Influence of California halibut (Paralichthys californicus) on the vertical distribution of dissolved oxygen in a raceway and a circular tank at two depths

The design and operation of aquaculture tanks should minimize stagnant areas especially in the immediate vicinity of the fish. In tanks with pelagic fish, mixing caused by the water flow and by fish swimming is sufficient to maintain dissolved oxygen and metabolite concentrations in the immediate vicinity of the fish that are similar to those in the main water body. Given the behavior of sedentary benthic species, such as the California halibut (Paralichthys californicus), and their tendency to remain motionless on the bottom of aquaculture tanks, often in layers that are several fish deep, water quality may stratify with the worse conditions occurring in the area where they fish are lying. The purpose of this study was to evaluate the influence that California halibut (450 g average weight) may have on the vertical profile of oxygen concentration in a raceway (239 cm long, 28 cm wide) and a circular tank (92 cm diameter) operated at two water depths (10 and 20 cm). Oxygen was measured at each centimeter of the vertical profile both in an area with fish and without fish to assess their influence.

Results showed a lower oxygen concentration in the near-bottom region of the raceway and circular tanks. The phenomenon was most pronounced in the raceway operated at a 20 cm depth, but was also observed in the circular tank operated at 20 cm and in the raceway at 10 cm.

Measurements were also taken in samples collected just in front of or directly from a fish's mouth. A zone of depressed oxygen concentration in the immediate vicinity of the fish was documented, with oxygen concentrations as low as 50% of the measured tank effluent concentration. The magnitude of the depression was greater in raceways than in circular tanks and in 20 cm water depth than in 10 cm depth. The fish remained sedentary in these zones of depressed oxygen concentration for extended periods of time and frequently exhibited hyperventilation. The oxygen concentrations in the vicinity of the fish were consistently lower than the concentrations measured in the tank effluent. Therefore, effluent measurements did not provide an accurate representation of conditions to which the fish were exposed.     

Comparative analysis of flow patterns in aquaculture rectangular tanks with different water inlet characteristics

The objective of the work is to improve the design rules of rectangular aquaculture tanks in order to achieve better culture conditions and improve water use efficiency. Particle tracking velocimetry techniques (PTV) are used to evaluate the flow pattern in the tanks. PTV is a non-intrusive experimental method for investigating fluid flows using tracer particles and measuring a full velocity field in a slice of flow. It is useful for analysing the effect of tank geometries and water inlet and outlet emplacements. Different water entry configurations were compared, including single and multiple waterfalls and centred and tangential submerged entries.

The appearance of dead volumes is especially important in configurations with a single entry. Configuration with a single waterfall entry shows a zone of intense mixing around the inlet occupying a semicircular area with a radius around 2.5 times the water depth. A centred submerged entry generates a poor mixing of entering and remaining water, promoting the existence of short-circuiting streams. When multiple waterfalls are used, the distance between them is shown to have a strong influence on the uniformity of the velocity field, increasing noticeably when the distance between inlets is reduced from 3.8 to 2.5 times the water depth. The average velocities in configurations with multiple waterfalls are very low outside the entrance area, facilitating the sedimentation of biosolids (faeces and non-ingested feed) on the tank bottom. The horizontal tangential inlet allows the achievement of higher and more uniform velocities in the tank, making it easy to prevent the sedimentation of biosolids.     

Format and mode of artificial substrate fixation affect the performance of Litopenaeus vannamei in high‐density rearing systems

We investigated whether the positive impacts of artificial substrates on shrimp performance are altered in any way by their format or mode of fixation in the tanks. To examine this question, substrates were fixed vertically in the water column in three different configurations: SCF treatment (Substrate Completely Fixed), SPF treatment (Substrate Partially Fixed) and SFF (Substrate in Frond Format). Another treatment received no substrate and served as control (WS = Without Substrate). The shrimp were cultured for 38 days in intensive biofloc culture tanks at a stocking density of 1,125 shrimp m^?3. In general, water quality variables were similar among treatments and remained within the appropriate range for shrimp culture. The final biomass was higher (8.5 kg m^?3) and the feed conversion ratio (FCR) lower (1.6) in all tanks with substrates when compared with the WS treatment tanks (final biomass = 6.3 kg m^?3 and FCR = 3.1). However, only shrimp from the SCF and SPF treatments had a higher survival rate (>95.0%) compared to those in WS tanks (75.9%), which was statistically similar to the SFF treatment (88.0%). These results show that substrate format and its mode of fixation in tanks can alter shrimp performance. In well‐aerated intensive tanks, substrates in frond format are constantly pushed to the tank surface, making it difficult for shrimp to adhere to the screens. In such situation, the extra surface provided by the substrates is not always available to the shrimp, a fact that minimizes the positive effects of substrate.     

工厂化对虾养殖池管式射流集污水力特性

工厂化养殖池内水体的流场分布特性直接决定了其对残饵、粪便等的排污性能。本实验研究了管式射流驱动模式下,射流角度与射流流速对养殖池内水体流场与污物聚集特性的影响。养殖池流场特性采用点式流速仪进行布点测量,利用MATLAB软件对流场特性进行分析。采用相机采集的养殖池集污效果图像,并利用Photoshop软件与自行开发的不规则图形面积分析软件对图像进行分析。研究表明,流速从池心向外呈"V"型变化,在射流角度固定的情况下,射流速度越大,池心低流速区域越小,污物向池心的聚集效果越好;在流速固定的情况下,存在一个最佳的射流角度,本实验在24 cm/s流速条件下,射流角度为40°时,池内污物聚集效果最优。结论认为在保证养殖对象生长的前提下,可尽量提高射流流速;在24 cm/s流速条件下,最佳射流角度在40°左右。本研究成果可为工厂化养鱼池、养虾池等管式射流水力驱动系统的优化设计提供参考依据。     

An effective and practical method of net settings in rearing tank to suppress hypermelanosis in Japanese flounder

In Japanese flounder aquaculture, hypermelanosis occurs widely on the blind side. Rearing flounders in a net-lined tank was recently reported to prevent hypermelanosis. To effectively apply this method to larger tanks for aquaculture farming, the net setting method was examined. Juvenile flounders without darkened areas on the blind side [total length (TL) 13 cm] were selected, and reared for 6 months (TL 32 cm). In the control tank without a net, the median value of darkened area ratio (darkened area to blind side area) was 46%. By only covering the tank bottom with slack net, the darkened area ratio was suppressed to 8%, less than 1/5 of that in the control tank. At the end of experiment, bottom coverage ratios of 0%, 10%, 30%, 50%, and 100% revealed a negative correlation between bottom net coverage and darkened area ratio. In this experiment, the darkening area in the tank with 50% bottom net coverage decreased to 1/3 of the control. Although the occurrence of hypermelanosis differs depending on the production lot, these results are expected to serve as a reference for selecting the suitable net size to meet the level of clearness of the blind side.

     

Water velocity in commercial RAS culture tanks for Atlantic salmon smolt production

An optimal flow domain in culture tanks is vital for fish growth and welfare. This paper presents empirical data on rotational velocity and water quality in circular and octagonal tanks at two large commercial smolt production sites, with an approximate production rate of 1000 and 1300 ton smolt/yr, respectively. When fish were present, fish density in the two circular tanks under study at Site 1 were 35 and 48 kg/m³, and that in four octagonal tanks at Site 2 were 54, 74, 58 and 64 kg/m³, respectively. The objective of the study was twofold. First, the effect of biomass on the velocity distribution was examined, which was accomplished by repeating the measurements in empty tanks under same flow conditions. Second, the effect of operating conditions on the water quality was studied by collecting and analysing the water samples at the tank’s inlet and outlet. All tanks exhibited a relatively uniform water velocity field in the vertical water column at each radial location sampled. When fish were present, maximum (40 cm/s) and minimum (25–26 cm/s) water rotational velocities were quite similar in all tanks sampled, and close to optimum swimming speeds, recommended for Atlantic salmon-smolt, i.e., 1–1.5 body lengths per second. The fish were found to decrease water velocity by 25% compared to the tank operated without fish. Flow pattern was largely affected by the presence of fish, compared to the empty tanks. Inference reveals that the fish swimming in the tanks is a major source of turbulence, and nonlinearity. Facility operators and culture tank designers were able to optimize flow inlet conditions to achieve appropriate tank rotational velocities despite a wide range of culture tank sizes, HRT’s, and outlet structure locations. In addition, the dissolved oxygen profile was also collected along the diametrical plane through the octagonal tank’s centre, which exhibits a close correlation between the velocity and oxygen measurements. All tanks were operated under rather intensive conditions with an oxygen demand across the tank (inlet minus outlet) of 7.4–10.4 mg/L. Estimates of the oxygen respiration rate in the tank appears to double as the TSS concentration measured in the tank increases from 3.0 mg/L (0.3 kg O₂/kg feed) up to 10–12 mg/L (0.7 kg O₂/kg feed). Improving suspended solids control in such systems may thus dramatically reduce the oxygen consumption and CO₂ production.     

Protection of crucian carp (Carassius auratus Gibelio) against septicaemia caused by Aeromonas hydrophila using specific egg yolk immunoglobulins

Egg yolk immunoglobulins (IgY) were obtained from laying hens immunized with inactivated Aeromonas hydrophila. The purified IgY was shown to inhibit the growth of A. hydrophila in vitro and the optimum concentration for inhibition of A. hydrophila‐specific IgY was 75 mg mL^?1. In a subsequent challenge trial, 100 carp (200~250 g) were assigned to one of ten tanks with ten carp per tank. The fish in one tank were unchallenged whereas the remaining 90 fish were injected intraperitoneally with 100 μL of A. hydrophila at a concentration of 10⁸ cfu mL^?1. For the next 21 days, all fish were moved in their respective groups to a clean tank for 20 min day^?1. The fish in four tanks (one unchallenged tank and three challenged tanks) received no treatment whereas the fish in the remaining six tanks were immersed in either 0.5 g L^?1 aqueous nonspecific IgY (N = 3) or 0.5 g L^?1 aqueous specific IgY (N = 3). Haemoglobin concentrations, white and red blood cell numbers as well as the mortality of specific IgY‐treated fish were significantly different from those of the control. These results suggest that passive immunization by immersion with pathogen‐specific IgY may provide a valuable treatment for A. hydrophila infection in carp.     

Size selectivity of the Spisula solida dredge in relation to tooth spacing and mesh size

A study was undertaken on the northwestern coast of Portugal to evaluate the selectivity effect of both tooth spacing and mesh size on the catch of the white clam (Spisula solida). Two typical clam dredges, equipped with different tooth spacing and mesh size, were towed side by side. Three tooth spacing (20, 40 and 60 mm) and three mesh sizes (35, 40 and 50 mm) were investigated. The dredge selectivity experiments were performed by attaching a cover bag with a 20 mm mesh to the gear. Individual hauls were analysed by the SELECT model. The Fryer’s mixed model of between-haul variation was also applied to data. The results showed that the space between teeth does not have an effect on selectivity. Tooth length was directly related to the dredge’s capture efficiency. The only factor that contributed to dredge size selection was mesh size. The 50% retention lengths estimated for mesh sizes 30, 40 and 50 mm were 25.68, 26.35 and 31.94 mm, respectively. For all mesh sizes, very narrow selection ranges were observed. The most appropriate mesh size to be enforced within the Portuguese northwestern S. solida fishery should be 40 mm.     

Stocking density and shelter type for the optimal growth and survival of western rock lobster Panulirus cygnus (George)

The growth and survival of three size classes of wild caught western rock lobster, Panulirus cygnus (post-pueruli: mean 2.14 ± 0.07 g, 13.2 ± 0.1 mm CL; year 1: post-settlement juveniles, 57.1 ± 1.1 g, 38.7 ± 0.28 mm CL; and year 2 post-settlement juveniles, mean 138.2 ± 2.26 g, 51.9 ± 0.25 mm CL) were examined at combinations of two stocking densities (post-pueruli: 50 and 100 m^− 2; year 1: 11 and 23 m^− 2; year 2: 10 and 19 m^− 2) and two shelter types (a novel rigid plastic mesh shelter or bricks) over a period of 6 months. Survival of lobsters held at the lower densities (90–95%) was significantly greater than for lobsters held at higher densities (post-pueruli = 78%, year 1 = 86%, year 2 = 88%). Post-pueruli survival was significantly higher in tanks with mesh shelters (91.7%) than brick shelters (75.8%) with a similar trend exhibited by year 1 and year 2 lobsters. Densities tested did not significantly affect lobster growth for any size class. Growth of post-pueruli was considerably higher in tanks with mesh shelters (641.7% weight gain; specific growth rate 1.07 BW day^− 1) (p < 0.05) but there was no difference in the growth of year 1 and year 2 lobsters between mesh and brick shelters. Feed intake (g pellet dry matter lobster^− 1 day^− 1) was not significantly different between densities. This study has shown that P. cygnus is well suited for aquaculture based on the collection and ongrowing of wild caught pueruli, as this species exhibits good survival at high densities (up to 100 m^− 2) without adverse effects on growth, and shows no captivity-related health problems. We recommend mesh shelters, with stocking densities of 50 m^− 2 for post-pueruli and between 20 and 25 m^− 2 for year 1 and year 2 juveniles, to maximise survival and production.     

Effect of tank proportions on survival of seven-band grouper Epinephelus septemfasciatus (Thunberg) and devil stinger Inimicus japonicus (Cuvier) larvae

We examined the effects of rearing‐tank proportions on early survival, surface death and growth of the seven‐band grouper Epinephelus septemfasciatus (Thunberg) and the devil stinger Inimicus japonicus (Cuvier). Fertilized eggs were introduced into three differently shaped 100 L rearing tanks. The three tanks had different water surface areas, and included a shallow tank (S; 71 × 26 cm in diameter and depth, respectively), an intermediate tank (I; 57 × 39 cm) and a deep tank (D; 44 × 70 cm). Both species showed their highest survival rate and the lowest numbers of surface death in the D tank (P<0.05). There were no significant differences between fish reared in the three tank shapes in notochord length, total length, growth rate and dry weight. Rearing‐tank shape affected larval movement in the water column, with the duration of larval movement under the water surface being the shortest in the D tank. These results suggest that using a rearing tank of a suitable shape could significantly reduce the surface death of marine fish larvae.     

小丑鱼室内循环水养殖设施与技术

为了实现规模化人工养殖小丑鱼(Amphiprioninae),研发了小丑鱼室内循环水养殖设施和技术。1组循环水养殖系统由10个玻璃钢养殖桶和1个水处理玻璃缸及管道系统组成,采用物理过滤、生化过滤、藻板过滤进行循环水处理。1组循环水养殖系统每3个月可养殖产出全长约3.5 cm的商品小丑鱼5 000尾,养殖存活率达80%以上。从2014年至2015年,利用该设施养殖生产出商品小丑鱼10余万尾。和常规的食用海水鱼循环水养殖设施相比,小丑鱼室内循环水养殖系统主要减少了蛋白分离器、气浮机、微滤机等设备,增加了藻板过滤设施。研究表明,小丑鱼室内循环水养殖系统建造成本低、运行能耗低、管理维护简单、水质稳定,可基本实现全封闭循环水养殖,适合进行小丑鱼等海水珊瑚礁观赏鱼类的规模化养殖生产。     

Determining and quantifying components of an aquaculture soundscape

Recirculating aquaculture systems have a unique anthropogenic-based soundscape which is characterized by the type of equipment utilized, the structural configuration of walls, tanks, equipment, the substrate the tanks are situated on and even the activities of the personnel operating the facility. The soundscape of recirculation facilities is inadequately understood and remains poorly characterized, although it is generally accepted that the dominant sounds found in such facilities are within the hearing range of fish. The objective of this study was to evaluate the soundscape in a recirculating aquaculture facility from an intra-tank perspective and determine how the soundscape is shaped by a range of characteristics within the facility. Sounds were recorded across an operating aquaculture facility including different tank designs. The sounds recorded fell within previously measured pressure level ranges for recirculating systems, with the highest maximum sound pressure level (SPL) recorded at 124 dB re 1 μPa²/Hz (with an FFT bin width of 46.9 Hz, centered at 187.5 Hz). The soundscape within the tanks was stratified and positively correlated with depth, the highest sound pressure occurring at the base of the tanks. Each recording of the soundscape was dominated by a frequency component of 187.5 Hz (corresponding centre of the 4th 46.9 Hz FFT analysis bin) that produced the highest observed SPL. Analysis of sound recordings revealed that this peak SPL was associated with the acoustic signature of the pump. The soundscape was also evaluated for impacts of tank hood position, time of day, transient sounds and airstone particle size types, all of which were found to appreciably influence sound levels and structure within the tank environment. This study further discusses the distinctiveness of the soundscape, how it is shaped by the various operating components and considers the aquaculture soundscape in relation to natural soundscapes found within aquatic tropical environments.     

Geosmin depuration from European eel (Anguilla anguilla) is not affected by the water renewal rate of depuration tanks

This study established that geosmin depuration from European eel is not affected by the water renewal rate of depuration tanks. A general fish bioaccumulation model extended with terms that account for effects of tank water renewal rate and system losses of chemicals, predicted strong effects of the water renewal rate of depuration tanks on geosmin depuration from European eel. Model predictions were validated in a depuration experiment with geosmin‐loaded European eel (n = 95) with a mean (SD) individual weight of 134.4 (5.0) g and a mean (SD) lipid content of 33.7 (2.8) % (w/w). Fish were depurated for 24, 48 or 72 h at three different tank water renewal rates (0.3, 3.3 and 33 day^?1). Treatments were installed by three different mean (SD) water flow rates (13.8 (1.3), 143.5 (9.2) and 1511 (80) L kg fish^?1 day^?1) over 30‐L tanks. Eels eliminated geosmin from their bodies, but unlike the model predicted, this was independent of the water renewal rate of the depuration tanks. Although being eliminated from the fish, geosmin hardly appeared and certainly did not accumulate in the water of the depuration tanks as the model predicted. This observation may be explained by geosmin being eliminated from eel as metabolite rather than the parent compound. Geosmin elimination from eel seems not to occur according to the generally accepted passive diffusion mechanism for excretion of lipophilic chemicals, and geosmin biotransformation by the eel seems indicated. Clearly geosmin depuration from European eel cannot be enhanced by increasing water renewal rates of depuration tanks.     

Acoustical monitoring of fish density,behavior, and growth rate in a tank

A challenge for the aquaculture community has long been the development of harmless techniques for monitoring fish in a tank. Acoustic telemetry has been used to monitor fish swimming behavior, and passive acoustics have been used to monitor fish feeding, but new techniques are needed to monitor non-invasively their numbers and growth rates. Recently, it has been demonstrated that the acoustical total scattering cross section of fish swimming in a tank can be measured from multiple reverberation time series. These measurements have been used successfully to estimate the number of fish in a tank in laboratory conditions, and to characterize their acoustical signatures. Here, we introduce a novel method for acoustically monitoring fish numerical density and behavior, and measuring their growth rates over long periods of time. These measurements can be performed remotely, without human interaction with the fish, and are harmless. To demonstrate the efficiency of these techniques, the number of sea bass, as well as the behaviors of sardines, rockfish and sea bass, in different tanks were monitored. Also, the growth rates of a group of starved sardines and a group of fed sardines were measured acoustically, over 1 month. For comparison, their average weight was measured once per week.     

Using a macroalgae Ulva pertusa biofilter in a recirculating system for production of juvenile sea cucumber Apostichopus japonicus

A simple indoor recirculating system for production of juvenile sea cucumber (Apostichopus japonicus) was operated on a commercial scale for 90 days during winter. The system consists of three 70 m³ sea cucumber rearing tanks and one biofilter tank where macroalgae (Ulva pertusa) was used as a biofilter in order to reduce water requirements. Effluent from the sea cucumber tanks drained into the macroalgae biofilter tank and were then returned to the sea cucumber tanks by a discontinuous-flow recirculation system. Survival and growth rates in the sea cucumber culture tanks were similar to those in the control tank (with one water exchange per day). The survival rate averaged about 87%. The average body weight increased from 3.5 ± 0.3 g to 8.1 ± 0.8 g and total sea cucumber biomass production over the experimental period was 745 g m⁻² after initial stocking densities of 375 g m⁻². The growth rate of U. pertusa was 3.3% day⁻¹. U. pertusa was efficient in removing toxic ammonia and in maintaining the water quality within acceptable levels for sea cucumber culture; there were only small daily variations of temperature, pH and DO. The U. pertusa tank removed 68% of the TAN (total ammonia-nitrogen) and 26% of the orthophosphate from the sea cucumber culture effluent; the macroalgae biofilter removed ammonia at an average rate of 0.459 g N m⁻² day⁻¹. It would be efficient to use the U. pertusa biofilter in a recirculating system for production of A. japonicus juveniles in winter.     

Seagrass habitat reduces vulnerability of red sea bream <Emphasis Type="Italic">Pagrus major</Emphasis> juveniles to piscivorous fish predator

ABSTRACT:   Predation experiments were conducted in mesocosms to test the hypothesis that habitat complexity affects vulnerability of red sea bream Pagrus major juveniles to piscivorous fish predators. Juvenile behavior was video-recorded for 6 h in two structurally different habitats: vegetated with sea grass Zostera marina , and unvegetated 0.5-t tanks. Association behavior with seagrass was observed in the vegetated tank throughout the trials. Predation experiments were conducted with 30 red sea bream juveniles (29.9 mm body length) exposed to two individuals of the piscivorous fish predator Chinese sea bass Lateolabrax sp. (261.6 mm), for 6 h in 1.0-t tanks. Predation rate (no. of fish predated on per predator per h) was significantly lower in the vegetated tank (0.02/predator per h) than in the unvegetated tank (0.27/predator per h). Present experiments indicate that habitat complexity reduces vulnerability of juvenile red sea bream to predation by piscivorous fish by serving as physical and/or visual barriers and limiting the predator's ability to pursue and capture prey. Seagrass beds in the shallow coastal waters around Japan are suggested to be an important nursery for red sea bream since they provide the juveniles with habitat complexity as well as serve as a feeding ground.     

Estimation of flow in a rearing tank of marine fish larvae by simplified numerical computation—a case of two-dimensional flow

Marine fish larvae are fragile against physical stress. However, few studies have been conducted to evaluate the flow field in a rearing tank, which is assumed to provide a high degree of physical stress to marine fish larvae. The flow field in a rearing tank (volume of 1 m³) is generated by aerators, which are commonly used to provide oxygen.

This paper is a report on the estimation of stationary flow in the rearing tank of marine fish larvae. The larvae are seven band grouper larvae of Epinephelus septemfasciatus, which have a very low survival rate immediately after the hatching of eggs. The experiments of rearing of seven band grouper larvae were carried out using rearing tanks with four aeration rates (1000, 200, 50 ml/min, and no aeration). The effects of aeration on the survival and floating death of seven band grouper larvae were examined. The experiments confirmed that the mass mortality of seven band grouper larvae depends on the flow rate in the rearing tank. Aeration at 200 ml/min resulted in the highest survival and growth rates of grouper larvae.

Larvae-rearing experiments provided evidence that the flow rates of the rearing tanks are very important design aspects of rearing tanks. The estimation of flow in a rearing tank for an aerating rate of 200 ml/min was carried out by numerical calculation. The computation was simplified by a two-dimensional flow based on experimental results. The calculated flow in the rearing tank was compared with the experimental one. The calculation of the stationary flow in the rearing tank showed good qualitative and quantitative agreement with the experimental results. The numerical estimation of the flow in a rearing tank of marine fish larvae was confirmed to be effective and satisfactory for the design of a tank that would provide optimum performance.     

以CFD-DEM为基础的养殖槽排污性能及底坡优化

由于能提高资源利用率,减少环境污染,低碳高效池塘循环流水养殖(IPA)作为一项新型养殖技术被大力推广。为了提高养殖过程中的集污排污效率,本研究拟采用构造负坡底面的方法对养殖槽结构进行优化。通过建立二维养殖槽简化模型,结合计算流体力学—离散单元法(CFD-DEM)模拟计算与核偏最小二乘(KPLS)建模方法,建立槽内垂向流速分布与底面坡度和粗糙度的关系模型。在此基础上结合泥沙运动理论,获得了槽内颗粒起动流速与单宽输沙率模型,在构建颗粒起动和输运两方面的性能指标后,利用基于偏好的多目标粒子群算法(DP-MOPSO)寻求最优底面坡度。寻优结果显示,随着底面粗糙度的增加,最优坡度略有减小,范围为0.013~0.015;仿真实验结果显示,构造最优底坡可有效提高颗粒的起动概率和槽体的颗粒运输能力,且对于表面较为粗糙槽体,坡型底面在颗粒起动方面的优越性更为显著,说明通过构建底坡来改变水流结构,从而实现养殖槽排污性能的优化是合理的。     

Analysis of mesh breaking loads in cotton gill nets: Possible solution to ghost fishing

ABSTRACT:   A small number of fishers in Chiba Prefecture of eastern Japan use cotton gill nets to catch Japanese spiny lobster Panulirus japonicus. To examine the advantages of cotton gill nets, we analyzed changes in mesh breaking load of a new cotton gill net used in a fishing operation. A new cotton gill net was also soaked in a seawater tank to simulate ghost fishing conditions. The average mesh breaking load of new cotton mesh was 50.3 N. This value decreased to 19.0 N after 38 days (∼912 h), and after 82 days (∼1968 h) the mesh could be easily torn (breaking load 0.07 N). Under fishing conditions, the cumulative soak time was only 744.4 h over 19 months. The average breaking load at the end of this period was 43.1 N, a strength 86% that of the presoaked mesh. The mesh breaking load of a cotton gill net continuously soaked for 744.4 h was 26.1 N, as estimated from tank experiment data. Thus, a cotton gill net maintains reasonable strength under typical use conditions, but will degrade if lost at sea.     

Gillnet selectivity of small coastal sharks off the southeastern United States

Gillnet selectivity parameters for the Atlantic sharpnose, Rhizoprionodon terraenovae, blacknose, Carcharhinus acronotus, finetooth, Carcharhinus isodon, and bonnethead, Sphyrna tiburo, sharks were estimated from fishery-independent catches in multi-panel gillnets with stretched mesh sizes ranging from 8.9 to 14.0 cm in steps of 1.3 cm, with an additional size of 20.3 cm. Mesh selectivities were estimated using a maximum-likelihood model, which fits a gamma distribution to length data for each mesh size using the log-likelihood function. The Atlantic sharpnose and finetooth shark exhibited the broadest selection curves. Peak selectivities for the Atlantic sharpnose were reached from 750 mm FL for the 8.9 cm mesh to 1150 mm FL for the 14.0 cm mesh in 50 mm FL increments per mesh. Peak selectivity for the finetooth shark was reached at 550 mm FL for the 8.9 and 10.2 cm meshes, increased to 650 mm FL for the 11.4 mesh, and 750 mm FL for the 12.7 and 14.0 cm meshes. Selectivity was highest at 1150 mm FL for the 20.3 cm mesh. The bonnethead and blacknose shark exhibited narrower selection curves, with peak selectivity occurring at 450 mm FL for the 8.9 cm mesh, 750 mm for the 12.7 cm mesh in 100 mm FL increments per mesh. Maximum selectivity for the 20.3 cm mesh was 950 and 1050 mm FL for bonnethead and blacknose shark, respectively. The θ₁ values for blacknose and finetooth shark were most similar (140.58 and 141.25), whereas the value calculated for Atlantic sharpnose was the highest (211.95) and that for the bonnethead (131.77) was the lowest. Values calculated for θ₂, a parameter that describes the variance of sizes by mesh, ranged from 27,259 for the bonnethead to 189,873 for the finetooth shark. Although gillnets used in this study were not directly constructed for use in estimation of gillnet selectivities, information on mesh selectivities estimated herein has direct applicability to commercial gillnets with meshes of similar sizes.