鱼类游泳速度分类方法的探讨

游泳速度是评价鱼类游泳能力的重要指标。本文对常用的几种鱼类游泳速度概念和分类方法进行了总结。目前,以时间长度作为分类标准可将游泳速度分为巡航游泳速度(Cruising swimming speed,T200min)、长时游泳速度(Prolonged swimming speed,200minT20s)、冲刺游泳速度(Burst swimming speed,T20s)3类。但由于时间长度的选择缺乏令人信服的依据,很多学者并未遵循这一时间界定标准进行各类试验,致使游泳速度的界定出现混乱,从而使以时间长度作为分类依据的方法失去意义。作者综合多数学者的研究方法,以鱼类的游泳状态作为依据,重新对鱼类游泳速度进行分类,并在这一分类基础上拓展引申出最大探顶游泳速度这一新概念,并对其推求方法和实际意义进行了探讨。根据新的分类方法,鱼类游泳速度主要分为5大类:最优巡航速度(Optimum swimming speed)、最大续航速度(Maximum sustained swimming speed)、临界游泳速度(Critical swimming speed)、最大探顶游泳速度(Maximum domed swimming speed)、冲刺游泳速度(Burst swimming speed)。其他游泳速度概念基本可以按照上述分类进行归并。借助最大探顶游泳速度这一概念,作者进一步探讨了其与最大续航速度、临界游泳速度3者之间的联系,认为在某种程度上这3种游泳速度作为评价鱼类游泳能力的指标具有一致性。     

Temperature and fatigue effect on the maximum swimming speed of jack mackerel Trachurus japonicus

Swimming performance of jack mackerel Trachurus japonicus (18.2 ± 0.8 cm fork length (FL), n = 185) was examined in a flume tank by measuring the stride length at low and high tail beat frequencies with electromyogram monitoring and a muscle twitch experiment. Stride length was analyzed by monitoring the tail beat frequency according to the swimming speed at different temperatures of 10, 15 and 22 °C. In the electromyographic observations, the initiation of ordinary muscle activity occurred between 71.4 and 99.6 cm/s, that is 3.7 to 5.3 FL/s, when the tail beat frequency was over 6 Hz. The swimming speeds increased rectilinearly with the tail beat frequency at each water temperature both for the low and high tail beat frequency. Lower stride length was observed at the lowest temperature (10 °C) tested. The forced swimming exercise significantly affected the muscle contraction time to become longer than the control fish, which indicated a reduction of the maximum swimming speed performance.     

Classification of sound-scattering layers using swimming speed estimated by acoustic Doppler current profiler

There are various techniques for identifying fish species, including the multi-frequency method, in situ target strength characteristics, and digital image processing methods. Acoustic Doppler current profilers (ADCPs) are able to determine multiple current fields simultaneously and have been used to observe the swimming speed and behavior patterns of shoals of pelagic fish under natural conditions. In this study, we evaluated a classification method that can be used to determine the swimming velocity of both the sound-scattering layer and pelagic fish shoals using an ADCP (153.6 kHz) and a scientific echosounder (38, 200 kHz). To calculate the actual swimming speed of the fish shoals, the mean swimming velocity vectors of each stratified bin must be compared with the mean surrounding three-dimensional (3D) current velocity vectors. We found the average 3D swimming velocity of the sound-scattering layer to be characterized by a deviation of >5.3 cm/s from the surrounding current field. The average 3D swimming velocity of Pacific saury Cololabis saira was calculated to be 91.3 cm/s, while that of lanternfish Diaphus theta was 28.1 cm/s. These swimming speeds correspond to 4.19- and 4.26-fold the body length, respectively. Thus, the use of ADCP swimming velocity data can be expected to be a valuable species identification method for various fishes distributed in a given survey area.     

Energy partitioning in cultured juvenile Pacific bluefin tuna,Thunnus orientalis (Temminck & Schlegel, 1844)

The characteristics of dietary utilization, energy conversion efficiency, metabolic rate and energy partitioning were measured for cultured Pacific bluefin tuna (PBT) juveniles fed on an artificial diet. Thirty‐one juveniles (1.1 ± 0.3 g BW) were stocked into each of two 2500 L tanks to measure oxygen consumption (?O₂), swimming speed, digestibility and growth performance. ?O₂ elevated until 2.5 ± 0.3 times of pre‐feeding level within 1.5 h after feeding, except for the first feeding, and returned to pre‐feeding level within 3 h. Swimming speed fluctuation was corresponded with the ?O₂ fluctuation, and both parameters were stable from 02:00 to 06:00 and also during the whole day for starved fish. These indicate that feeding has strong influence on their metabolic rate. Energy partitioning for faecal, urinary and branchial, heat increment and voluntary activity, standard metabolism, and retained energy were calculated to be 17.2%, 5.9%, 14.9%, 41.3% and 20.7% of total ingested energy, respectively. The results indicate that, unlike other fish, juvenile PBT distribute large amount of energy for maintenance, which allows only a little proportion of ingested energy available for growth.     

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.     

Effect of lordosis on the swimming performance of juvenile sea bass (Dicentrarchus labrax L.)

The critical swimming speed (U_crit, cm s⁻¹) value of normal juvenile sea bass (Dicentrarchus labrax) (64.7±6.33) was significantly higher than that of fish with lordosis (60.3±6.66) (P<0.05). Both normal fish and those with lordosis showed increased endurance with increasing length during exercise at a fixed water velocity of 50 cm s⁻¹. This speed was used to investigate the separation possibility of deformed specimen from the normal developed fish based on their swimming performance, and 28% separation was achieved without losing any normal fish by the end of the 10th minute. To achieve a better separation of the deformities from the normal, first grading of the juveniles and then a swimming endurance test is advised.     

Completion of the Pacific bluefin tuna Thunnus orientalis (Temminck et Schlegel) life cycle

Tuna aquaculture is currently dependent on the wild capture of juveniles for production. The development of hatchery technology for bluefin and other tunas would be a major step forward in improving sustainability of their aquaculture. The present study overviews the technology in the life cycle completion of the Pacific bluefin tuna (PBT) Thunnus orientalis (Temminck et Schlegel) under aquaculture conditions in Kinki University, and the problems to be solved for the establishment of tuna hatchery technology. On 23 June 2002, broodstock of PBT that were artificially hatched and reared spontaneously spawned in captivity. The resulting eggs hatched and were subsequently reared to the juvenile stage. The spawning fish were the result of a research project started in 1987 to rear wild‐caught juvenile PBT that were several months old. Fertilized eggs were obtained from these fish in 1995 and 1996. Resulting juveniles (the artificially hatched first generation) were reared to maturity and spawned in 2002. Over the summer of 2002, 1.63 million eggs from these fish were used for a mass rearing experiment, and 17 307 juveniles were produced and transferred to an open sea net cage. Of these artificially hatched second‐generation PBT, 1100 grew to approximately 95 cm total length and 14 kg body weight in 22 months. This procedure means the completion of PBT life cycle under aquaculture conditions, which was first attained among large tuna species. The problems awaiting solution in PBT hatchery production are their unpredictable spawning in captivity, to improve survival during the first 10 days post hatch, to reduce cannibalism in larval and juvenile stages, and to solve collision problem causing high mortality during the juvenile stage.     

Effect of rearing density on the growth and welfare indices of juvenile spotted wolffish,Anarhichas minor (Olafsen)

The goal of this study was to determine the optimal stocking density for rearing juvenile spotted wolffish, Anarhichas minor (Olafsen), at two different sizes and assess the welfare status in relation to density. No major growth impairment was observed, although smaller fish (50–100 g) were significantly affected by density during the 120 days of the experiment, with final mean weights of 119.6 ± 11.6, 118.0 ± 5.8 and 88.7 ± 0.6 g for initial rearing densities of 10, 20 and 40 kg m^?2 respectively. No effect of rearing density was seen for larger fish (100–160 g) during the 90 days of the experiment, with final mean weights of 160.2 ± 5.9, 159.7 ± 3.7 and 163.7 ± 11.5 g at fixed rearing densities of 20, 30 and 40 kg m^?2 respectively. Our results suggest that the optimal rearing density of juvenile spotted wolffish is below 40 kg m^?2 for smaller size fish (～50–100 g) and probably ≥40 kg m^?2 for the larger fish (100–160 g). Furthermore, it appears that the range of rearing density used did not have a significant effect on a selection of stress indicators (Na⁺ and K⁺ concentration, haematocrit, hepatosomatic index, total amount of plasma proteins and liver and muscle water content) and immunity response (plasma lysozyme activity) of juvenile spotted wolffish, making it a very tolerant species to crowding.     

鲢幼鱼非疲劳贴网及姿态转换行为研究

流速是影响鱼类生存与繁衍栖息的重要因子。为了了解鱼类在中等流速区的游泳行为,探究其对水流的适应特征,利用鱼类游泳能力测定装置,以鲢(Hypophthalmichthys molitrix)幼鱼[体重(9.82±3.81) g,体长(8.56±1.11) cm)]为对象,采用递增流速法,分析了游泳过程中的非疲劳贴网行为与姿态转换行为。结果表明,在22℃水温条件下,鲢幼鱼平均相对临界游泳速度(critical swimming speed,U_(crit))相比其体长(body length,BL)为(6.00±0.93)BL/s;其非疲劳贴网速度(no-fatigue impingement speed,U_(imp))与临界游泳速度呈线性正相关,U_(crit)=1.03 U_(imp)+1.26 (R~2=0.86,P0.01);姿态转换速度(gait transition speed,U_(tran))与临界游泳速度呈线性正相关,U_(tran)=0.59 U_(crit)+1.55 (R~2=0.43,P0.01);非疲劳贴网速度与姿态转换速度呈线性正相关,U_(tran)=0.51U_(imp)+2.72 (R~2=0.41,P0.001)。在中等流速范围内(2～4 BL/s),实验鱼摆尾频率(tail beat frequency, TBF)和单次摆尾周期前进距离(stride length, SL)均随流速增加而增大。首次出现非疲劳贴网的流速为4.62 BL/s,流速增至5.08 BL/s时出现姿态转换行为。发生姿态转换后,TBF开始下降,而SL随流速增加快速增大。研究显示,鲢幼鱼非疲劳贴网行为对游泳能力和姿态转换速度有显著影响,且非疲劳贴网行为与姿态转换行为之间也存在相互影响。研究结果可为自然环境中鱼类生态行为研究、鱼类资源保护及渔业管理提供参考。     

Temperature effect on heart rate of jack mackerel Trachurus japonicus during swimming exercise

The heart rate of jack mackerel [16.5–21.2 cm fork length (FL), n = 24] was examined through forced swimming exercise in a flume tank by 10-min step-ups of speed levels in 1.5–6.0 FL/s range at different temperatures of 10, 15, and 22 °C. Electrocardiograph (ECG) monitoring was conducted by comparing the heartbeat pattern in still water without flow as a control, and continuously during exercise by speed levels until fatigue and during the recovery phase. Average heart rates in the control at each temperature were 36.5 beats/min at 10 °C, 56.1 beats/min at 15 °C, and 75.2 beats/min at 22 °C. The heart rate of jack mackerel significantly increased as the swimming speed was increased in each temperature. At the lower swimming speed of 1.5–2.4 FL/s, the heart rate was the same level as the control value at each respective temperature. The heart rate started to increase at swimming speeds of 2.3–2.5 FL/s at all temperatures. The higher heart rate in the range of 150–200 beats/min was achieved at a swimming speed of 6.0 FL/s at 22 °C. The recovery time after the maximum heart rate at high speed became longer at high temperatures.     

水流对杂交鲟幼鱼游泳行为的影响

在26℃水温下,使用特制的鱼类游泳行为测定装置研究了杂交鲟(Huso duricusGeorgi♂×Acipenser schrenc-kiBrandt♀)幼鱼在0 m/s、0.1 m/s、0.3 m/s、0.5 m/s 4种流速条件下的游泳行为。结果显示:杂交鲟幼鱼随着流速的增加趋流率增大,且在90min内趋流率达到100%所需的时间明显缩短。实过过程中0.3 m/s和0.5 m/s两组摆尾频率都高于0.1 m/s和静水组,但在各个时段内,摆尾频率与趋流率均没有显著的相关。杂交鲟幼鱼的游泳状态明显受到所处流速的影响:0.1 m/s流速下以逆流前进为主,约占时间比例56.3%;0.3 m/s流速下以逆流静止为主,约占时间比例58.1%;0.5 m/s流速下以逆流后退为主,约占时间比例80.7%。在逆流前进和逆流静止两种游泳状态下,杂交鲟幼鱼的游泳速度和摆尾频率呈线性相关。但在逆流后退和顺流而下两种状态下,两者之间却没有显著的相关,且此两种状态下其游泳速度和摆尾频率随流速增加的变化趋势也不一致。     

Bottom feeding and net chasing improve foraging behavior in hatchery-reared Japanese flounder Paralichthys olivaceus juveniles for stocking

We examined the effect of bottom feeding and net chasing as means to improve the maladaptive off-bottom swimming of hatchery-reared Japanese flounder Paralichthys olivaceus juveniles for stock enhancement. Three treatments were tested: (1) a bottom feeding treatment in which fish were fed near the bottom; (2) a net chasing treatment in which fish were chased by a hand net two to four times a day, and (3) a surface feeding treatment in which fish were fed from the surface (control treatment). Foraging behavior at the surface with surface feeding was analyzed 1 day before initiating these treatments and 2 weeks thereafter. A comparison of the pre- and post-treatments revealed that off-bottom swimming was less in the bottom feeding treatment and net chasing treatments. A comparison of off-bottom swimming in the bottom feeding treatment between the pre- and post treatments revealed that bottom fed fish showed less off-bottom swimming than surface fed fish. These findings suggest that fish behavior can be manipulated during the rearing period and that such manipulations can be used to improve the behavior of fish for release.     

Effects of pectoral fin ray removal on Siberian sturgeon <Emphasis Type="Italic">Acipenser baerii</Emphasis> swimming performance

Fin ray biopsies are commonly used to age sturgeon, but the effects of fin ray biopsy on Siberian sturgeon Acipenser baerii swimming performance are unknown. Therefore, the effects of two fin ray biopsy methods on swimming performance of hatchery-reared, juvenile Siberian sturgeon were evaluated in this study. Treatment fish were subjected to one of two biopsy methods: removal of a 2- to 4-cm section from a marginal pectoral fin ray, or full removal of a marginal pectoral fin ray. Control fish were only subjected to a sham operation. A modified 2,936-l Brett-type swim tunnel was used to evaluate 10-min critical station-holding speeds (C _SHS) and behavioral swimming characteristics of sturgeon immediately after fin ray biopsies were carried out. Fish sizes (range 65- to 84-cm fork length) were comparable among treatments. Mean 10-min C _SHS (mean ± SE) were 113 ± 3.4, 109 ± 2.5, and 111 ± 2.8 cm s^?1 for the segment removal treatment, full removal treatment, and control treatment, respectively. ANOVA indicated that the two methods had no significant effect on the 10-min C _SHS of Siberian sturgeon compared to the control treatment. Results indicate that fin ray removal has no effect on Siberian sturgeon swimming performance.     

The effect of shade on water quality parameters and survival and growth of juvenile fluted giant clams, Tridacna squamosa, cultured in a land-based growth trial

Juvenile fluted giant clams, Tridacna squamosa (mean shell length (SL) 15.7 ± 1.9 mm, mean total wet weight (TWW) 0.3 ± 0.1 g), were cultured for 4 months in land-based nursery tanks under four shade regimes (0, 10, 55 and 80 % shade). After 4 months, there were no significant differences in survival which ranged from 98.8 ± 0.1 % in juveniles cultured under 55 % shade to 100.0 ± 0.0 % in the other treatments. The SL of unshaded juveniles was only significantly greater than that of juveniles shaded 55 and 80 %. However, the TWW of juveniles cultured with no shade was significantly greater than that of the juveniles from all other treatments. The mean growth rate of juveniles varied from 5.0 mm month^?1 for juveniles shaded 80 % to 6.9 mm month^?1 for juveniles not shaded. There was no significant difference in mean growth rate of unshaded juveniles and juveniles shaded 10 %, but only the growth rate of juveniles cultured unshaded was significantly greater than that of juveniles shaded 55 and 80 %. The organic tissue content of juveniles cultured either unshaded or shaded 55 % was significantly higher than that of juveniles shaded 80 %. In contrast, the condition index (CI) of juveniles shaded 80 % was significantly higher than the CI of juveniles cultured either unshaded or shaded 55 %. However, there was no significant difference between the CI of juveniles shaded 80 and 10 %. Growth rate of juveniles was significantly correlated with temperature, dissolved oxygen content and flow rate of seawater. While low degrees of shade increase the growth rate of juvenile T. squamosa, they also negatively affect tissue condition. A shade level of 10 % appears to be optimal as it maximises growth and maintains a high CI.     

人工繁殖圆口铜鱼幼鱼游泳能力与游泳行为研究

利用鱼类游泳能力测定装置,以人工繁殖的圆口铜鱼(Coreius guichenoti)幼鱼为实验对象,体重(3.05±0.99)g,体长(5.94±0.66)cm,采用递增流速法研究其游泳能力与游泳行为。结果表明,在(20±1)℃水温条件下,圆口铜鱼平均相对临界游泳速度(critical swimming speed,U_crit)为(8.41±1.56)BL/s;其步态转换速度(gait transition speed,U_chg)与临界游泳速度呈线性正相关:U_chg=0.63 U_crit+0.21(P<0.01,R 2=0.86);摆尾频率(tail beat frequency,TBF)与进口流速(inlet velocity,U_in)呈线性正相关:TBF=0.48 U_in+2.53(P<0.01,R 2=0.95);进口流速(inlet velocity,U_in)与步长(step length,SL)呈线性正相关:SL=0.11 U_in+0.41(P<0.01,R 2=0.99)。实验鱼的摆尾幅度(tail beat amplitude,TBA)、冲刺次数、各进口流速下的摆尾时间百分比及稳定摆尾与非稳定摆尾比例,均随进口流速改变而变化。随流速增大,摆尾幅度呈现先增大、再减小、最后又增大的趋势;冲刺次数也是先增加、随后逐渐下降;摆尾时间百分比最初是快速增大,随后基本保持不变,流速增至8 BL/s高流速时,再次随流速增加而快速增大。当流速与临界游速比值(U/U max)为0.38时,实验鱼出现非稳定摆尾行为;比值为0.58时,实验鱼稳定摆尾行为与非稳定摆尾行为比例为1∶1;比值增至0.78时,稳定摆尾行为消失。人工繁殖的圆口铜鱼游泳能力较强,在不同流速下,通过改变游泳行为以保持更长的游泳时间及距离。研究结果可为以圆口铜鱼为过鱼对象的鱼道建设以及养殖流速优化提供参考。     

Effect of feeding frequency on growth,feed efficiency and nutrient utilization of juvenile flounder (Platichthys flesus luscus)

An experiment was conducted to determine the effects of optimal feeding frequency on growth performance, feed efficiency, nutrient utilization and body composition in juvenile flounder. Four feeding frequencies of one (F1), two (F2), three (F3) and four (F4) times a day were evaluated as treatments in triplicate for a period of 60 days. Forty-seven hatchery produced juveniles (2.5 ± 0.7 g) were stocked into 160-L rectangular fiberglass tanks and fed with a commercial diet containing 54 % protein to satiation. At the end of the experiment, the final weight of F1, F2, F3 and F4 were 5.06 ± 0.29, 5.91 ± 0.42, 6.24 ± 0.42 and 6.16 ± 0.46 g, respectively. Growth rates and feed consumption were highest in F3 and lowest in F1. Feed conversion ratios ranged between 0.83 ± 0.04 and 0.90 ± 0.03. There were no significant differences in either moisture or ash content of the fish groups. Protein content decreased with increasing feeding frequency, while lipid content partly increased with increasing feeding frequency. Nitrogen intake and lipid intake were significantly higher in fish fed under F3 and F4 treatment, whereas the nitrogen gain decreased and lipid gain increased with increasing feeding frequency. It is concluded that the flounder juveniles can achieve maximum growth performance and better nutrient utilization when they are fed a given ration two times a day. The findings have practical significance toward establishing an appropriate flounder nursery rearing and will directly benefit the nursery operations.     

不同运动状态下鳙幼鱼的游泳特性研究

为了探究鱼类反复运动疲劳后游泳能力和游泳行为的变化,以鳙(Aristichthys nobilis)幼鱼为研究对象,利用环形水槽实验装置,采用流速递增法,通过LoliTrack视频软件分析鱼类游泳行为,研究不同运动状态下鳙幼鱼的游泳特性变化情况。结果表明,鳙幼鱼疲劳前耗氧率随游泳速度的增大显著增加,且临界游泳速度恢复过程中最大耗氧率小于突进游泳速度恢复过程中最大耗氧率。视频分析显示,摆尾频率与游泳速度呈线性正相关关系。同时发现,在2组反复疲劳运动状态下,连续进行2次临界游泳速度测定过程中,鳙幼鱼频繁使用爆发-滑行游泳行为,而在经过突进游泳速度测试后转入临界游泳速度测定过程中,实验鱼并未使用这一游泳行为。经过一次临界游泳速度测试后,第二次临界游泳速度显著大于突进游泳速度测试后的临界游泳速度(P0.05),第一次突进游泳速度与第二次突进游泳速度无显著差异(P0.05)。无氧运动消耗导致鳙幼鱼有氧运动能力显著下降,而有氧运动消耗过程对无氧运动能力无显著影响。     

Early life history of yellow puffer,Chonerhinos naritus (Richardson, 1848) from Sarawak,Northwestern Borneo

The growth and morphological development including fins, spine distribution and pigmentation of larval and juvenile of hatchery‐reared yellow puffer, Chonerhinos naritus were described to provide essential information on the early life history of this species. The total length (TL) of newly hatched larvae was 3.42 ± 0.23 (mean ± SD) mm, reaching 5.66 ± 0.38 mm on 5 days after hatched (DAH), 7.80 ± 0.28 mm on 11 DAH, 9.88 ± 0.40 mm on 27 DAH and 10.92 ± 0.58 mm on 30 DAH. The yolk was completely absorbed in preflexion larvae at 4 DAH. The mouth opening started at 3 DAH of yolk sac larvae, while the teeth appeared starting from preflexion larvae at 7 DAH. Overall aggregate fin ray numbers including caudal fin attained full complement in postflexion larvae at 27 DAH. Several melanophores with appearance of small stellate were first appeared dorsally on the head of flexion larvae at 13 DAH, expanded at the dorsal region of the head, above the eye in juveniles at 30 DAH. The spines first appeared in preflexion larvae of C. naritus at 7 DAH, covering the ventral skin region below pectoral fin base and expanded to the ventral part of the body and nearly covered the whole abdomen region before the anus and below the eyes in juveniles. C. naritus remain as larvae for approximately 29 days, during which they metamorphose to the juvenile stage prior to sexual maturation. Observations in larvae development of C. naritus revealed similar characteristics with other Tetraodontidae species.     

An intensive hatchery rearing protocol for larvae of the bullseye puffer,Sphoeroides annulatus (Jenyns)

In this study bullseye puffer, Sphoeroides annulatus larvae were reared from hatching through to 1 or 2 months after weaning on an experimental scale in three replicate 600 L tanks and on three occasions during the spawning season (nine tanks in total). The rearing protocol used was green water (Nannochloropsis oculata and Isochrysis sp.) 100 000 cells mL^?1 from 0 to 11 days after hatch (DAH), 5–10 rotifers, Brachionus rotundiformis mL^?1 from 2 to 26 DAH, 0.1–1 Artemia mL^?1 from 21 to 34 DAH and weaning from 29 to 34 DAH. Survival to a month after weaning was 1%, a total of 3153 juveniles were produced with an average wet weight of 0.29 ± 0.07 g and a length of 27.5 ± 0.82 mm.     

High incidence of death due to collision of hatchery-reared Pacific bluefin tuna Thunnus orientalis juveniles in sea cages, as revealed by head and spinal injuries

High mortality of hatchery-reared Pacific bluefin tuna (PBT) juveniles is frequently observed in sea cages after transfer from indoor tanks, and is likely due to collisions of the fish with the net wall of the sea cages. Because collisions frequently cause bone injuries due to physical impact, such injuries can be considered an index of obvious collisions. In the present study, bone injuries of dead juveniles were investigated in order to examine the prevalence of net collision as a cause of death among hatchery-reared PBT juveniles in sea cages. Juvenile PBT were reared in sea cages for 90 days after transfer, and dead fish were collected and examined by the clearing and staining method or by dissection to detect injury to the vertebral column and parasphenoid. The prevalence of injuries to the vertebral column and parasphenoid until day 30 after transfer was low (0.0–12.0 %) when the fish were 5.5–15.2 cm in total length (TL). However, it drastically increased to 17.8–78.0 % for fish 21.0–39.2 cm TL from day 31 onward after transfer. These results suggest that the mortalities of juvenile PBT larger than 20 cm TL in cages were induced by net collisions.