14 days. The proximate analysis showed significant changes in the composition of L. plantarum -fermented tuna (LPFT) and B. licheniformis -fermented tuna (BLFT) from the unfermented raw materials. Fermentation of tuna waste has resulted in a significant ( P <0.05) increase in the protein content of tuna waste between days 6 and 12. All the amino acid contents in BLFT increased during fermentation, whereas, in LPFT the levels of serine, histidine, tyrosine, methionine, cystine and phenylalanine contents were decreased. A marginal increase in calcium and phosphorus levels was recorded in the fermented products. The results of the study suggest that LPFT or BLFT can be used as a novel aquafeed ingredient for different fish species.