Water-tank experiment and static numerical analysis of the mooring system of a controllable depth cage

Recent variations in marine environments have increased the risk of aquaculture accidents at sea. This risk can be reduced by installing fish cages at the desired depth, based on environmental conditions such as wave height, the vertical profiles of water temperature, algal concentration, and dissolved oxygen concentration. Most submergible fish cages can be located at only two depths: the sea surface and a submerged depth. In the present study, a fish cage installed at various depths, i.e., a controllable depth cage (CDC), is proposed for avoiding undesirable environments. A water tank experiment is conducted to measure the drag of the cage and the static deformation of the mooring system using a scale model of the actual cage. Then, a simple numerical model based on the balance of forces on each component is developed to analyze the position and the attitude of the cage and the mooring tension of the system. The numerical model is verified by comparing the experimental and numerical results. The outcome showed that the cage and floats moved downstream at an increasing velocity. The results of the numerical simulation supported those of the water tank experiment. However, the simulated vertical positions of a cage and floats were higher compared with experimental results. Additionally, the inclination of angle increased alongside increasing velocity in the numerical simulation, whereas a complex variation was observed in the experiment. This happened because of underestimating the drag on the mooring rope in lower water current velocities; additionally, cage lift was not considered in the numerical model. Despite these discrepancies, the tension of each mooring rope was well predicted because of the dominant tension of the horizontal component. In future studies, the balance of forces on the rope should be predicted more precisely, and variations in cage drag and inclination angle should be included in the numerical model. Additionally, the effect of waves should be considered alongside water currents to ensure the safety of the CDC.     

大冶湖水体光学特性的季节变化及其影响因子

基于2018年对大冶湖全湖四季的野外调查与水质监测,结合大冶湖的形态,将研究范围划分为西区和东区两大部分,均匀设置15个采样点,在1、4、8、10月对各点位进行水样采集;通过分析水体光学衰减系数(K_d)、真光层深度(Z_(eu))及其他水体光学特性重要参数,得到大冶湖周年的光合有效辐射(PAR)衰减特性,可为大冶湖流域生态管理及沉水植物恢复提供参考。结果显示:(1)大冶湖的K_d在季节上以冬季最低,夏季最高,平均值为3.17/m,由春至秋的季节变化不明显;(2)大冶湖水体光学特性最主要的影响因子为叶绿素a(Chl-a)、悬浮物(SS)和溶解性有机物(DOM),相关系数最高分别达到0.606、0.445和0.419,是限制水体K_d的主要因子,关系式为:K_d=0.019Chl-a+0.011SS-0.029DOM+2.364,降低水体Chl-a、SS和DOM浓度成为改善大冶湖水体光学特性的重要内容;(3)大冶湖适宜进行沉水植物恢复的季节为春季,全湖其他季节的光学阈值(真光层深度/水深,Z_(eu)/D_w)均小于1,不利于沉水植物的萌发生长,恢复区域以磊山、蚌壳岭以东湖区、南部湖区沿岸浅水地带以及湖西侧的河道浅滩为宜。     

吊养在池塘不同水层中的三角帆蚌的存活和生长

2005年5～11月在浙江省诸暨某池塘研究了吊养在池塘水表层下10 cm、40 cm和80 cm处三角帆蚌(Hyri-opsis cumingii)的生长。实验结果表明:吊养在不同水层的三角帆蚌成活率无显著差异,但吊养在表层下40 cm水层的蚌蚌壳长、蚌壳宽和蚌重增长速度明显高于吊养在表层下10 cm和80 cm水层的蚌,吊养在表层下10 cm处的蚌生长最慢。     

Effects of depth and eutrophication on the zooxanthella clades of Stylophora pistillata from the Gulf of Eilat (Red Sea)

• 1. The symbiotic association between corals and zooxanthellae has been a major contributing factor in the success of reef‐building corals.
• 2. The diversity of zooxanthellae harboured in the hermatypic coral Stylophora pistillata in the Gulf of Eilat (Aqaba), northern Red Sea, was investigated on the basis of the length and sequence variation of the 18S small subunit ribosomal DNA. Of several clades found worldwide, only clades A and C were found to be associated with this coral species in the Gulf.
• 3. No correlation between the pink and brown colour morphs of the coral S. pistillata and the zooxanthella clade was found. Shallow‐water colonies (5 m) harbour clade A while deeper‐water colonies (17 m) harbour either clade A or C. This coral is a brooding species that associates with different zooxanthellae at different depths.
• 4. Eutrophication from fish farms did not affect clade identity associated with Stylophora.

Copyright © 2008 John Wiley & Sons, Ltd.     

东太平洋长鳍金枪鱼个体大小与钓获深度的关系

长鳍金枪鱼(Thunnus alalunga)具有重要的经济价值,其个体大小与钓获深度有一定的关联。本研究利用2013年东太平洋渔业观察员的数据,将渔获物分为76~85 cm、86~95 cm、96~105 cm、106~115 cm和116~125 cm 5个叉长组,运用悬链线公式得到钓获深度,统计各叉长组渔获个体钩位差异和各钩位渔获个体大小差异,分析个体大小与钓获深度的关系,得到结果如下:渔获物钓获深度范围为61.3~236.1 m。各叉长组主要渔获钩位随叉长增大而增大。各钩位主要渔获个体随序号增大而增大。76~85 cm叉长组的渔获物主要分布在61.3~121.8 m水层,平均钓获深度为(110.6±11.7)m;86~95 cm叉长组的在102.3~174.4 m,平均钓获深度为(144.1±4.5)m;96~105 cm叉长组的在189.4~202.8 m,平均钓获深度为(184.9±1.6)m;106~115 cm叉长组的在214.3~223.6 m,平均钓获深度为(193.8±3.8)m;116~125 cm叉长组的在230.5~236.1 m,平均钓获深度为(219.3±7.2)m。研究结果表明:东太平洋长鳍金枪鱼随叉长增大,栖息深度也加深,二者有相关关系。生产时调整钓具深度可有效保护资源可持续发展。     

The effect of environmental factors, depth and position on the growth and mortality of raft-cultured blue mussels (Mytilus edulis L.)

One‐year‐old rope grown blue mussels (Mytilus edulis L.) were grown in experimental lantern nets at two depths (2 and 6 m below the surface) in two different positions (inflow and outflow) off a raft in Loch Etive on the west coast of Scotland. Shell and tissue growth, and mortality were recorded. Water temperature, salinity and food availability were also monitored over the experimental period. There were no significant differences in the length, live weight, wet meat weight, dry meat weight and ash‐free dry meat weight between depths (P > 0.05). However, position had a significant effect on these parameters as mussels located at the inflow of the raft differed significantly from those at the outflow of the raft (P < 0.05). Particulate organic matter (POM) and chlorophyll a (Ch a) were significantly higher at the inflow than the outflow of the raft (P < 0.05), but depth had no effect on POM and Ch a (P > 0.05). The results show that food concentration was higher in the inflow of the raft than the outflow. In the light of these results, recommendations for better management of Scottish raft mussel cultivation are discussed.     

基于海表温和水深的南海北部短尾大眼鲷渔场分析

根据2014—2017年南海北部200m等深线以浅海域渔业资源调查的短尾大眼鲷(Priacanthusmacracanthus)数据,结合遥感获得的海表温度(seasurfacetemperature,SST)数据,首次将渔场水深(D)与SST的乘积(SSTD)引入作为新的变量,采用灰色关联度方法筛选与渔场关联度最高的变量,将关联度最高的变量与标准化后的单位捕捞努力量(catch per unit effect, CPUE)采用一元非线性回归建立模型,对模型理论CPUE与实际CPUE的时空变化进行分析。结果表明,南海北部短尾大眼鲷渔场随季节变化明显,各季节CPUE随SST呈先增大后减小的趋势;不同季节CPUE最高的海域水深不同,春季为160 m,夏季为140 m,秋季为60 m,冬季为140 m; CPUE与SST、D、SSTD进行灰色关联度分析发现,各季节CPUE与SSTD关联度最高且关联度均超过0.5;将SSTD作为变量, CPUE作为表征渔场好坏的指标值,建立环境因子与CPUE的关系模型,模型理论CPUE高值区有明显的季节变化,且实际CPUE高值区的分布与理论CPUE高值区分布一致,以上结果 P值均小于0.05,模型预测准确。     

The effects of water depth on prey detection and capture by juvenile coho salmon and steelhead

Abstract –  We used three-dimensional video analysis of feeding experiments to determine the effects of water depth on prey detection and capture by drift-feeding juvenile coho salmon ( Oncorhynchus kisutch ) and steelhead ( O. mykiss irideus ). Depth treatments were 0.15, 0.30, 0.45 and 0.60 m. Mean prey capture probabilities for both species were constant across all treatments (coho = 0.51, steelhead = 0.39), and did not differ significantly between species. In deeper treatments, capture probabilities were lower nearer the surface than they were nearer the substrate, particularly at the lateral edges of the foraging area. In deeper treatments coho had greater capture probabilities nearer the surface than did steelhead. It is unclear if this was a species difference, or one based on the relative amount of foraging experience the fish had in the wild prior to capture. Prey capture manoeuver characteristics were very similar for both species, including positive relationships between water depth and both prey detection distance and prey interception swimming speed, and no relationship between depth and speed of return to the focal point. Because prey encounter rate is expected to increase with increasing water depth, we used capture probabilities to predict capture rates for coho and steelhead, which increased linearly with water depth. We conclude that any benefit of foraging in deeper water is more likely due to increased encounter rate rather than to increased capture probability.     

Movements and behaviors of swordfish Xiphias gladius in the United States Pacific Leatherback Conservation Area

This study reports on the movements of swordfish tagged within the Pacific Leatherback Conservation Area (PLCA), an expansive region (>500,000 km²) off the U.S. West Coast that has been seasonally restricted to drift‐gillnet fishing since 2001 to reduce leatherback sea turtle (Dermochelys coricea) interactions. Thirteen swordfish were outfitted with satellite‐linked archival tags scheduled for short (2–20 days, n = 11) and longer‐term (150 days, n = 2) data collection. All tags were deployed on basking swordfish using traditional harpoon‐based methods during the fall of 2012–2013, near offshore seamounts (35.6°N/122.9°W to 37.4°N/123.5°W). Depth and temperature data from 11 swordfish (~90 to 150 kg) resulted in <251 days of movement information from the PLCA region. All tagged individuals exhibited surface‐oriented nocturnal movements, spending >99% of the night above the average thermocline depth (37.5 m), with an average night depth of 8.3 ± 1.6 m. Daytime depth distribution was greater and more variable (mean 107.1 ± 21.2 m), with fish primarily displaying three behavioral patterns: (i) basking activity, 16.7% of the day, (ii) a mixed‐layer distribution between 3 m and the thermocline (26.8% of the day), and (iii) prolonged dives below the thermocline, 56.5% of the day. For seven of the tracks, daytime basking rates increased when thermocline depth was <37 m. As fish moved offshore, there was less variability in vertical movements with a reduction in both basking activity and mixed layer occupancy, as well as an increase in average daytime depth. These data are discussed with respect to the potential development of alternative fishery options for the PLCA.     

Behavioural ecology at sea of Atlantic salmon (Salmo salar L.) kelts from a Newfoundland (Canada) river

For migratory fishes such as Atlantic salmon, temporal variation in the utilization of thermal and depth habitats in the marine environment is likely to occur at both broad and fine scales, reflecting both ecological and metabolic factors. To test this, we implanted 26 Atlantic salmon kelts (i.e., post‐spawners) descending Campbellton River, Newfoundland, with data storage tags that recorded date/time, internal and external temperatures, pressure, and light. The salmon that returned to Campbellton River after an average of 62 days at sea were consecutive spawning kelt. Detailed data were recovered from eight of these fish and revealed distinct patterns in their utilization of thermal and depth habitats. Water temperatures experienced over the period at sea showed a wide range for all fish (from <0 to nearly 20°C); however, there were two clear frequency modes; one at 6–7°C and the other at 11–12°C. The cooler mode was indicative of daytime profiles and the warmer mode of night‐time profiles. This corresponded with the depth profiles, which indicated that salmon dove more frequently below 5 m (mean ± SD = 23 ± 0.9 dives per day) and spent less time (approximately 18%) near the sea surface (<1 m) during the day than during the night (4 ± 0.4 dives per night; approximately 45% time <1 m). The diurnal pattern may be indicative of a reliance on vision for feeding at depth and the short duration of dives (<10 min) may be a strategy that allows salmon a metabolic advantage (e.g., swimming capacity) over their prey living constantly at depth in cool waters.