Current status and future perspectives of Italian finfish aquaculture

Currently available data show that shellfish and finfish production in Italy, derived both from fisheries and aquaculture activities, is on the order of 474,000 tons, each activity representing 50 % of the total amount. In this context, the finfish aquaculture industry contributes on average 31 % to the national aquaculture production and on average 59 % of its value, giving a total amount of 72,000 tons and a value of around 351 million € (2010). According to FEAP statistics, Italy is the fourth largest finfish producer in EU27, after the UK, Greece, and Spain, while it is also one of the six largest finfish producers among the non-EU and EU member countries, together with Norway, UK, Greece, Turkey, and Spain. Presently, fish culture activities are mainly focused on rainbow trout (Oncorhynchus mykiss, 55.5 %), followed by European sea bass (Dicentrarchus labrax, 13.6 %), gilthead sea bream (Sparus aurata, 12.2 %), gray mullet (Mugil cephalus, 5.3 %), sturgeon (Acipenser spp., 2 %), and European eel (Anguilla anguilla, 1.7 %). Over the last 20 years, freshwater fish production and aquaculture (trout, carp, and eel) have decreased in Italy, with the exception of sturgeon. In contrast, marine fish production has significantly increased during the same period, and the two leading species, European sea bass and gilthead sea bream, presently contribute 25.8 % of the finfish production. From 1,900 tons in 1990, production reached 19,000 tons in 2010, with a 900 % increase, at an average percentage of 4.5 %. In addition, new marine fish species were successfully cultured over the same period. This review outlines the past and present situation of finfish culture in Italy and discusses future developments and priorities, with particular emphasis on new, emerging aquaculture species.     

Zooplankton research in the North Sea

Summary Water types of the North Sea with different plankton are the thermally stratified northern and central regions with a relatively nutrient rich inflow of Atlantic water, a mixed region in the southern North Sea with a poor inflow of Channel water, and turbid narrow coastal zones with inflow of nutrient rich river water. Plankton studies reveal that the primary production starts early, in February, in the southern region, but is delayed in the coastal zones by turbidity. In stratified areas the algal spring bloom is delayed by mixing towards the greater depth and usually starts with the onset of thermal stratification. The spring bloom soon declines and the algae remain on a low density level in summer, presumably due to depletion of nutrients in the euphotic zone. The coastal zones and the frontal zones between mixed and stratified water have a relatively high summer primary production.The herbivores (mainly planktonic copepodes and the tunicateOikopleura dioica) grow and increase in number when the temperature rises and food is available. There is a considerable mismatch with the algal spring bloom, which comes too early and is too short in most regions. The best coincidence occurs in the coastal zones and maybe the frontal zones. Carnivores build high biomasses in late summer and fall in the coastal regions and compete with fish larvae for food and also kill many fish larvae. The large scyphomedusae are most important in this respect.The overall yearly primary production of the North Sea is estimated to be about 100 mg C.m^–2. The estimates for herbivores and fish are 20 and 1 mg C.m^–2. Considering a growth efficiency of 20%, the herbivores must consume all algae produced. The indication of a low consumption due to bad phenological coincidence in most regions leads to the assumption, that either primary production is underestimated or there is a considerable influx of organic matter from the Atlantic Ocean. During June–July 1979 the carnivore consumption was estimated in the coastal zone of the Southern Bight to be 39 mg C.m^–2.d^–1 at a copepod production of 20 mg C.m^–2.d^–1. Consumption by fish larvae and large jellyfish (Cyanea lamarckii) was 15% and 74%, respectively.It seems clear that the productivity of the North Sea depends highly on coastal and frontal zones, where herbivores find sufficient food at optimal growth conditions. Most organic matter will at the end be consumed by invertebrate carnivores, which urge fish populations to reproduce early in spring or to recruit at remote places.     

真江蓠(Gracilaria verrucosa)对网箱养殖海区的生态修复及生态养殖匹配模式

2006年8～9月,在浙江象山港花鲈(Lateolabrax japonicus)养殖网箱中吊养真江蓠(Gracilaria verrucosa)对网箱养殖造成的水体富营养化进行生态修复研究.通过45d内的平面监测、定点跟踪监测和断面监测,结果表明:该网箱养殖区水体呈严重富营养化状态,营养状态指数(E)为32.00,其营养盐分布由高浓度的中心区向周围150m非养殖水域扩散;真江蓠对养殖区的富营养化海水具有较好的修复效果:江蓠生态修复区及其相邻网箱中水体PO4-P、NO2-N、NH4-N和NO3-N含量显著低于非修复区(P＜0.01),修复区海水PO4-P、NO2-N、NH4-N和NO3-N浓度比非修复区分别降低22%～58%、24%～48%、22%～61%和24%～47%.养殖真江蓠45d后,修复区水体DO浓度和透明度显著高于非修复区(P＜0.05),DO平均提高28%,透明度平均提高30%;而修复区水体Chl-a浓度显著低于非修复区(P＜0.05),平均降低49%.通过建立基于N平衡的鱼藻生态养殖模式,每收获1kg花鲈至少需要匹配江蓠4.7 kg wet wt才可实现对鱼类排放N的完全吸收.因此网箱内栽培江蓠的混合生态养殖模式,可平衡因经济动物养殖所带来的额外营养负荷,有利于实现动物养殖环境的自我修复.     

Environmental determinants of the altitudinal variations in relative group densities of Japanese macaques on Yakushima

Altitudinal variations in relative group densities of the Japanese macaques on Yakushima were studied. This is an ideal place for studying resource limitations because it avoids various complicating factors that are difficult to quantify but might affect animal densities, such as predation, interspecific competition, and past catastrophes. The relative group density was high in the coastal forest (0–400m), while it did not differ among the higher zones (400–800, 800–1200 and 1200–1886m). To examine this variation, three habitat variables were analyzed: total basal area of food trees per unit area, seasonal variations in fruit abundance, and total annual fleshy fruit production. All of these variables indicate that fruit and seeds are most available in the coastal forest. Thus, altitudinal variations in the density of Japanese macaques on Yakushima are determined by the total annual food abundance.     

Mass cultivation of seaweeds: current aspects and approaches

A word-wide overview is presented of the current state of mass cultivation of seaweeds. In comparison with a total annual commercial production of fish, crustaceans and molluscs of about 120 × 10⁶t, of which one-third is produced by aquaculture, the production of seaweeds is about 10 × 10⁶t wet weight; the majoirty of this comes from culture-based systems. The Top Ten Species List is headed by the kelp Laminaria japonica with 4.2 × 10⁶t fresh weight cultivated mainly in China. The productivity of a well-developed, multi-layered, perennial seaweed vegetation is as high as dense terrestrial vegetation, and even higher annual values for productivity have been reported for tank cultures of macroalgae. Epiphytes provide a major problem for the seaweed cultivator, but can be controlled by growing plants at high densities in rope cultures in the sea, or, more easily, in seaweed tank cultures on land. The main environmental problem of animal (fed) aquaculture is the discharge of nutrient loads into coastal waters, e.g., 35 kg N and 7 kg P t^–1 aquacultured fish. Integration of fish and seaweed farming may help to solve this problem, since seaweeds can remove up to 90% of the nutrient discharge from an intensive fish farm. Mass culture of commercially valuable seaweed species is likely to play an increasingly important role as a nutrient-removal system to alleviate eutrophication problems due to fed aquaculture.     

The emergy ecological footprint for small fish farm in China

Mariculture, especially cage aquaculture remains a growing, vibrant and important production sector for high protein food in coastal regions in China. A quantitative evaluation of the mariculture system is an essential step to documenting its’ sustainability. The method of emergy ecological footprint is applied to evaluate the environmental sustainability of an offshore small fish farm (Great Marine fish farm, GMFF) in the East China Sea. All input needed to support fish farming were accounted and converted into biological space, to estimate the natural capital demand for the rearing process in terms of global hectares. The emergy ecological footprint of GMFF was 1953.19 ha, meaning that nearly 2000 ha of ecologically productive lands were needed to support the fish framing. The largest component of emergy ecological footprint was forage (1183.64 ha), which took up 60.60% of the total footprint; the second and third largest footprint components were fingerlings and fuel. In a word, emergy ecological footprint can serve as a practical and meaningful tool for comparing and monitoring the environmental impact of fish farming. The strong dependence of external contributions of exploiting the wild fish resources affects strongly the level of environmental sustainability of fish farming.     

Mariculture development in Kenya: alternatives to siting ponds in the mangrove ecosystem

Pond culture of brackishwater prawns and shrimps has recently generated much interest in Kenya. The mangrove areas are the target zones for the construction of these ponds. With the increasing awareness of the unique ecological role played by the mangroves, there is an urgent need to stop the conversion of mangrove swamps into aquaculture ponds. To develop pond aquaculture without destroying the mangroves, a shift from tide-fed to pump-fed pond systems is recommended in order to divert the farming from the mangroves to higher grounds. Mangrove-friendly mariculture practices like pen, cage and raft culture are discussed. Methods of efficiently utilising the already destroyed mangrove areas are considered.     

Cultivation ofMonostroma nitidum (Chlorophyta) in a river estuary,southern Japan

The green seaweedMonostroma nitidum Wittrock is cultivated in brackish waters of southern Japan and an ecological survey of its cultivation was carried out in the estuary of R. Shimanto over three years. Artificial seed culture began by collecting many gametes in April. The zygotes adhered to a plastic settlement board (20–30 cm long and 10 cm wide). The cultivated zygotes in the indoor tank increased gradually in size from 8 to 40m till early September. Maturation of the zygote was promoted by providing dark conditions for 2–3 weeks. The production of a concentrated zoospore solution was achieved by adding freshwater 2–3°C above that of the zygote culture tank and a photon flux density of 100mol m^–2 s^–1).The culture nets were set our horizontally at a level exposed for 4 h and were harvested 3–4 times till March. The total production was approximately 6–10 kg dry weight per net during culture periods. The total production of nets harvested 3–4 times per year was greater than that of nets harvested only once.     

Diatom bloom in the tidal freshwater zone of a turbid and shallow estuary,Rupert Bay (James Bay,Canada)

Phytoplankton biomass and species composition were studied from June to September 1991 at the mouth of four major rivers and in the freshwater (sal. 0 %), the estuarine (sal. 2–10%) and the coastal (sal. 10–12%) zones of Rupert Bay, located at the southeast tip of James Bay, Canada.A chlorophyll a maximum (5–14 µg 1^–1) was observed in the freshwater zone from July to September. Chlorophyll values were low at the mouth of the rivers and in the estuarine and coastal zones (chl a < 1.00 µg 1^–1). Diatoms were dominant in the freshwater zone (30–80 % abundance), with flagellates dominating in the estuarine and coastal zones (60–95% abundance). Diversity was low (H: 1.5–2.5) in the freshwater zone and decreased seaward (H: 0.5–1.5).The diatom bloom was composed almost exclusively of the autochthonous planktonic diatom Cyclotella meneghiniana Kütz., which contributed 25–85% of the species composition, and of the subdominant benthic species Diploneis smithii, Navicula lanceolata and Surirella robusta. Peak abundance occurred upstream of the turbidity maximum, in the tidal freshwater zone. In this zone the mean photic depth was 1 m and residence time was from 7 to 8 days during the bloom. Residence time is considered to be the dominant factor controlling the phytoplankton bloom, with light not acting as a limiting factor. The high turbidity due to resuspension and shallow depth of the bay controlled the species composition.     

Low Temperature Tolerance of Pacu, Piaractus mesopotamicus

Pacu, Piaractus mesopotamicus (=Colossoma mitrei), is a South American warm water fish species found in the temperature range of 15–35°C. The culture of a warm water species in temperate regions demands knowledge on its temperature requirements. Pacu introduction into the Israeli fish culture system is being considered. Temperature range in the region is 8–33°C, thus the minimum winter water temperatures might be a limiting factor. To determine what is the minimum temperature pacu would tolerate, and hence which overwintering operations in warm-temperate regions are required for this warm water species, low temperature tolerance tests in the laboratory and observations in the field were carried out. Laboratory experiments reducing temperature by 1–3°C per day were carried out with fish of 150–200g, about the size pacu reach after one culture season. The field observations compared survival of two-year-old pacu of 1.3kg mean weight overwintered in outdoors and in greenhouse ponds. For one-year-old fish 7.5°C was found to be the lower temperature tolerance limit. Two-year-old fish withstand short exposures to this temperature rather well and their lower tolerance limit might be lower. This indicates that in warm-temperate regions pacu should survive in outdoors ponds. In this case some loss of weight should be expected, and suspension of feeding when temperature drops below 16–18°C is recommended to avoid wasting feed that the fish will not consume anyway. To be in the safe side, inflow of the warmest available water into the ponds is recommended if maximum water temperature drops to 10°C or below. Overwintering in greenhouses or other heated facilities would be recommended if an exceptionally cold winter is expected and for regions with lower winter minimum temperatures.     

Food safety and products from aquaculture

Aquaculture is currently one of the fastest growing food production systems in the world with production increasing at an average rate of 9.6% per year over the past decade. As world fish stocks are reaching the limits of exploitation, we shall rely to a far greater extent on products from aquaculture as food sources of high nutritional value. Approximately 90% of global aquaculture production is based in Asia, where it provides an important source of dietary animal protein of the region and income for millions of small-scale farmers. Commercial aquaculture contributes significantly to the economies of many producing countries, where highly valued species are a major source of foreign. Many different aquaculture systems exist world wide, ranging from small family-sized fish ponds to intensive cage culture industries as used in salmon fishing. There has been an expansion in the use of integrated farming systems, especially in Asia, where animal and human faeces are used to fertilise ponds. This paper will review global aquaculture systems used in the production of finfish and crustaceans and will focus on potential hazards arising from biological contamination of products that pose risks to public health.     

Spatial distribution of Pleuragramma antarcticum (Pisces: Nototheniidae) near the Filchner- and Larsen ice shelves (Weddell sea/antarctica)

Summary In the zooplankton collected during three German Antarctic expeditions to the southern Weddell Sea in 1979/80, 1980/81 and 1983, the post-larvae of the nototheniid fish Pleuragramma antarcticum were found in 71–94% of the samples and represented 85–98% in numbers of all fish caught. In 1983, the abundance of post-larvae was up to 88 ind./m² (corresponding to 3 ind./m³). Highest concentrations were generally found over the continental slope and innershelf depressions. More than 70% of the post-larvae were caught in the upper 50–100 m water layer, in the well stratified Summer Water of -1.3 to -0.5°C. The Summer Water is shifted towards the outer edge of the shelf by Ekman transport and accumulation of post-larvae in the slope front and eddies can be explained by this drift. Older Pleuragramma antarcticum of age 1 resemble in their vertical distribution the juveniles and adults, 40–60% of which were caught on the shallower parts of the shelf in cold Ice Shelf Water of -1.8 to -2.1°C in depths below 200 m. Mean abundances of the yearclasses varied by a factor of 16 in age 0 fish and by factor of 10 in age 1. The size of a yearclass may be related to the varying appearance and persistence of an ice free coastal polynya.     

The potential of fish production based on periphyton

Periphyton is composed of attached plant andanimal organisms embedded in amucopolysaccharide matrix. This reviewsummarizes research on periphyton-based fishproduction and on periphyton productivity andingestion by fish, and explores the potentialof developing periphyton-based aquaculture.Important systems with periphyton arebrush-parks in lagoon areas and freshwaterponds with maximum extrapolated fish productionof 8 t ha^–1 y^–1 and 7 t ha^–1y^–1, respectively. Experiments with avariety of substrates and fish species havebeen done, sometimes with supplemental feeding.In most experiments, fish production wasgreater with additional substrates compared tocontrols without substrates. Colonization ofsubstrates starts with the deposition oforganic substances and attraction of bacteria,followed by algae and invertebrates. Afterinitial colonization, biomass density increasesto a maximum when competition for light andnutrients prevents a further increase. Often,more than 50% of the periphyton ash-free drymatter is of non-algal origin. Highest biomass(dm) in natural systems ranges from 0 to 700g m^–2 and in aquaculture experiments wasaround 100 g m^–2. Highest productivity wasfound on bamboo in brush-parks (7.9 gC m^–2 d^–1) and on coral reefs (3 gC m^–2 d^–1). Inorganic and organicnutrients stimulate periphyton production.Grazing is the main factor determiningperiphyton density, while substrate type alsoaffects productivity and biomass. Better growthwas observed on natural (tree branches andbamboo) than on artifical materials (plasticand PVC). Many herbivorous and omnivorous fishcan utilize periphyton. Estimates of periphytoningestion by fish range from 0.24 to 112 mg dm(g fish)^–1 d^–1. Ingestion rates areinfluenced by temperature, fish size, fishspecies and the nutritional quality of theperiphyton. Periphyton composition is generallysimilar to that of natural feeds in fishponds,with a higher ash content due to the entrapmentof sand particles and formation of carbonates.Protein/Metabolizable Energy (P/ME) ratios ofperiphyton vary from 10 to 40 kJ g^–1.Overall assimilation efficiency of fish growingon periphyton was 20–50%. The limited work onfeed conversion ratios resulted in valuesbetween 2 and 3. A simple simulation model ofperiphyton-based fish production estimates fishproduction at approximately 2.8 t ha^–1y^–1. Together with other food resources infishponds, total fish production with thecurrent technology level is estimated at about5 t ha^–1 y^–1. Because grazingpressure is determined by fish stocking rates,productivity of periphyton is currently themain factor limiting fish production. Weconclude that periphyton can increase theproductivity and efficiency of aquaculturesystems, but more research is needed foroptimization. Areas for attention include theimplementation and control of periphytonproduction (nutrient levels, substate types andconformations), the ratio of fish to periphytonbiomass, options for utilizing periphyton inintensive aquaculture systems and with marinefish, and possibilities for periphyton-basedshrimp culture.     

Seasonal Dynamics of Plankton in Zones with Different Thermal Structures of Water off the Southern Kuril Islands

Based on plankton samples collected from 100–0 m depths in the years 1979–1980, the duration of biological seasons was determined and the seasonal succession of plankton was followed in five surface water modification zones: Kuril, Subarctic, Subtropical transformed, Oyashio (cold), and Soya (warm). The bloom of phytoplankton off the southern Kuril Islands begins in the months March–May and continues until June in waters of warm currents and until September in cold waters. Succession rates vary among the zones. In cold-water zones, the change in the plankton community from the spring to the summer state occurs about half a month later than in warm waters. The reason for this lag is the slower sea surface heating and the later stratification in cold-water zones.     

The present situation and problems of mariculture in Japan

Summary Recently, rearing techniques for various kinds of fish have advanced markedly, and the number of fish species in commercial production increases every year. The establishment of methods for stable, reproducible mass culture of live foods that are highly nutritious is still necessary to improve the survival and growth rates of larval fish. Since, however, the mass propagation of live foods requires costly equipment and depends upon weather conditions, the development of artificial larval diets to replace live foods will be essential. In mariculture local trash fish are commonly used as a feed for juvenile fish because of their low cost and high acceptability to the cultured fish. However, this frequently results in deterioration of water environments, leading to the appearance of fish diseases and pollution. The development of artificial diets such as moist pellets will also improve these conditions.Mass-cultured fish seed are mainly used for the culture of commercial-sized fish, even though they are generally poorer in taste than wild fish. They are also used for release into coastal waters to promote inshore fishery, but it is difficult to evaluate the effect of stocking on the total catch. Another type of mariculture depends upon raising wild juveniles, though there are clearly too few caught to supply enough fish seed to satisfy the ever-growing demands of fish breeders. Thus, the cultivation of broodstock to produce high-quality eggs is important.     

Marine finfish hatchery technology in the USA – status and future

In 1993, about 52% of the 433 698 tons of thetotal US aquaculture production came from theproduction of freshwater catfish. Excludingsalmonid culture, the percentage of marine finfishculture in total aquaculture production in the UShas been negligible. Commercial scale production ofmarine finfish in hatcheries is very limited in theUS.Studies on eggs and larvae of marine finfishspecies in the US have stemmed from theconsideration of fisheries management rather thanaquaculture. Most of the marine finfish larvaeproduced in the laboratory has been for the purposeof providing materials for other academic relatedstudies. Results of these studies can be applied inthe development of marine finfish hatcherytechnology. Hatchery technology for several marinefinfish species has been developed for stockenhancement, technology transfer and aquaculture. This paper reviews the current hatchery technologyof striped mullet (Mugil cephalus), dolphinfish (Coryphaena hippurus), red drum (Sciaenops ocellatus), and other potentialaquaculture species.     

Coastal upwelling, primary production and mussel growth in the Rías Baixas of Galicia

The Rías Baixas are four flooded tectonic valleys located on the northwest Iberian Peninsula that support a mussel production of about 250 × 10⁶ kg y^–1 from 3337 mussel rafts. Mussel production in this region is the highest in Europe and one of the most intensive in the world, giving employment to 9000 people directly and 20000 indirectly. The causes of this high mussel production are discussed through the analysis of published and some unpublished information. The interaction between coastal upwelling and the circulation patterns in the Rías, which channel the 3-dimensional variability of the open ocean into a 2-dimensional system, promotes a massive response in the productivity of phytoplankton populations inside the Rías, even during weak upwelling events along the coast. Coastal upwelling in the area typically occurs between March–April and September–October. The mean value of gross primary production during the whole upwelling season is 1.4 g C m^–2 d^–1, although high sporadic values of 4 g C m^–2 d^–1 may occur during upwelling relaxation events, when phytoplankton export to the coastal shelf is restricted. Mussel growth occurs mainly during the upwelling season. It is estimated that mussel harvest extracts 10% of the primary production. The phytoplankton response to upwelling provides food of high quality (f 0.5) that determines high absorption efficiency (0.6), whereas the characteristics of the Rías maintains the seston concentration at levels (0.5 – 1.3 mg TPM l^–1 and <5 mg Chl a m^–3) below the threshold of pseudo-faeces production. The physiological behaviour of mussels indicates that the high yield of mussel culture in the Rías of Galicia is a consequence of the particular characteristics of the seston.     

On the age and growth of flounder Paralichthys orbignyanus (Jenyns, 1842) in Bahía Blanca Estuary, Argentina

Estimates of age and growth of flounder Paralichthys orbignyanus (Jenyns,1842) were made by analysing fish from commercial catches in Bahía Blanca estuary (39° LS). A total of 823 fish caught by fishing fleets operating in the estuary were collected between February 1997 and January 1998. Age was determined via scale reading and growth estimation parameters using von Bertalanffys equation. Annulus formation occurred in August. The age for the total population, which ranged from 7.7 to 87.5 cm TL was 0–7. Age ranged from 0 to 6 years (21.5–71.9 cm TL) in males and from 0 to 7 years (24.9–87.5 cm TL) in females. Length–weight relationships were W(g)=0.0093 L(cm) exp. 3.03 for the total population, W(g)=0.0147 L(cm) exp. 2.91 for females, and W(g)=0.0168 L(cm) exp. 2.87 for males, respectively. As from age three, females were longer and heavier than males. The growth parameters estimated for the total population, for females and for males were L=83.29, 79.66, and 46.11 cm, respectively; k values were 0.18, 0.23, and 0.92, respectively, and t₀ values were –1.87; –1.54, and –0.62, respectively. Similarly to other flounders,P. orbignyanus is a typical inhabitant of estuaries and coastal regions. Adults stay in the study area mainly during spring and summer, they go outside or offshore during the spawning period and then they return to the estuary for feeding and recovering.     

我国水产养殖事业的发展及今后的努力方向

我国水产养殖事业的发展及今后的努力方向曾呈奎（中国科学院海洋研究所青岛２６６０７１）我国水产养殖事业在新中国成立以前,只有个别零星古老的,传统的事业。在养殖海产鱼虾方面北方有几百年来的＂港养对虾和鱼＂,而在南方也有类似的鱼塘。在海藻栽培方面,则有几百年来的福建金门县的礁养海萝及平潭县的礁养紫菜等。这些古老的传统养殖方法虽然产生一些效果,但产量较小。     

汕头南澳养殖区表层沉积物中生源要素的分布及其污染评价

于2010年9月采集南澳海域鱼类养殖区、贝类养殖区、大型藻类栽培区和对照区4个功能区表层沉积物样品,分析沉积物中生物硅(BSi)、总有机碳(TOC)、总氮(TN)和总磷(TP)4种生源要素的含量。结果表明,南澳海域生源要素含量与国内外养殖区相比属于中等水平。鱼类养殖区TOC、TN、TP的含量最高,而BSi的最高含量出现在藻类栽培区(平均含量为0.30%)。鱼类养殖区BSi、TOC、TN和TP的平均含量分别为0.24%、0.89%、0.13%和0.097%。根据沉积物中TOC/TN比值分析,发现鱼类养殖区的TOC主要来源于水生,贝类、藻类和对照区TOC则主要来源于陆源。生源要素污染评价表明,4个功能区的TN均属于Ⅱ类污染,鱼类和贝类养殖区的TP属于Ⅱ类轻度污染。