Freshwater resources are fundamental
Water and atmosphere are the most fundamental materials of the world for the existence of humans and all other organisms. Water covers 2/3 of the earth's surface and is the most abundant structural component of organisms. Life is essentially based on the continuous exchange of water between an organism and its environment. Freshwater constitutes only 3% of the total water of the world; of which 77.2% is stored as ice, 22.4% as underground and soil water, and about 0.4% is in lakes, rivers and other water bodies (i.e., pond, bogs, etc.) (IIED, 1987). Although freshwater is much less abundant than salt water, it is the essential material for terrestrial organisms and fundamental to the civilization of humans. It also serves as the bridge between terrestrial and marine ecosystems. In recent years, at least 43 countries have experienced water shortage, and one hundred million people are confronted with worse drinking water. At present, water shortage has become one of the most serious global environmental problems which threatens human society.
China is poor in water supply, with the average annual supply for each person around 2,600 tons, which is only 1/4 of the world's average (IIED, 1987). The quantity of freshwater resources is also declining, while the demand for it is increasing; moreover, about half of the lakes have already been polluted to some extent by the increasing industrialization, human and agricultural wastes. Because increasing freshwater shortage is becoming an important limiting factor for the economic growth and development of China, it is important for our existence to protect freshwater resources.
Freshwater fishes are important food resources
Fish make up the most abundant group of vertebrates, and there are in excess of 22,000 described species. Global fish production exceeds that of cattle, sheep, poultry or eggs, and is the largest source of either wild or domestic animal protein for the world's expanding human population (Norse, 1992). Of total world fish landings in 1989, marine landings comprised 86.2% while inland fisheries (aquaculture and capture fisheries) accounted for the remaining 13.8% (FAO, 1991). In 1990, total fish landings in China was 1,236 tons, while inland fisheries accounted for as high as 42%. Therefore, freshwater fishes comprised one of the fundamental elements for the existence of Chinese people. There are around 8,400 described species of freshwater fishes in the world, which comprise approximately 40% of total fish species. Around 1,500 species of freshwater fish have been described in Asia (Nelson, 1984). In China, there are in excess of 800 primary freshwater fishes (those confined to freshwater), among which about 500 species are endemic.
It is estimated that there is around 100,000km3 of water for each marine species but only 15km3 for each freshwater species; and that the population level of a marine species may be around 109. but ranges down to around 106 for freshwater species (Groombridge, 1992). Compared with marine ecosystems, freshwater ecosystems are usually smaller, more isolated from each other, and the distributions of freshwater fish species are more limited. Freshwater ecosystems are also less stable, and much more susceptible to environmental disturbance. For example, more than one decade ago in Lake Luguhu (located in the intersectional area between Sichuan and Yunnan provinces), local people had tried to introduce grass carp. However, by mistake they introduced Pseudorasbora parva, which not only have little commercial value but also made three species of Schizothoracine fishes endangered to near extinction (Liang, pers. comm.).
It is estimated that at least 20% (ca. 1,800 species) of the world's freshwater fish species are seriously threatened or extinct, which is mainly caused by habitat modification (competition for water, drainage, pollution), introduced species and commercial exploitation (Groombridge, 1992). In North America, nearly 1/3 of the total fish species are endangered, threatened or listed as species of special concern (Williams et al., 1989). In China, there are 92 vulnerable of endangered species of freshwater fishes which account for 10% of the total number of freshwater fish; Cyprinus yilongensis, an endemic species in Yunnan Province, has become extinct, and Psephurs gladius, Macrura reevesi and Hucho bleekeri are in danger of extinction (Le, 1995a,b).
Lake ecosystems in the middle and lower basins of the Yangtze River
The total surface area of all Chinese lakes is around 74,300km2, of which 42% is in humid eastern China. The Yangtze River, located in the eastern plain, is the longest river in China and the third longest river in the world. It has a total length of more than 6,300km. The middle and lower basins of the Yangtze are one of the major distribution areas of freshwater lakes in China, and the surface area of lakes constitutes around 1/8 of the total surface area of this region.
The Yangtze basin is the most important commercial fishing base in China with a fish yield comprising around 2/3 of the total (Liu and He, 1992). In addition, waters of the Yangtze basins are not only essential for water supplies (drinking water, irrigation, etc.) but also for many other purposes such as recreation, and are therefore, very important for the economic development.
The middle and lower basins of the Yangtze River are influenced by the wet monsoon, and the lakes of the basin were formed by flood of the river in the Late Tertiary. These lakes are shallow (without thermal stratification), and interlaced with the main river and its branches into a unique complex river-lakes ecosystem. These shallow lakes usually have a high productivity, abundant vegetation cover, and a developed littoral zone communities. Because of the differences between the river and the lakes, and the flooding caused by the wet monsoon there are obvious changes in water level. In this environment have evolved unique migrating fish, for example: Hypophthalmichthys molitrix, Aristichys nobilis, Ctenopharyngodon idellus, Mylopharyngodon piceus, Ochetobius elongatus, Luciobrama macrocephalus, Squaliobarbus curriculus, and Parabramis pekinensis. Among these are some of major commercial importance in China and the world (The Fourth Laboratory of the Institute and the Tunghu Fish Farm, 1976). In the Yangtze basin, there are about 300 fish species, of which more than half belong to the Cyprinidae, the most commercially important fishes are also cyprinids (Liu and He, 1992). The Yangtze basins are considered to be the center of origin and evolution of many freshwater fishes in East Asia, preserving some remnant fish species (Cao and Chen, pers. comm.). Natural lakes of the Yangtze basins, superior in water quality and abundant in species diversity, provide essential freshwater resources for our existence.
Biodiversity problems of lake ecosystems
For several decades, inappropriate construction of dikes, dams and levees, unreasonable fishing and fishery management, and the extremely strong pressure of the rapidly increasing human population on lake ecosystems, have brought severe damage to the biodiversity from genetic to ecosystem levels. This has resulted in the destruction of many natural resources of the lakes and have posed a threat to the stability of our society and the sustainable development of the economy.
1. Shrinking and fragmentation of lake ecosystems
Fragmentation of lakes by continuous subsidence of mud from river water and by extensive reclamation of farmland from the lakes is a serious problem in China. Not only are the lakes in the western part of China shrinking, but those in the eastern part are also contracting. In the western regions, drought has fragmented many lakes into smaller lakes (Shi, 1989). While in the eastern regions, shrinking of lakes is mainly caused by subsidence of mud from river water, and by reclamation of farmland from lakes. In the eastern part of China, the superior natural conditions for agriculture has been accompanied with a rapid increase in human population, and consequently a rapid decline in land area per capita, so conversion of lakes for farmland has become an historical by-product. For example, the total surface area of Lake Dongtin was 4,350km2 in 1949, but declined to 2,619km2 in 1983, due to continuous subsidence of river water and extensive reclamation of farmland from the lake. The total surface area of Lake Honghu was about 760km2 in the early 1950's, but subsequent portioning and reclamation work in the 1960's and 1970's around the lake caused the continual shrinkage of the surface area of the lake to 350km2 of 1979. The Gianghan Plain contained 1,066 various sized lakes (surface area of 8,300km2 ), however, by the early 1980's, only 309 lakes (surface area of 5,600km2) were left due to similar reasons (Shi, et al., 1985).
Welcomme (1979) reported that the number of fish species present in subtropical and tropical rivers was highly correlated with the area of the river basin. Temperate rivers showed a similar pattern, although the number of species rises more steeply with increasing basin area in tropical systems than in higher latitudes. Two factors are important here: (1) the area of a lake sets an upper limit to the maximum population size of each fish species and (2) small populations are inherently more prone to extinction than large ones. From this alone it can be predicted that shrinking of lake ecosystems will contract the living spaces of many freshwater species, and therefore make more and more fish species endangered or extinct.
2. Destruction of lake-river ecosystem by severance of lakes from the Yangtze River
Severance of lakes from the river by construction of sluices and dikes have led to the impoverishment of the natural fish resource--especially of migratory fishes--causing the lakes to be dominated by species of small size. The Yangtze basin was originally a network of water systems, including all of the large and small tributaries of the river, and many interconnected shallow lakes. This complex river-lakes system provided superior living conditions for many commercially important migratory fish species (including four domestic carp) which can spawn only in the river and regularly migrate into lakes for feeding (Fish Laboratory, 1976). Over the past decades, most lakes have been artificially severed from the river by hydroelectric and irrigation projects, and as a result, the migratory fish can no longer enter into these lakes from the river. This has lead to a sharp decline of their population size in both the lakes and the river.
In the middle and lower region of the Yangtze River, there averages around a hundred fish species in natural lakes, but only 30-40 species in severed lakes. For instance, according to a survey of fish species made in the 1990's in Honghu Lake, there were no less than 90 species in the lake. In 1958, a sluice was constructed in the canal linking the lake with the river, thus severing the intercommunication between the river and the lake. The survey in 1964 listed 74 species, in 1981-1982, 54 species, of which only 33 species were obtained from the lake, while the remaining 21 species were riverine fishes carried into the lake during the channeling of the Changjiang River water for the purpose of irrigation.
There is also a shortage of large-sized economic fish species which are mostly migratory between the lake and the river (Honghu Research Group, 1991). Fishery resources of the three domestic carp (siluer, bighead and grass carp) in the Yangtze River are also decreasing: the catch of marketable-sized fishes in the 1980's was just half as much as that in the 1950's, whereas the catch of natural fries was only one quarter as much as that in the 1960's. Changes in composition of the catch also occurred: proportion of migratory fishes declined, whereas that of small-sized lake-dwelling species increased (Li, et al., 1990).
Fingerlings of the four domestic carp used as stocking are now mainly from artificial reproduction, but retrogression due to successive inbreeding has occurred (i.e., both growth and mature age decline, adult fish become smaller, and incidence of diseases become higher). For example, in natural populations of silver carp, the mature age of females is 3-4 years with an average body weight of 4.85kg, and the mature age of males is 3 years with a average body weight of 3.81kg. After inbreeding for five generations, the mature age of females declined to 2 years with a body weight of only 1.25kg (the minimum reached as low as 0.3kg) and the mature age of males declined to one year with a body weight of only 0.69kg. Moreover, the inbreeding offsprings had a higher malformation rate, a weaker constitution, and an eleven times higher incidence of diseases. Therefore, it is harmful to inbreed for many generations, and it is essential to restore the population by natural fingerlings. At present, adult fishes of the Yangtze River are mainly from Lake Boyang and Lake Dongtin which have not yet been severed from the river, and fate of these two lakes remains unclear (Liang, pers. comm). Severance of lakes from the river by hydroelectric and irrigation projects has changed or disrupted dispersal and migration of drastic changes in environments will accelerate distinction of remnant species, and consequently decrease the abundant biodiversity.
3. Decline in biodiversity of fish species by reckless over fishing
Over fishing is commonly occurring in large lakes which are too large to cultivate fish and manage fisheries efficiently. In addition to the severance of lake from the river, reckless over fishing of natural fish populations has resulted in severe decline of species diversity, and decreased drastically the population sizes of commercially important large-sized fish species (mostly migratory fishes). Consequently, a lack of top consumers (carnivores) has usually caused an explosive population increase of small-sized fishes (swamping also favors small sized species), leading to a low fish yield, low fish quality, and low profit (Liang, pers. comm.).
For instance, fish production of Honghu Lake witnessed a steady decline. In the 1950's, the annual fish yield was around 10,000 tons and the dominant species were the four-domestic-carp, Cyprinus carpio and Parabramis pekinensis. However, excessive overfishing and inadequate protection of the spawners caused the diminution both in the size of fish species and in the size of individual fish, the lake being dominated by species of small size as well as by population of stunted growth. In the 1980's, the annual fish yield declined to 3,000-4,000 tons, and 87% were composed of the small-sized Carassium auratus auratus, Pseudobagrus fulvidraco and Culter erythropterus (Honghu Research Goup, 1991).
In Lake Dongtin, the maximum recorded annual fish yield reached as high as 45,000 tons, and the average annual yield was 30,700 tons in the 1950's, but declined to 15,000 tons in the 1980's. The composition of fish yield also changed obviously: the proportion of migrating fishes declined (mainly the four domestic carp), while that of lake-dwelling fishes (Cyprinus carpio, Carassium auratus auratus and Silurus asotus) greatly increased. Of the major economic fishes captured, the proportion of young ages increased and the average individual size of a certain age was also decreased. A large number of young individuals and small-sized species became the targets of fishing (Shan, et al., 1990).
A quite similar phenomenon also occur in some of the lakes of the middle and lower basins of the Yangtze River such as in Lake Caohu, Lake Taihu and Lake Hongzhe, where three small-sized species of Coilia became dominant (Liang, pers. comm.; Shun & Huang, 1993).
4. Seconday Extinction following destruction of climax macrophyte communities
Over-stocking of plant-eating carps has usually caused severe destruction of climax macrophyte communities, and consequently led to a series of extinction of animal species. Like our ancestors who had advanced from collecting wild plants and animals to cultivating crops and domestic animals, artificial breeding, stocking and cultivating fish have developed rapidly. We have succeeded in changing the fish composition for our purposes and cultivating some commercially important fish species. This kind of fishery has applied not only to ponds but also to small/middle-sized lakes (especially urban lakes), and has made it possible to greatly increase fish yield, and to lessen the danger of over-capture on natural populations of some species. However, it has been accompanied with serious problems. In order to get short-term profit over-stocking of fish has led to overgrazing of prey organisms, which has conversely exerted great impact on the whole ecosystem. The most significant events are the destruction of vegetation cover, especially submerged macrophytes, and the dominance of increased r-selected organisms.
In many lakes, the over-stocking of plant-eating carp (especially grass carp) has usually led to destruction of macrophyte communities, and led to the shift of dominant primary producers from macrophytes to phytoplankton. For instance, in the Guozheng area of Lake Donghu, the biomass of macrophytes was as high as 1,779.8g/m2, but declined to 5.8g/m2, which was mainly due to over-stocking of the grass carp. The high destructive power of grass carp on macrophytes is not only due to their low digestion of macrophytes but also because the grass carp suppress the recovery of macrophytes by grazing on new shoots. To macrophytes, algae are r-selected species with a small body and a high turnover rate, and their ability to store nutrients is low. Therefore, in lakes dominated by macrophytes, since a lot of nutrients are stored in macrophytes, the growth of algae is suppressed, and the water is clear. This process is sometimes referred to as the cleanup ability of macrophytes. As the abundance of macrophytes declines, nutrients stored in macrophytes are released into lake water by grazing and excretion of the grass carp. This favors the growth of phytoplankton. Moreover, increased phytoplankton biomass decreases both water transparency and compensative depth of macrophytes, which in return decreases living extent of the macrophytes. Such a vicious circle usually leads to less and less macrophytes of even their extinction. Now, in the Guozheng area of Lake Donghu, the climax macrophyte community in the 1950's has disappeared completely, and consequently, it is followed by secondary succession of the primary producer community in which the dominants are r-selected algae, which is just like the consequences of deforestation for agriculture (Liang, pers. comm.).
Secondary extinction. Macrophyte communities are associated with many periphyton and mollusc, and also serve both as living space and as the substrata for the spawning of many fishes. Therefore, an abundant macrophyte community is accompanied with a high biodiversity of the whole community of plants and animals. The destruction of macrophytes not only leads to the crash of the grass carp populations, but also causes secondary extinction of those organisms depending on these macrophytes and of those fish associated with these directly-related organisms. For example, with the disappearance of macrophytes, many periphytons and molluscs are prone to extinction. This causes the spawning substrata of Cyprinus carpio, Carassium auratus and Ophicephalus argus to also be decreased. The destruction of living environments for molluscs then results in the decline of the available food for Cyprinus carpio and Mylopharyngogon piceus. The dominance of grass carp is then replaced by filter-feeding planktivorous fishes, and so on.
Decline in stability of ecosystems. Disappearance of macrophytes not only causes increase in nutrient concentrations, but also leads to obvious decline in biodiversity of plankton community. In four lake areas (with different nutrient levels) of Lake Donghu, three biodiversity indicies (Margalef, Sympson and Shannon-Weaver) and the number of species of diatoms showed obvious negative relationships with nutrient levels (Lei Anping, Unpubl.). In two sampling stations of Lake Donghu, the Margalef Index of rotifers in 1991 (with high nutrient levels) was only 1/3 that in 1992 (with low nutrient levels). A similar decline in species number was also observed, and eutrophication of the lake decreased the biodiversity of rotifers (Zhuge Yan, Unpubl.). Similar phenomenon might have occurred to other groups of plankton.
However, it remains unknown why eutrophication decreases biodiversity of plankton communities. On the other hand, in hypertrophic waters (probably due to low biodiversity) stability of the systems declines severely, and an outbreak of a few algal species, typically the cyanobacterium water bloom, frequently occurs. In recent years, the frequent outburst of blood poisoning fish disease by bacteria in the lakes of the middle and lower regions of the Yangtze River is probably related to the simplification of the aquatic community (it is also possibly attributed by too high fish stocking density and decreasing water quality). This is very similar to the frequent outburst of insect or disease in the highly-simplified agricultural ecosystems with only one or a few crops.
5. Chemical pollution and accelerated eutrophication in urban lakes
In terrestrial ecosystems, human agricultural activities usually lead to decline in soil nutrients. From southern to northern parts of China, nutrient concentrations in the soil cultivated for 200-500 years, are only half that in the original soil covered with primary vegetation (Heilongjiang Term, 1982). However, during the ontogeny of a lake, organic and inorganic nutrients from the surrounding terrestrial ecosystems accumulates continuously. Trophic status of the lake changes from oligotrophic to eutrophic, and the lake finally appears as land. The process of natural eutrophication is very slow, usually on the time scale of centuries or more. But in recent decades (especially in many urban lakes) the rapid increase in the human population around lakes has resulted in outpouring of untreated industry and organic wastes into the lakes. Also because of unreasonable fishery management, eutrophication in these lakes have been accelerated at an extremely fast rate (i.e., it only needs decades, or even several years to change the lakes from mesotrophic to hypertrophic levels). Lake Donghu in Wuhan City and Lake Xuanwu in Nankin City are good examples of this. The water in lake ecosystems is directly comparable to the soil in terrestrial ecosystems. Soil is important because it supports terrestrial animals and plants. However, water in freshwater ecosystems is important not only because it provides aquatic food but also because it supplies freshwater resources. Severe eutrophication in aquatic ecosystems results in destruction of water supply, and although productivity of the ecosystem may be high, decreasing water quality may be a great threat to human health.
At present, nearly all urban lakes have been seriously eutrophicated. This is not only due to the rapid increase in surrounding human population, but also related with unreasonable fish cultivation. Overstocking of plant-eating carp has resulted in the destruction of submerged macrophytes. This practice is more or less intentional because it is thought that the deforestation by grass carp not only makes people get high fish yield, but also increases the fish yield of the planktivorous silver and bighead carp (as more nitrogen and phosphorous change into plankton biomass). For example, in 1963 in the Guozheng area of Lake Donghu, the biomass of macrophytes was 1779.8 g/m2, and phytoplankton production was only 1 g/m2/day. However, in 1975, the biomass of macrophytes declined to 5.8 g/m2, while phytoplankton production increased to 4.1 g 0.2/m2 (Chen, 1989). During the same period, fish yield of Lake Donghu increased from 93.8 to 276.0kg/ha (Liu, 1984), and proportion of silver and bighead carp was 83% (Section of Fish Ecology, 1976) (now more than 98%). On the other hand, water quality of Lake Donghu became worse and worse: low transparency, outburst of cyanobacterium water bloom, stinking odor of the polluted lake water (Jao & Zhang, 1980). These decreased the lakes multi-functions such as providing drinking water supply and recreation.
Recent studies further indicate that algal blooms (specifically, microcystins) a symptom of lake eutrophication, are harmful to the human liver (Falconer, 1983; Hasser, 1989; Mirura, 1991; Runneger, 1987, 1991). Microcystins not only inhibits the activity of protein phosphates, but also acts as a tumor promoter (Erikson, 1990; Honkanen, 1990; Matsushima, 1990; Nishiwaki-Matsushima, 1991; Yoshizawa, 1990), and is a health threat to humans..
Conclusions
Habitat loss, modification or fragmentation, reckless overfishing, overstocking of plant-eating carp and deterioration of the ecological environment appears to be the most serious current threats to biodiversity in freshwater lakes and species are faced by several of these threats operating simultaneously. Generally, aquatic ecosystems have received little attention in comparison with terrestrial ecosystems, and only a few scattered surveys (mainly on fish) have so far been conducted. Severe decline in species diversity of fish, coupled with inadequate knowledge of freshwater faunas, indicates that biological diversity in aquatic systems require increased conservation attention.
Acknowledgments
We wish to express deep thanks to Prof. Liang Yangling who kindly provided the unpublished manuscript titled: "On the current status and the future of Chinese fisheries from ecological viewpoint." Thanks are also to Mmes Lei Anping and Zhuge Yan for the kind provision of their unpublished data.
