Transaction costs in contract farming models for mussel and oyster farming in South Africa: Organisational and management implications

Abstract

This study examines transaction costs faced by small mussel and oyster growers to elicit the appropriate organisational design of contract institutions. The theoretical premise is that high transaction costs essentially constrain the participation of small growers in mussel and oyster mariculture. Transaction cost analysis is applied to contract farming models and this approach is first discussed conceptually. Experience with contract farming by small growers in a developing context is synthesised and used in guiding the design of contract farming models. This is followed by a brief overview of the mussel and oyster industries, which is used to gain insight into transaction costs associated with contract institutions. Contracting models and implications are assessed separately for each industry. For mussel mariculture, four farming models are examined and the appropriate model is subsequently selected. For oyster mariculture, the transaction costs of contract fanning are examined to assist in organisational design.     

三倍体长牡蛎浮筏养殖技术的研究

本文报道了两个相邻海区近５００ｈｍ＾２三倍体长牡蛎Ｃｒａｓｓｏｔｒｅａ ｇｉｇａｓ（Ｔｈｕｎｂｅｒｇ）浮筏养殖的研究结果。三倍体牡蛎的采苗器为栉孔扇贝壳,每片采苗器采苗１０￣１２个,以１５￣２０ｃｍ间距夹于３股聚乙（丙）烯蝇间并吊养在浮筏上。三倍体牡蛎在个体重、出肉率和条件指数等养殖经济性状方面比二部体分别增加４２．１％、１６．２％、５６．７％,成活率也有一定的提高。三部体在生长等方面性状优势的生     

Comparison of intensive shrimp farming systems in Indonesia, Philippines, Taiwan and Thailand

The estimated production of cultured shrimps for 1995 in Taiwan, the Philippines, Indonesia and Thailand was 20 000, 40 000, 80 000 and 220 000 tonnes, respectively. Intensive shrimp ponds in the Philippines (71%) and Indonesia (63%), which are developed in the tidal and mangrove areas, cannot be properly treated by complete drying, owing to seepage from supply and drainage, nor by removal of the fouled layer by heavy machines such as bulldozers. Intensive farms in Thailand and Taiwan are owned by small-scale operators operating 2-3 ponds simultaneously, each ranging from 0.16 to 1.0 ha, which is the optimal size for efficient farm management and lower overhead and investment costs compared with larger farms such as those found in Indonesia and the Philippines. In Taiwan, 90% of pond water supply is mixed open sea water with underground fresh water. Pond salinity, which is kept constant at 10-15%o, causes Taiwanese farmers to encounter an array of problems which include high cost of underground water pumping, land subsidence, salinization, more pathogens and rapidly fouled bottom. Water loss by seepage in Thailand is minimal (average 23 cm in the final month), compared with Indonesia and the Philippines, because pond dikes are tightly compacted by heavy machines and high clay content (86%). Circular water movement in ponds in Thailand, facilitated by heavy aeration (13.3 hp ha^?1), aids in the settling of waste in pond centres for easy removal. Indonesia and the Philippines still maintain high water exchange systems (335 cm and 470 cm in the final month, respectively) which introduce viruses, other pathogens, excess organic loads, ammonia and other toxic particles released by nearby farms through the incoming water. Despite serious crop failures in other countries within the past few years, the annual shrimp production in Thailand still remains high because farmers have readily adopted new, environmentally friendly and locally suitable, water exchange systems such as less water exchange, and closed, full-strength seawater and freshwater systems, overcoming heavy viral and disease infections. Approximately 30% of shrimp production in Thailand comes from the freshwater areas, sometimes 200 km from the sea. Half of the Philippine farmers rely on imported feeds; this has caused high shrimp mortality owing to toxins produced from expired feeds kept in humid conditions.     

Assessment of spat collector ropes in Galician mussel farming

The large amount of mussel seed required to support the present mussel farming production levels in Galicia (NW Spain) forces the development of new designs in artificial spat collectors for continual improvement of mussel seed gathering. In the present study, we have assessed both settlement and recruitment of Mytilus galloprovincialis on four different collector ropes in the Ría de Ares-Betanzos (Galicia). Besides the traditional collector ropes (lacing without loops and non-filamentous structure; NF-NL), three new rope designs with different lacing and structures were evaluated; ropes with a filamentous loop complement (F-L), ropes with a non-filamentous loop complement (NF-L) and filamentous ropes without loops (F-NL). Ropes with loops showed higher settlement densities (53,925 ± 4625 and 42,433 ± 5525 indiv./m for F-L and NF-L, respectively) than ropes without loops (26,475 ± 3875 and 13,033 ± 1136 indiv./m for F-NL and NF-NL, respectively). This may be explained by the increase in available surface area provided by the loops. Several studies recognized the importance of filamentous substrata for mussel spat settlement, which may help to explain greater settlement densities on filamentous structures between ropes with the same lacing. In recruitment evaluation, ropes with filamentous loops showed the highest densities expressed in indiv./m (5493 ± 587) as was the case of settlement. However, when density was expressed in kg/m, the ropes with non-filamentous loops had a higher yield (8.48 ± 0.22 kg/m), that could be a result of differences in adjusted shell length between ropes. Intra-specific competition and predation were identified as important factors affecting post-settlement mortality. The latter factors could also influence population length distribution. Ropes with rigid loops (NF-L) may supply refuges for spat from predators and therefore, enhance the recruitment of larger individuals, although other factors like size selective settlement could play a significant role in this result.     

Assessment of spat collector ropes in Galician mussel farming

The large amount of mussel seed required to support the present mussel farming production levels in Galicia (NW Spain) forces the development of new designs in artificial spat collectors for continual improvement of mussel seed gathering. In the present study, we have assessed both settlement and recruitment of Mytilus galloprovincialis on four different collector ropes in the Ría de Ares-Betanzos (Galicia). Besides the traditional collector ropes (lacing without loops and non-filamentous structure; NF-NL), three new rope designs with different lacing and structures were evaluated; ropes with a filamentous loop complement (F-L), ropes with a non-filamentous loop complement (NF-L) and filamentous ropes without loops (F-NL). Ropes with loops showed higher settlement densities (53,925 ± 4625 and 42,433 ± 5525 indiv./m for F-L and NF-L, respectively) than ropes without loops (26,475 ± 3875 and 13,033 ± 1136 indiv./m for F-NL and NF-NL, respectively). This may be explained by the increase in available surface area provided by the loops. Several studies recognized the importance of filamentous substrata for mussel spat settlement, which may help to explain greater settlement densities on filamentous structures between ropes with the same lacing. In recruitment evaluation, ropes with filamentous loops showed the highest densities expressed in indiv./m (5493 ± 587) as was the case of settlement. However, when density was expressed in kg/m, the ropes with non-filamentous loops had a higher yield (8.48 ± 0.22 kg/m), that could be a result of differences in adjusted shell length between ropes. Intra-specific competition and predation were identified as important factors affecting post-settlement mortality. The latter factors could also influence population length distribution. Ropes with rigid loops (NF-L) may supply refuges for spat from predators and therefore, enhance the recruitment of larger individuals, although other factors like size selective settlement could play a significant role in this result.     

关于牡蛎的养殖技术之一太平洋牡蛎塘养快长高产试验

由于太平洋牡螨养殖区域及养殖方式的不同,其产量和达到可收获的商品规格所需时间调然有异,出肉率、稳定性及抗逆性大不相同,经济效益相差很大。为了探索太平洋牡蛎快长高产的养殖方法,充分挖掘深水上塘的养殖容量潜力,受浙江省海洋水产养殖研究所和温岭市水产局的委托,温岭市江厦乡于1998年10月～1999年2月在乐清湾沙山海区东门虾蛙蛐混养的深水土塘挂养太平洋牡领秋苗,将对虾、缢蛙、泥蛐和太平洋牡烟四大海产经济品种混养,获得成功,经济效益非常明显。单就太平洋牡烟而言,这种养殖方法有五大优点：生长快、出肉率高、投入产出…     

Experimental insight of trace metal environmental pollution problems in mussel farming

A survey was made on the content of nine trace metals in waters of 12 predetermined coastal stations in the Island of Penang, Malaysia, with the object of establishing mussel culture farms in the near future. The results indicated low concentrations of these trace elements except for Cr and Ni, which ranged from trace level to 35 ppm and BDL (below detectable level) to 2.07 ppm, respectively. These values are slightly above the upper limits (Cr, 0.05 and Ni, 1.0 ppm) set by the national Division of Environment, Ministry of Science, Technology and Environment, under the Environmental Quality (Sewage and Industrial Effluents) Regulations 1978.Investigations on tissue level bioaccumulated trace metal contents in mussel from an experimental culture farm indicated BDL values for As in all tissues, Cd highest in the intestines and stomach (6.24 ppm), Co in the mantle (59.85 ppm), Cr in the siphon (1869.85 ppm), Cu in the mantle (23.94 ppm), Fe in the intestines and stomach (218.82 ppm), Mn in the style (365.28 ppm), Ni in the mantle (23.94 ppm), Pb in the mantle (279.3 ppm) and Zn in the style (152.2 ppm). However, culture experiments at 10 ppm concentration stresses of the different trace elements over a 48-h period did not result in a similar tendency in tissue bioconcentration, except for Cd (378.35 ppm) and Fe (419.66 ppm) in the intestine and stomach, and Mn (748.5 ppm) in the style. Co was highest in the style (187.35 ppm), Cr in the gill (2924.91 ppm), Cu in the gill (457.22 ppm), Ni in the intestines and stomach (222.44 ppm), Pb in the gill (1897.2 ppm), and Zn in the mantle (1077.73 ppm).Short-term stress studies over a 48-h period with trace metal concentrations ranging between 10 and 300 ppm indicated the patterns of biodeposition for Cd, Cr, Co, Pb, Mn and Zn to be closely related even though their toxicity levels varied. Cu indicated a bioaccumulation maximum after 12 h of incubation followed by a regulatory mechanism, while Fe and Ni demonstrated a drastic spiked absorption after 6 h exposure followed by the normal trend of biodeposition as observable for the other trace metals. The biodeposition factors at most instances, were highest for the 10 ppm concentration cultures of 48 h.     

Integrated rearing system approach in the farming of mud crab,shrimp, fish,oyster and periphyton in bracksihwater pond

Diversification of production systems has been identified as one of the major strategies for the long‐term sustainability of brackishwater aquaculture. On the context of diversification, a 180‐day culture experiment was carried out where four farming systems were tested: monoculture of mud crab, Scylla serrata (T1), polyculture of mud crab, with mullet Mugil cephalus and shrimp Penaeus indicus (T2), polyculture of mud crab with mullet, shrimp and oyster Crassostrea cuttackensis (T3), polyculture of mud crab with mullet, shrimp, oyster along with periphyton net (T4) covering 10% of the water spread area. There was no significant difference in the growth performances of mud crab, shrimp and oyster among the treatment but a significantly higher (p < .05) final average body weight (ABW) of M.cephalus was recorded in T4 (121.19 ± 2.09 g). Production considering only the fed animals (excluding oyster) was significantly (p < .05) higher in T4 (2,115 kg/ha) compared to other treatments. The overall water quality parameters do not vary significantly (p > .05) among the treatment ponds. Although there was no significant difference in total ammonia nitrogen (TAN) and nitrite‐N, highest value was recorded in T2 (56.23 ± 39.47 µg/L and 185.53 ± 110.71 µg/L) and T1 (55.78 ± 32.26 µg/L and 180.83 ± 103.65 µg/L). The results from the current experiment based on water quality and growth of animals showed that there is no negative interaction between the mud crab and other animals stocked in the polyculture pond which indicated that polyculture provides an efficient and sustainable utilization of resources.     

枸杞岛筏式贻贝养殖海域鱼类群聚特征

为探究筏式贻贝养殖生境鱼类资源养护功能,于2020年9－12月对枸杞岛筏式贻贝养殖生境(分表层区和底层区)及周边岩礁生境的鱼类进行了多网目组合刺网采样。应用相对重要性指数IRI、多样性指数并结合等级聚类、非度量多维标度排序(nMDS)分析鱼类组成和群落结构。结果显示,在2种生境中共采集鱼类55 种,隶属于10目34科47属,其中贻贝养殖生境采集到37种(底层和表层分别为29和20种);岩礁生境共采集到40种。在鱼类组成上,中国花鲈和鲻是贻贝养殖生境表层区的典型优势种,而黄姑鱼和褐菖鲉是贻贝养殖生境底层区的典型优势种;褐菖鲉同时也是岩礁生境的典型优势种。多样性分析表明,贻贝养殖生境的总体鱼类多样性要高于岩礁生境,但尚不显著。多元分析显示,各月份2种生境中的鱼类群落格局均可分为3种类型,虽与养殖表层区、养殖底层区和岩礁区并非严格一致,但各群落间差异依然显著。研究表明,大规模筏式养殖设施的存在吸引了众多中上层鱼类及底层鱼类,也为岩礁生境优势鱼类提供了额外的栖息环境,发挥了近似于浮鱼礁系统的资源养护作用。研究结果可为岛礁海域海洋牧场目标种的选择和人工生境构建模式的应用提供重要参考。     

Effects of oyster farming service vehicles on an intertidal sand flat

A Tasmanian tidalflat receiving differing amounts of vehicle traffic associated with the servicing of oyster farm leases was sampled for sediment properties and benthic community structure. There was a gradient of vehicle usage in both the littoral zone (LZ) and the intertidal zone (IZ), with the highest usage in the intertidal zone equating to the lowest usage in the littoral zone (IZ2=LZ1). Results indicate that the littoral sediments were less compacted where vehicles were present. Species richness and diversity were reduced in the littoral sites (70%) and one intertidal site (50%) where there was vehicle traffic. Multivariate analyses confirmed the univariate findings. Large bivalves and epibenthic snails were generally less abundant where there was vehicle traffic. However, in the intertidal zone one snail showed the opposite pattern. There was less change in the community structure at sites where vehicles spread out across the foreshore than at sites where all vehicles travelled in a narrow lane, but this reduction in impact was offset by a greater area being affected. This study provides preliminary evidence of a measurable impact on sediment and community composition at 16 vehicle trips per day and suggests that management needs to consider the tradeoff between magnitude of impact and area affected.     

Test of size-specific mass selection for Nile tilapia, Oreochromis niloticus L., cage farming in the Philippines

One generation of mass selection based on the collimation procedure (early culling of large fry) was applied on Nile tilapia, Oreochromis niloticus L., in net cages set in Laguna de Bay, Philippines. The objective was to test the effectiveness of a low-cost, small-scale broodstock improvement procedure in this culture environment. Directional selection was performed in two steps after initial removal of large fry at 21 days. Selection of parents and testing of the offspring were also conducted in hapa net cages set up in Laguna de Bay. The selection resulted in a significant positive response of 3% relative to the control, which represents a projected 34% gain over 5 years in Laguna cage culture. The realized heritability is ≈16%.     

Quantifying sponge erosions in Western Australian pearl oyster shells

This paper describes several methods to estimate the amount of sponge erosion in pearl oyster shells (Pinctada maxima Jameson, 1901). Internal erosion can be calculated from non‐destructive surface area estimates because a clear relationship was found between the eroded area on the surface of the shells and internal erosions. Grading shells into three different categories, according to the severity of erosion, also proved to be a suitable method to quickly assess the amount of sponge erosions. All methods were more accurate in shells with larger amounts of erosions. Using the described methods, the Lacepedes fishing ground was more diverse in bioeroding sponge species compared with sites at the 80‐Mile fishing ground locations. Pione velans Hentschel, 1909 was the dominant bioeroding sponge species but Cliona dissimilis Ridley and Dendy, 1886 eroded more shell material than P. velans. Observed patterns suggest that P. velans had a more recent recruitment than C. dissimilis. We suggest that visual grading together with estimates from the shell surface will be sufficient to determine the general long‐term trends in bioeroding sponge severity. However, results of monitoring programmes need to be supplemented by histological examination to confirm sponge activity and species identity.     

不同混养鱼类和投喂方式对鱼蚌综合养殖水体化学特征的影响

选用草鱼(Ctenoparyngodon idellus)、银鲫(Carassius gibelio)、鲢(Hypopthalmichthys molitrix)、鳙(Aristichthysnobilis)作为本研究的混养鱼类,通过围隔实验检验了不同混养鱼类组合(草鱼+鲫+鲢+鳙或鲢+鳙)和配合饲料投喂方式(投喂或不投喂)对三角帆蚌(Hyriopsis cumingii)养殖水体化学特征的影响,实验为期155 d。结果表明,实验设计的混养鱼类组合可显著影响鱼蚌综合养殖水体的Ca2+、总碱度(Alk)、总硬度(HT)和总磷(TP)水平;投喂配合饲料可显著影响Ca2+、Alk、HT、氨氮(TAN)、TP和化学耗氧量(CODMn)。采用混养组合(草鱼+鲫+鲢+鳙)且投喂配合饲料的围隔内溶氧(DO)水平和透明度(SD)较低,Ca2+、HT、TN、TP、TAN和CODMn水平较高;采用混养组合(鲢+鳙)且不投喂的围隔内DO和SD水平较高,而Ca2+、HT、TN、TP、TAN和CODMn水平较低。随养殖时间延长,各组围隔内Ca2+水平下降,TN、TP、TAN和CODMn水平升高。实验期间,DO与SD水平呈正相关;DO和SD与TAN、TN、TP、Ca2+和CODMn水平呈负相关。因此,鱼蚌综合养殖中,控制CODMn和TAN浓度,维持Ca2+稳定及较高的DO水平是水质管理的基本目标。     

Recruitment of the threatened mussel Margaritifera margaritifera in relation to mussel population size,mussel density and host density

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Artificial upwelling to stimulate growth of non‐toxic algae in a habitat for mussel farming

Large‐scale artificial upwelling was tested as a method to enhance the environmental conditions for the growth of non‐toxic algae in a Norwegian fjord (61°0′N, 6°22′E). The experiment was designed to evaluate if nutrient‐rich seawater, brought up from below the mixed zone of a stratified fjord to the euphotic zone by air bubbling, would stimulate the growth of non‐toxic relative to toxic algae. Pumping 44 m³ min^?1 of air at 1 atm through a pipe diffuser submerged at 40 m depth formed a buoyancy flux that lifted 60 m³ s^?1 of deep water to the upper 17 m over a period of 21 days. The supply of silicate, inorganic nitrogen and phosphate to the upper 10 m in the fjord increased, and a significant increase in the biomass of non‐toxic algae was observed. The upwelling gave an increased growth of the non‐toxic dinoflagell ates Ceratium furca and C. tripos. After termination of the experiment, the phytoplankton biomass decreased significantly, whereas a distinct increase occurred in the relative biomass of the potentially toxic Dinophysis spp. The result is considered promising when it comes to creating controlled geographical areas with non‐toxic food for mussel production.     

Economic analysis of stake and rack-hanging methods of farming oysters (Crassostrea iredalei) in the Philippines

The cost efficiency of stake and rack-hanging methods of farming oysters are compared. Annual costs and returns data for both methods were obtained from a socioeconomic survey conducted in 1991 at the Aquaculture Department of the Southeast Asian Fisheries Development Center (SEAFDEC/AQD). On average, 14 kg m⁻² (shell-on) and 22 kg m⁻² (shell-on) were produced using the stake and rack-hanging methods, respectively. The translog cost function for 58 oyster farms was estimated using iterative seemingly unrelated regression (SUR). Statistical analysis indicates that the higher production from the rack-hanging method is significantly different (=0.10) from the production using the stake method. The cost-efficient method varied based on production levels. At output levels less that 9044 kg (shell-on), the rack-hanging method had lower average cost per unit, and for higher production levels the stake method had the lower average costs. Similar conclusions were drawn from the marginal cost analysis. Results imply when production is less than 9044 kg, then the most efficient technology is the rack-hanging method and if production is greater than 9044 kg, the most efficient method is the stake method.     

An experimental assessment of the potential impacts of longline mussel farming on the infauna in an open coastal embayment

• 1. The existence of perceived ecological impacts and development of adaptive management solutions to mitigate these problems are important issues in sustainable aquaculture. This paper examines the general impacts of two newly established trial longline installations on the infauna in Twofold Bay, a large, coastal embayment in south‐east Australia.
• 2. We hypothesized that the physical presence of these longline installations and the biological activities of the mussels they supported would result in temporal changes in densities of infaunal taxa below installations differing from those at undisturbed control sites. We also predicted different patterns of variability in infauna between longline and control sites from before to after the longlines were stocked. These hypotheses were tested by using a beyond‐BACI sampling design and asymmetrical analyses of variance to compare changes in densities of taxa at several different spatial scales below the proposed longline sites with those at two adjacent control sites, before and after the longlines were stocked.
• 3. After 18 months of longline operations, there was no evidence of any impact on total number of taxa, nor densities of individual taxa. Short‐term temporal trends in densities in plots at control sites from April to May 2001 were often as different from each other as from those at the longline site. This indicates that densities of taxa at the farm site were within the range typically found at undisturbed sites, so there were no ecological impacts from the farm.
• 4. These results do not concur with previous studies on impact of mussel farming in semi‐enclosed coastal waters. Differences in location, scale of production, duration of operation and assimilative capacity of the environment probably contributed to this discrepancy. The infauna in Twofold Bay either do not respond to this form of disturbance or have not yet been exposed to disturbance of a sufficient magnitude, or for a sufficient period of time, to elicit a detectable response. Better definition of the potential ecological impacts associated with aquaculture, plus their scale and magnitude in different environments is needed to design experiments and monitoring programmes to detect specific impacts. This activity can only be considered sustainable once we know that these impacts are localized, reversible and short‐term.

Copyright © 2006 John Wiley & Sons, Ltd.     

褐菖鲉幼鱼对贻贝养殖生境的利用规律初探

嵊泗海域规模庞大的贻贝养殖生境发挥着养护幼鱼的海洋牧场功能,而探查当地趋礁鱼类在幼体阶段对该生境的利用规律可为当地海洋牧场的设计提供重要参考,为此2018年5—7月对舟山北部枸杞岛贻贝场的褐菖鲉幼鱼、附着生物进行了逐月调查,使用自制网具采集目标生物样品,应用相对密度指数和相关分析法分析了褐菖鲉幼鱼在该生境中的栖息密度、栖息方式和空间分布特征,采用胃含物分析法确定其食性特征,并结合耳石轮纹探究其利用该生境的生长周期。结果显示,褐菖鲉幼鱼的栖息密度时空特征上,5—7月在养殖区贻贝串上的栖息密度分别为(10±6)、(7±5)和(5±5)个/串,时间上呈现逐月递减趋势,随时间的推移养殖区外侧幼体逐渐迁移至内侧近岸区直至消失。栖息方式选择上,褐菖鲉幼鱼栖息数量与贻贝串孔隙大小和附着海藻生物量之间皆呈正相关,但并不显著。饵料利用上,养殖区褐菖鲉幼鱼总体维持较高的摄食强度,各月份平均摄食强度皆高于对照区,主要摄食对象为麦秆虫和钩虾,重量百分比为70.9%和28.3%。栖息周期上,养殖区内褐菖鲉幼鱼耳石平均轮纹数为(57±12)个,其利用养殖贻贝串的周期一般约为2～3月,且该生境中幼鱼耳石纹路间距均匀性和条纹清晰度均明显优于岩礁生境。研究表明,枸杞岛的贻贝养殖生境成为褐菖鲉幼鱼额外的栖息环境,在增殖养护褐菖鲉资源方面发挥了积极作用,因此嵊泗海域在建设海洋牧场时可依据贻贝养殖生境的资源养护原理,通过设置浮体结构强化海洋牧场的饵料场和庇护功能,以提高褐菖鲉种群的补充量,为当地休闲海钓的持续发展提供部分保障。     

夏季北黄海贝类养殖区环境因子、长牡蛎糖原和免疫指标变化的调查研究

为解析夏季环境胁迫和长牡蛎（Crassostrea gigas）机体响应特征,实验针对2021年北黄海夏季高温降雨期的长牡蛎养殖海区开展了2次调查研究和对比分析。结果显示,7月份海区表层水温分别为25.3 ℃和24.9 ℃,盐度分别为26.41和27.87;表层海水中硅藻丰度分别为3.6×103和3×103 个/L,低于往年平均水平,甲藻多样性增加,表层海水中细菌的总丰度分别为2.10×109和2.77×1010 拷贝/L,弧菌丰度分别为3.37×108和5.40×108 拷贝/L;长牡蛎肝胰腺糖原含量分别为40.96和31.58 mg/g、闭壳肌糖原含量分别为6.63和8.91 mg/g;鳃MDA含量分别为13.74和8.69 nmol/mg prot、鳃T-AOC分别为0.46和0.77 U/mg prot;海区养殖长牡蛎血淋巴细胞中CgIL17-5和CgTNF-1的mRNA表达量在第一次调查时相对室内暂养长牡蛎均剧烈升高;血淋巴细胞中CgCaspase3的mRNA表达量升高但差异不显著。综上,2021年北黄海贝类养殖区在经历6—7月夏季高温降雨期后出现水温升高和盐度降低现象,硅藻丰度较往年平均水平下降而甲藻多样性增加,细菌总丰度较入夏前水平降低而弧菌丰度显著升高。与室内暂养对照组长牡蛎相比,海区养殖长牡蛎的糖原含量下降,并发生轻度氧化应激。研究结果有助于我们进一步理解夏季养殖环境的变化规律和长牡蛎机体响应特征,为预防夏季大规模死亡发生提供理论依据和参考。