Citation Detail: YANG Junxing. 1996. The Alien and Indigenous Fishes of Yunnan: A Study on Impact Ways, Degrees and Relevant Issues. in: Conserving China's Biodiversity (II) (PETER Johan Schei, WANG Sung and XIE Yan eds.). China Environmental Science Press. Beijing. 157-168p.

The Alien and Indigenous Fishes of Yunnan: A Study on Impact Ways,
Degrees and Relevant Issues
YANG Junxing

(Kunming Institute of Zoology, Chinese Academy of Sciences, Kunming, Yunnan P. R.China 650223)

¡ïIntroduction
¡ï1 the indigenous fishes of Yunnan: its composition, status and biological characteristics vulnerable to threats
¡ï2 Species and distribution of alien fishes
¡ï3 Spreading means of alien fishes at various aquatic ecosystems
¡ï4 The biological characters of alien fish species
¡ï5. Influence Degree and means of alien fish species on indigenous fish species
¡ï6 Management of alien fish species and protection of indigenous fish species


Abstract. The specimens confirm that Yunnan, which is the richest province in China in terms of Chinese fish species, has 432 recorded indigenous fish species. Since the beginning of the 1960s, in order to augment the fishery sector, 34 alien fish species have been introduced or brought into Yunnan from the provinces of Guangdong, Guangxi, Hunan, Hubei, etc. Among them 18 fish species were not native to Yunnan. The rest of the 16 alien fish species were introduced or brought into the middle and upper reaches of rivers in Yunnan from Pearl River and the middle and upper reaches of the Yangtze River. However these 16 alien fish species were also distributed naturally in the lower reaches of some rivers in Yunnan. The distribution of Yunnan indigenous fishes in 6 river systems is uneven. The Nanpanjiang River has 147 fish species, the richest in all the rivers. The Lancangjiang River is secondary with 130 species. A total of 94 species is distributed in lakes. Dianchi and Fuxian Lakes have the most species, respectively with 25 species. The lakes, mid-upper reaches of rivers have usually simple fish fauna with small populations and few predator fishes. The fish species are quite different between different fauna and they have different feeding habits. Their reproduction habits are peculiar. The background of simple fauna has formed a lower inter-species competition pressure among fishes of Yunnan. The basic biological characters of these indigenous fishes evolved from the low competitive background and have provided lower competition and survival capabilities to the indigenous fishes. When competing with alien fish species from a more highly competitive background, Yunnan indigenous fishes are very vulnerable. Yunnan Province has 432 documented fresh water fish species. However, there are about 130 fish species (or 30% of total species in Yunnan) of which no specimen has been caught for the last 5 years. About 150 fish species (or 34.7% of total species) which were common species in 1960s have now become rare species. The remaining 152 species have an evident decline in population size compared to that of 1960s. The condition of the fishes in the lakes is worse. Among 94 fish species in the lakes, 2 species (Cyprinus yilongensis and Anabarilius macrolepis) are extinct, 60 species are endangered which no specimens have been caught for the last 5 years. The extinct and endangered species account for 66% of total lake fishes. Two massive transplants and introduction of alien fishes have occurred. The first massive introduction of economic alien fishes such as "the four major domestic carps" and the non-economic fish species such as topmouth gudgeon and goby took place from 1963-1970. The second massive introduction is the introduction of Taihu Lake icefish and piper halfbeak into Dianchi and Xingyun lakes during 1982-1983. The alien fishes spread by different ways in upland lakes and rivers after introductions. The distribution of alien fishes in rivers exhibit a disjunct pattern, that is to say, alien fishes mainly occur in the slow waters and are sparse in the rapid currents. In contrast, the alien fishes in lakes are continuously distributed in shallow waters along the shore, middle-lower layer of deep water or middle-upper layer of open water. The economic alien fishes are usually characterized by having a large body size, a long lifecycle and slow reproductive speed or incapability of natural reproduction in various waters of Yunnan. The non-economic alien fishes are typical "R" survival strategists with common biological characteristics of small body size, short lifecycle and rapid reproduction speed. The documented fluctuation in fish resources in Erhai, Lugu and Chenghai Lakes indicates that among the four threats (the spread of alien fish species, land reclamation from lakes, irrigation and over fishing), alien fishes play the most important role in the sharp decline in population or endangering indigenous fish species. In terms of degrees, the species number and population size of alien fish species has an absolute dominance in water bodies such as lakes and reservoirs where most of indigenous fish species have become endangered. The impact of alien fishes on indigenous fish species in lakes and reservoirs is greater than that in rivers. The impacts of alien fishes on indigenous fish species in eastern rivers (Jinsha River, Nanpan River and Yuanjiang River) is greater than that in western rivers (Lancang River, Nujiang River and Yiluowadi River). The alien fishes yield impacts on the indigenous fish species by 3 ways: devouring their spawns, competition for food and a change in fishing methods. Actions should be taken to lessen the impacts of alien fish species and conserve indigenous fish resources. The proposed actions include fishery management, conservation of aquatic ecosystems, strict control of the introduction of alien fish species and implementing artificial cultivation and fry release of indigenous fishes.

Key words: Yunnan, alien fish species, indigenous fish species, biological characteristics, inter-species relationship, endangered

Introduction

Yunnan is the richest province in China in terms of biodiversity. The province has the most fresh water fish species in China, accounting for about 40% of total species of China. Moreover, Yunnan lies in the east wing of the connected belt where the India tetonic plate collided with Eurasia tetonic plate and where the Indian fish fauna meets East Asian fish fauna. Owing to its rich species and complicated history background of fauna, Yunnan draws the attention of ichthyologists. However, the populations of indigenous fish species keep declining and many species are almost extinct since 1960s. The threats include 4 artificial factors: the spread of alien fish species, land reclamation from lakes, irrigation engineering and over fishing. What role do the alien fish species play among the 4 threats? By which ways do the alien fish species spread after entrance to various waters in Yunnan? How do the alien fish species impact on the indigenous fish species? Now we are trying to tentatively discuss these questions according to the information collected in Yunnan since 1960s.

1 the indigenous fishes of Yunnan: its composition, status and biological characteristics vulnerable to threats

1.1 Composition of fish fauna and species distributions
According to the results of specimen identification as well as the data from former researches (WU et al., 1963, 1964, 1977; CHU et al., 1989, 1990; ZHU, 1995; CHEN, 1998; CHEN et al., 1998; YUE et al., 2000), there are 432 indigenous fish species in Yunnan and 34 alien fish species, which were artificially introduced or brought into Yunnan Province from other provinces. Among 34 alien fish species, 18 were not naturally distributed in Yunnan. The rest of the16 alien fish species were introduced or brought into the mid-upper reaches of rivers in Yunnan from Zhujiang River and the mid-upper reaches of the Yangtze River while they occur naturally in the lower reaches of some rivers in Yunnan. The distributions of 432 indigenous fish species in 6 river systems of Yunnan are as follows (see Table 1): Nanpanjiang River has 147 fish species, the richest in species in Yunnan; Lancangjiang River is secondary with 130 species; Jinshajiang River has 92 species, Yuanjiang River has 83 species, Irrawaddy River has 52 species, Nujiang River has 43 species, the least species in Yunnan. The fact that the richest fish species occur in the Nanpanjiang River and the Lancangjiang River demonstrates the most diverse habitats are present in the two rivers.

Table 1 Distribution of Indigenous Fish Species of Yunnan

The distribution of various groups in 6 water systems is also uneven. For example, Cypriniformes fishes have 95 species in Lancangjiang River, 73.1% of total species and have 126 species in Nanpanjiang River, 85.7% of total species. On the contrast, Siluriformes fishes (Catfishes) has 27 species in Lancangjiang River, distinctly more than that in Nanpanjiang River (11 species). In terms of family, Cyprinidae has a maximum of 88 species in Nanpanjiang River and 64 species in Lancangjiang River. As far as subfamilies in the family Cyprinidae are concerned, Lancangjiang River, Nujiangjiang River and Irrawaddy River have many more species in Danioninae, Barbinae and Labeoninae. However the Yuanjiang River, Nanpanjiang River and Jinshajiang River have more species in Cultrinae, with Barbinae and Labeoninae species, being also common. The rich species diversity and uneven distribution are the result of both environmental diversity and history origins.

In terms of a single river, the distribution of fish species shows another character: more species occur in the lower reaches and less species occur in the mid-upper reaches.

Besides the many river systems, Yunnan is characterized by many upland lakes. Yunnan has 19 large lakes possessing 94 fish species of the total 432 species (CHU, 1989, 1990; CHEN et al., 1982; YANG et CHEN, 1995; WANG et al., 1981; CHEN et al., 1983; 1998). The total numbers of fish species in these lakes are as follows: Dianchi and Fuxian Lakes respectively has 25 fish species; Erhai and Yangzonghai Lakes respectively have 18 species£»Chenghai Lake 15 species; Xingyun Lake 14 species; Yilong Lake 12 species; Qilu Lake 11 species; Qingshuihai, Datunhai, Cibihu and Jianhu lakes respectively have 7 species; Luguhu Lake 5 species; Changhu and Yuehu lakes 2 species; Bitahai, Shuduhai and Napahai lakes have 1 species.

1.2 The status of indigenous fishes
Since 1960s, the population of indigenous fishes has been declining. Among 432 indigenous fish species, the specimens of about 130 fish species (or 30% of total species) in Yunnan have not been caught for the last 5 years. About 150 fish species (or 34.7% of total species) which were common species in the 1960s have now become rare species. The remaining 152 species have an evident decline in population as compared to that in the 1960s.
The indigenous fish species in the lakes has been declining more rapidly than those in the rivers. There were 94 fish species in lakes. However, 2 species £¨Cyprinus yilongensis and Anabarilius macrolepis£©are extinct and 60 species are threatened and no specimens have been caught for the last 5 years, which accounts for 66% of the total lake fish species.

1.3 Basic biological characteristics and the cause of being vulnerable to threats
Simple fish fauna in lakes and mid-upper reaches of rivers. Yunnan rivers are all the upper and middle reaches of headwaters. In terms of a single water body such as a lake or a single tributary, the common character of fish faunas is that they all have few fish species. For example, Dianchi and Fuxian Lakes have only 25 fish species and the lake fish faunas in the mid-lower reaches of Yangtze River are usually composed of about 70-90 species. The simple fish faunas of Yunnan form a lower interspecies competitive pressure. The basic biological characters of those indigenous fishes evolved from the low competitive background and have provided lower competition and survival capabilities to these fishes. When competing with alien fish species from a high competition background, Yunnan indigenous fishes are very vulnerable to the invasions.

Few predating fishes. Few predating fishes are another character of Yunnan fish faunas which is particularly distinct in the mid-upper reaches of rivers or lakes. For instance, among 25 indigenous fish species in Fuxian Lake, there are only 2 species of predating fishes£¨Percocypris pingi regani and Silurus grahami£©, 8% of the total species, conspicuously less than that in lakes of the mid-lower reaches of Yangtze River. As a result of low pressure from predating fishes, the competition and survival capabilities of Yunnan indigenous fishes are very low.

Intensive differentiation of fish species. Due to the high degree of geographical isolation between water bodies, fish species are intensively differentiated. Fishes in the same genus usually form into different species among neighboring lakes or tributaries. The specific differentiations of Cyprinus, Anabarilius, Discogobio, Yunnanilus etc. in Yunnan-Guizhou Plateau are just some typical examples. These fishes are usually endemic to some very small water bodies with a narrow distribution and a small population; therefore they easily become endangered or extinct because of being vulnerable to environmental interference.

Diverse morphology. The fishes of Yunnan have evolved various means to adapt to rapid currents. Sisorid catfishes and Hillstream loaches have flat bodies. Their heads and ventral chests form into a sucking disc-liked structure, which can help to effectively suck rocks in the rapid streams. The slender hind body and longitudinal ridge-like horny projection on the back can also effectively reduce the influence of turbulent flows. The pectoral fin and ventral fin are well developed and flat, and have also sucking effect together with the flat head. The gill opening is small or reduced into a minute pore on the dorsal pectoral fin, which is the result of adaptation to the rapid stream: high speed of flow and large exchange through the gill per unit time. No scales at all or rudiment scales on the body makes them more flexible. The body is colorful and usually with round stripes. Some other fishes such as Garra and Nemachilus has a cylindrical body. The ventral side of the head of the Garra forms into a sucking disc and can effectively adhere on rock in rapid streams. The Nemachilus has colorful body with stripes. Some other fishes such as Schizothorax have a compressed body. To adapt to rapid streams, the Schizothorax fishes have an elongated body with minute or rudimental scales on body.

Differentiation in feeding habits. There are carnivorous fishes such as Sisorid catfishes feeding on fingerlings and aquatic insects, as well as alga feeders such as Labeoninae fishes and Hillstream loaches feeding on filamentous algae. Species with a close genetic relationship usually differentiate distinctly in their feeding habits. For example, 3 species in the subfamily Schizothoracinae with close genetic relationship in Lugu Lake have differentiated into omnivorous, phytophagous and carnivorous species.

Peculiar reproduction habits. Pangasiid catfishes are long-distance migrating fishes in reproduction season. However the majority of the remaining species are short-distance migrating fishes. Therefore most fish species can complete the whole lifecycle in small water bodies such as the exit of a spring or underground rivers. Their spawns, which adhere to gravel or disperse between gravel are mainly viscid or sink-viscid and vulnerable to alien polyphagous fishes such as Pseudorasbora parva and Gobies which like to eat these spawns and greatly threaten the reproduction of indigenous fishes.

2 Species and distribution of alien fishes

Besides 432 indigenous fish species, there are 34 species introduced from provinces in the mid-lower reaches of the Pearl River, Yangtze River and Northeast China (Table 2). About 18 alien species such as Mylopharyngodon piceus, Ctenopharyngodon idellus, Hypophthalmichthys molitrix, Aristichthys nobili, Neosalanx taihuensis etc. are not naturally distributed in Yunnan. The remaining 16 alien fish species such as Pseudorasbora parva, Hypseleotris swinhoni, Ctenogobius giurinus, etc. are naturally distributed in the lower reaches of some Yunnan rivers, but were introduced from other provinces into the mid-upper reaches of Yunnan water bodies. The first massive transplant and introduction of alien fish species occurred during 1963-1970. To improve the fish output of Yunnan upland lakes and develop aquaculture in ponds, the fishery departments purchased fries of "the four major domestic carps" (Mylopharyngodon piceus, Ctenopharyngodon idellus, Hypophthalmichthys molitrix, and Aristichthys nobilis) and Cyprinus in large quantities and release into all main upland lakes and ponds in Yunnan. When the economic fishes were introduced, some non-economic fishes such as Pseudorasbora parva, Hypseleotris swinhonis, Ctenogobius giurinus, etc. were brought into the upland lakes and most water bodies in Yunnan by mix with the fries of economic fishes. The second massive introduction occurred during 1982-1983 when Neosalanx taihuensis and Hyporhamphus intermedius, etc. were introduced into Dianchi Lake, Xingyun Lake, etc. The result is that Neosalanx taihuensis had spread all over the main lakes in Yunnan.
As the above stated, alien fishes in Yunnan are generally classified into economic alien fishes and non-economic alien fishes. Economic alien fishes were introduced intentionally for industrial purpose. However non-economic alien fishes were unintentionally brought into Yunnan and usually were the side effects of economic introduction.

Table 2 A List of Alien Fish Species in Yunnan

Items with "*"refer to alien fish species not naturally distributed in Yunnan; items without "*"refer to alien fish species naturally distributed in the lower reaches of some rivers in Yunnan but introduced from other provinces into the mid-upper reaches of rivers in Yunnan.

3 Spreading means of alien fishes at various aquatic ecosystems

All Yunnan alien fishes were introduced with an economic approach intentionally or unintentionally. After they had entered various water bodies in Yunnan, they invaded and spread further into various aquatic ecosystems by the following means.

3.1 Alien filter feeders are mainly Hypophthalmichthys molitrix, Aristichthys nobilis, Neosalanx taihuensis, etc. Once they had entered lakes and reservoirs, they rapidly spread to mid-upper layers of lakes and reservoirs competing for an ecological niche with indigenous filter feeders (such as Racoma taliensis and Cyprinus megalopththalmus in Erhai Lake, Anabarilius albrunops and Anabarilius polylepis in Dianchi Lake, Anabarilius grahami in Fuxian Lake, etc.) and resulted in a rapid decline in population of indigenous filter feeders.

3.2 The grass carp Ctenopharyngodon idellus is the main alien phytophagous fish species. After they had entered lakes and reservoirs, they dispersed along shallow water and competed with indigenous phytophagous fishes (such as Spinibarbus yunnanensis in Fuxian Lake of Yunnan) for food and resulted in the over-consumption of aquatic grasses. The ecosystem where indigenous fries live along shallow water had changed and the reproduction of indigenous fish species suffered great impacts.

3.3 Alien benthic omnivorous fishes mainly include Cyprinus carpio, Pseudorasbora parva, Hypseleotris swinhonis, Ctenogobius giurinus, Ctenogobius cliffordpopei, etc. Once they had entered lakes and reservoirs, they first occupied shallow waters and then spread into deep water or go up to the source of the upper tributaries, presenting a continuous distribution pattern. However, shallow waters and upper tributaries are just the spawning places for the indigenous fishes. Alien Pseudorasbora parva, Hypseleotris swinhonis, Gobies devour large quantities of indigenous fishes' spawns and endangered the survival of indigenous species.

3.4 There is a notable difference in spread means between alien fish species in the rivers and in the lakes and reservoirs. Those in rivers are mainly benthic omnivorous fishes. They usually dispersed from upper reaches to mid-lower reaches after they had entered the rivers. Most of them gathered in slow flows, and few were distributed in rapid flows. The disjunct distribution pattern is distinctly different from the continuous distribution pattern in shallow areas of lakes and reservoirs. Alien fish species have more effect on those indigenous species lived in slow streams such as Barbinae, but less effect on those mainly distributed in rapid streams such as Sisorid catfishes and hill-stream loaches, etc., because the majority of alien fishes prefer the slow flows and streams.

4 The biological characters of alien fish species

There is an evidently difference between economic alien species and non-economic alien species. The common biological characters of economic alien fishes such as "the four major domestic carps" and Cyprinus carps are as following: large body size, long life cycle, slow spawn speed and most species disable to spawn naturally in Yunnan water bodies. However, Neosalanx taihuensis and Protosalanx hyalocranius are two exceptions. Though they have high economic value, they are small-sized fishes.

The common biological characters of non-economic alien fish species are as following: small body size, short lifecycle, rapid spawn speed and the typical "R" survival strategists in terms of ecology. Those species such as Pseudorasbora parva, Hypseleotris swinhonis, Gobies, ect. are typical. They usually become the dominant fishes in water bodies for their strong reproduction ability and large population size.

Both economic and non-economic alien fish species came from the mid-lower reaches of rivers with the background of high competition pressure. These alien fish species usually have higher viability and more predominance than the indigenous species, because their original distribution areas are rich in terms of fish species and high in interspecies competition pressure.

5. Influence Degree and means of alien fish species on indigenous fish species

Alien fishes are a major threat to Yunnan indigenous fishes. There are four major threats to Yunnan indigenous fishes: the invasion of alien fish species, land reclamation from lakes, irrigation engineering and over fishing, which are the main factors in other countries as well. £¨Diamond, 1989; Miller et al., 1989£©. Among the four major factors, to what degree do the alien fishes influence the indigenous fishes in the appointed areas? It's a difficult question in the study on conserving biodiversity all over the world. The data on fish population changes in lakes of northwestern Yunnan may provide some useful approaches to answer the question.

Erhai, Chenghai and Luguhu Lakes are the major lakes in northwestern Yunnan. A total of 34 indigenous fish species have been identified in the three lakes (19 species in Erhai Lake, 15 species in Chenghai Lake and 5 species in Luguhu Lake). Our data from field surveys indicate that among numerous factors leading to the decline and endangering indigenous populations of fishes in northwestern Yunnan lakes, the spread of alien fish species, land reclamation from lakes, irrigation works and over fishing are the major threats. Which is the main factor in the four threats? According to the data in the Table 3, 4, 5, the rapid declines in indigenous fish populations occurred subsequent to the introduction of alien fish species. The outputs of indigenous fish species in Erhai Lake were generally stable before these introductions. Since the introduction of "the four major domestic carps" at the beginning of the 1960s, some non-economic alien fish species such as gobies and Pseudorasbora parva were unintentionally brought into Yunnan by mixing with economic fish fries. Some indigenous fishes such as Racoma taliensis suffered the first impact with rapid declines in output by about 50-1000 times. Simultaneously the increase in fishing ships and fishing intensity by only about one time. At the same duration, almost no activities in land reclamation from lakes and dam building happened and had less effect on the degradation of habitats (Table 3). Until the middle of the 1980s, the population of non-economic alien fish species such as gobies and Pseudorasbora parva had an evident decline, while the output of indigenous fish species had increased. With the introduction of Neosalanx taihuensis at the end of 1980s and had some output at the beginning of 1990s, the indigenous fishes in the lake suffered the second impact and almost became extinct (Table 3).

The change of fish output in Luguhu Lake verified the same correlation between indigenous fishes and alien fishes. Before 1973, fish output of Luguhu Lake was always about 200-250t and the product was the snow trout Schizothorax, which is endemic to Luguhu Lake. With the introduction of Mylopharyngodon piceus, Cyprinus carpio, Carassius auratus auratus, etc. in 1973, large quantities of Pseudorasbora and gobies were brought into the lake and devoured lots of the snow trout spawns on the gravel in shallow water. These alien fishes greatly impacted indigenous Schizothorax fishes. Since 1974 the output of the snow trout had declined conspicuously (Table 4) while no devastating activities such as land reclamation from lakes and dam building took place.

The change of fish resources in Chenghai Lake indicated the similar correlation. Before 1989 indigenous fish output of Chenghai Lake was about 500t (Distoechodon 30%, Cyprinus 20%, Erythroculter 15%, Anabarilius 20% and the others 15%). Although the introduction of Cyprinus carpio and Carassius auratus auratus in 1989 did not have great impact on indigenous fishes, the output of indigenous fishes began to decline sharply just after the introduction of Neosalanx taihuensis in 1989 and had output in 1992 (Table 5).

The above data indicated that alien fish is the major threat to indigenous fishes among the 4 main threats (alien fish species, land reclamation from lakes, irrigation works and over fishing).

Table 3 Output change of indigenous fishes in Erhai Lake

Table 4 Correlation between fish resources in Luguhu Lake and their relative environment factors

Table 5 Changes of fish outputs in Chenghai Lake

Degree of impact from alien fishes on indigenous fishes. Compared to the 432 indigenous fish species, alien fishes have only 34 species with a ratio of 12.7:1 and don't dominate in the number of species. However, in lakes and reservoirs the number of alien fish species and their populations dominate absolutely and to some degree contribute to the fish output (Table 6), while most indigenous fishes are endangered. The output change of indigenous fishes in Dianchi Lake is a typical example. According to our survey in 2000, there are only 5 indigenous species in lakes: Carassius auratus auratus, Sinocyclocheilus, Anabarilius albrunops, Yunnanilus plenrotaenia, Misgurnus. The remaining 19 indigenous fish species have disappeared from the lake. Only Carassius auratus auratus has some output among the 5 existing indigenous species. Since the end of the 1960s, the fish products had been gradually taken up by 29 alien fish species. In the 1950s the fish output in Dianchi Lake increased from 300t to 1,050t by 1959 and are all indigenous fish species (Anabarilius albrunops, Anabarilius polylepis and Xenocypris yunnanensis 50-70%, the remaining part includes Cyprinus carpio, Carassius auratus auratus, Silurus asotus, Sinocyclocheilus, Channa argus, etc.). This increase is yielded from increasing fishing intensity, increase of engine-boat-dragged trawling nets and sailboat-dragged trawling nets. "The four major domestic carps" were introduced in 1958 including 350,000 large-sized fries such as Hypophthalmichthys molitrix, Aristichthys nobilis and Ctenopharyngodon idellus and 880 small fries. In the 1960s the output of indigenous fish species decreased evidently because more domestic carps were introduced and the aquatic grass had declined. In addition, with large-scale land reclamation from lakes, the area of Dianchi Lake decreased from 474,000mu to 450,000mu and the area of Caohai (the northern end of the Lake) decreased from 2,7400mu to 1,1500mu. The fish product in Dianchi was 1500t in 1964 and reached 5500t by 1969. With the growing output of "the four major domestic carps" and declines in aquatic grass, the output of indigenous fishes and water quality decreased distinctly, while non-economic alien fish species such as Pseudorasbora parva and gobies increased. Anabarilius albrunops, Xenocypris yunnanensis declined in their output. Anabarilius polylepis, Silurus asotus, Pseudobagrus medianalis and the shrimp Caridina dianchiensis are endangered and hard to find. Along with the entry of "the four major domestic carps", the introduced shrimps (Macrobrachium nipponense and Palaemon modestus) and the small fish Hyporhamphus intermedius began to form populations. The population of two shrimps (Macrobrachium nipponense and Palaemon modestus) and the Crucian developed very fast in 1970s when the phenomenon of more shrimps and less fish took place during 1973-1983. During 1971-1974, the ratio of alien fishes to total output were as follows: non-economic alien fishes such as Pseudorasbora and gobies accounted for 32.4-52.56%, Cyprinus 0.17-0.2%, Ctenopharyngodon idellus 0.04-0.85%, Hypophthalmichthys molitrix and Aristichthys nobilis 1.0-17.8%; the ratio of indigenous fish species to total output were as follows: Anabarilius and Xenocypris yunnanensis accounted for 35-64%; Carassius 0.7-2.6%. In 1975 indigenous Anabarilius and Xenocypris yunnanensis 63%, Carassius auratus auratus 13.8%, alien Ctenopharyngodon idellus, Hypophthalmichthys molitrix and Aristichthys nobilis 5.7%. Until 1978, indigenous Anabarilius and Xenocypris yunnanensis accounted for 13.0%, Carassius auratus auratus 59.4%, alien Ctenopharyngodon idellus, Hypophthalmichthys molitrix and Aristichthys nobilis 17.9%. Neosalanx taihuensis was introduced at the beginning of 1980s from Taihu Lake into Dianchi Lake and largely produced in 1982. At the same time Carassius auratus auratus maintained a high output, while the shrimp output gradually declined since 1983. Therefore the pattern of one big-sized Carassius auratus auratus and two alien small-sized Neosalanx taihuensis and shrimp appeared in the 1980s. At the same duration, except for Carassius auratus auratus, Sinocyclocheilus, Anabarilius albrunops, Yunnanilus plenrotaenia, and Misgurnus, the remaining 20 indigenous fish species disappeared from Dianchi Lake. By the 1990s, the population of alien Cultrichthys erythropterus became dominant in Dianchi Lake and at the same time the output of Neosalanx taihuensis declined. The output of Cultrichthys erythropterus was 500t in 1990 and up to 2000t by 1992.

The output change of indigenous fishes in Fuxian Lake is similar to that in Dianchi Lake. Anabarilius grahami was always the main fish product in Fuxian Lake, generally accounted for 80% of the total output (Table 7). Some non-economic alien fish species such as Pseudorasbora parva and gobies entered into Fuxian Lake while "the four major domestic carps" were introduced from other provinces in the middle of the 1960s. However, alien fish outputs were lower all the time and not greatly impacted the indigenous fishes. This phenomenon didn't change greatly until Neosalanx taihuensis was introduced and produced in the 1980s. The output of Neosalanx taihuensis was 7.5% of the output of Anabarilius grahami in 1987. However in 1988, the output of Neosalanx taihuensis was 180t and that of Anabarilius grahami was 190t. Until 1992, the output of Neosalanx taihuensis accounted for 77.4% of the total output and 5.6 times of the output of Anabarilius grahami.
Alien fish species influenced indigenous fish species to different degrees in terms of water quality and geographical location. The indigenous fish species in lakes and reservoirs suffered more impacts than those in rivers. More impacts were suffered by those indigenous fish species in eastern river systems (Jinshajiang River, Nanpanjiang River and Yuanjiang River), than those in western river systems (Lancangjiang River, Nujiang River and Irrawaddy River).

Table 6 Fish output changes of some Yunnan Lakes

Table 7 Fish output changes in percentage in Fuxian Lake

Impact means from introduction on indigenous fishes. Alien fish species have three impacts on indigenous fish species: devouring spawns of indigenous fishes, competition for food with indigenous fishes and a change of fishing methods. Take Erhai Lake for example, gobies and Pseudorasbora parva which were brought into Erhai Lake along with introduction of "the major domestic carps" in 1960s mainly lived in the shallow water and devoured indigenous fish spawns laid in shallow water, resulting in a rapid decline of the population of indigenous fish.. With the decline in population of non-economic alien fishes in the middle of the 1980s, the indigenous output increased again. This verifies that the main cause of the decline in indigenous fish species is that alien fish devoured indigenous fish spawns. From the point of output change in Erhai, Chenghai and Fuxian lakes after the introduction of Neosalanx taihuensis, filter feeder Neosalanx taihuensis competed for food, not only with indigenous fish larvae, but also with the indigenous filter feeders such as Culter mongolicus elongatus, Cyprinus megalophthalmus, Cyprinus longipectoralis, Cyprinus pellegrini and Anabarilius grahami. These are the best examples to demonstrate the food competition between alien fishes and indigenous fishes. When Neosalanx taihuensis entered upland lakes, the fishing methods changed and trawling nets were used in large numbers. Trawling nets caught not only Neosalanx taihuensis, but also indigenous fish larvae and fries in large quantities. According to our pilot surveys in Fuxian Lake during 1989-1990, there were 200-350 individuals of fries of Anabarilius grahami in 1kg catch of Neosalanx taihuensis. This demonstrates that the new fishing methods also threatened indigenous fish species. The introduction of alien fish species resulted not only in food competition between alien and indigenous fishes, but also the change of fishing methods, which finally rendered the decline in population size and endangered status of indigenous fish species.

6 Management of alien fish species and protection of indigenous fish species

To relieve the impact of alien fishes and protect indigenous fish resources, it's necessary to take action in fishery management, aquatic biodiversity conservation, strictly control alien fish species as well as aquaculture and the release of indigenous fish fries.
6.1 Improve fishery management. It should focus on controlling fishing intensity, setting rational catches and sanctuary time, forbidding illegal fishing, establishing sanctuary areas, etc.
6.2 Strictly control the introduction of alien fish species. The introduction of alien fish species should be deliberately dealt with or strictly prohibited, due to the competition between alien and indigenous fish species for living space, food and reproduction sites.
6.3 Set the lowest water level in lakes. Stable water level is the precondition of keeping the balance of the lake ecosystem and a guarantee in exerting multi-function of lakes. The fluctuation of water level has a great effect on the areas of shallow water along lakes (sometimes even rendered disappearance of some important areas for indigenous fishes to spawn and look for food) and threatened the reproduction of indigenous fishes. Most Yunnan upland lakes are enclosed or half-enclosed. Water storage is mainly static storage, while dynamic storage is very limited. Therefore water level control is very important.
6.4 Protect shallow waters along banks. Shallow waters along banks are an important element in lake ecosystems. These areas are not only rich in aquatic grass, benthic animals and planktons, but are also the spawning and food-seeking places for indigenous fishes. So any activity of reclamation and invasion of shallow waters should be strictly prohibited.
6.5 Control the population of non-economic alien fishes. Due to the great impact of non-economic alien fishes such as Pseudorasbora parva and gobies on indigenous fishes, the use of some special fishing gears (such as feeding cage) should be encouraged to catch these alien fishes.
6.6 Artificial proliferation of indigenous fishes. Because of the importance and a small population of indigenous fishes, the artificial cultivation and release of indigenous fish fries is the best measure to protect indigenous fishes. There is a successful experience. In 1999, we successfully hatched and released the fries of Anabarilius grahami in Fuxian Lake (released 6,000,000 fries in 1999 and 5,000,000 fries in 2000), which has had a positive effect.

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