4.1. Species Composition
Gehu Lake is a shallow lake, in which the fish community composition is dominated by sedentary fish and influenced by artificial fishing and pen culture to a great extent. According to the survey in the 1950s, 12 orders, 21 families, and more than 60 fish species were living in Gehu Lake [
28]. Nevertheless, only thirty species of fish in seven orders and nine families were reported in 2008 [
7] and, in 2017–2018, a total of thirty-six species of fish in four orders and eight families were collected in Gehu Lake [
29]. This survey was conducted for the first time after all net enclosures were removed in 2019, and a total of fourty-four species of fish in seven orders and ten families were collected. In addition, an alien species (
Cirrhinus mrigala) and a hybrid species (
Cyprinus carpio ×
Carassius auratus) was detected. The additions consist mainly of migratory fish and include
E. bambusa, which has not been surveyed previously but has become common-specific at this time. The number of fish species increased, as that of carp increased from 26 to 34, and accounted for a greater share of the total. Fish species composition and quantity could directly reflect the change in characteristics of the fish community. After the removal of the net enclosure, a significant change in fish species number and composition occurred in Gehu Lake.
The introduction and use of non-native species play one of the most critical roles in the rapid development of the aquatic industry [
30]. The majority of non-native fish are introduced voluntarily for the aim of fisheries development, and they play an important role in guaranteeing food security [
31]. However, non-native fish have become a widespread hazard to native fish across the world, and measures are needed to manage invasive fish while safeguarding native species.
Gehu Lake is located north of the Yangtze River, east of Taihu Lake, and west of Changdang Lake. The well-developed network of water systems and intensive human activities, coupled with the high dispersal ability of alien aquatic animals, result in aquatic ecosystems that are vulnerable to disturbance by invasive alien species.
C. mrigala was discovered as an alien species in Gehu Lake for the first time in this survey, which is native to India, Bangladesh, and other locations, and its introduction into our nation was first used as a food fish [
32]. When the consumer market shrank,
C. mrigala was employed as bait for carnivorous species in the southern area. In recent years, due to abandonment and escape during the breeding process, a large number of
C. mrigala have entered natural waters and flourished in the lake, competing for food and space with indigenous fish and causing long-term harm to the lake ecosystem [
33]. Because Gehu Lake lies closer to a residential area and the northern part is a parkland scenic region,
C. mrigala is more likely to be the product of random abandonment by the nearby population. Indeed, there are numerous cases of alien fish invasion. For example, the number of alien fish species in the Dongguan section of the Dongjiang River, one of the main streams of the Pearl River system, has been increasing year by year and, in 2015, Tilapia and
C. mrigala became the dominant species of fish resources, with entry routes including blind release, farming escape, and random abandonment [
34]. The survey also included the tetraploid hybrid fish species
C. carpio ×
C. auratus, which was created by the Institute of Aquatic Biology of the Chinese Academy of Sciences and has some commercial significance. The investigation discovered that
C. carpio ×
C. auratuss was from the northern part of Gehu Lake, the majority of which are park scenic areas that are specially set up to release aquatic animals in the water in the scenic area. Therefore, there is a possibility of blind release by local residents in the case of not understanding the type of fish, the removal of the net enclosure to further expand the living space and, to some extent, to increase the survival rate of released fish. While the introduced alien fish meet the protein demand and boost the market economy, the invasion of a small portion of alien fish can cause some damage to the global aquatic ecosystem, especially after the removal of net enclosures, which on the one hand promotes the enhancement of habitat connectivity of local species but, at the same time, increases the potential for the movement and spread of non-native species [
35].
The removal of the enclosure nets has the potential to increase the risk of invasion of exotic species (farm escapes and anthropogenic releases) into Gehu Lake. China is the largest freshwater aquaculture country in the world, and many of the areas are set near natural waters. The risk of invasive alien species in many lakes in China has increased significantly since 2020 [
36], probably due to the failure to regulate farming activities and the removal of seine nets in the corresponding natural lakes, which has implications for other lakes in terms of the management of pen culture. So, it is necessary to strengthen the scientific management of exotic fish in fisheries management measures to promote the development of aquaculture and the ecological safety of lakes, as well as to do a good job of popularizing alien species to the surrounding residents and farmers.
4.2. Dominant Species
Whether it is cage culture, net culture, or pen culture, there is an aggregation effect of fish due to the accumulation of residual bait and feces during the culture process, which enriches the nutrients and thus attracts detritivorous and zoobenthivorous fish to feed [
37]. As pen culture encroaches on the habitat and food resources of other fish species, when the net enclosure is removed, this part of the encroached spatial and nutritional niche is released, which inevitably triggers competition from other fish species. From the results, the IRI of carnivorous fish such as
C. nasus,
C. dabryi, and
C. mongolicus, and herbivorous fish such as
M. amblycephala and
P. pekinensis increased, indicating that they won the competition for spatial and trophic niches. This indicates, to some extent, that the removal of enclosure nets improved the survival environment of these fish and expanded their feeding range, and also improved the protection of spawning grounds in the natural environment which was beneficial to the recovery of their populations.The IRI of baitfish such as
T. swinhonis,
H. bleekeri,
P. fulvidraco, and
P. nitidus decreased, indicating that they were at a disadvantage in the competition.
According to R/K selection theory [
38], after the net enclosures were removed, the short-cycle growing fish, such as
C. nasus, had quicker growth and greater reproductive capacity in the early stage and faster resource recovery in the later stage, whereas the long-cycle growing fish had delayed recovery and faster recovery in the latter stage due to a shortage of food in the first stage. With the change in diet from zoophytoplanktivores to carnivorous, the width of the food niche has become wider. At the same time, the fast growth and high fertility of the
C. nasus, coupled with the implementation of the retreat policy, has led to an explosive increase in the number and size of
C. nasus in the last two years, which has reduced the survival space of other small fish.
C. dabryi and
C. mongolicus are top predators in aquatic ecosystems and, in the absence of fishing, populations expand and feed heavily on smaller fish, leading to declines in the numbers of
T. swinhonis, and
H. bleekeri. After the removal of the net enclosure in Gehu Lake, the situation of the quickly expanding small fish, such as
C. nasus, will encroach on the living area of other fish. This is not conducive to the stability of the lake ecosystem and should be governed by reasonable management and control.
O. rubicundus and
P. eupogon were not discovered in this survey when the species of the captures were compared to the survey data from 2017 to 2018. O. rubicundus is an estuary fish that spends the majority of its life cycle in environments that are semi-saline, have sand and gravelly bottoms, and have clear water [
39]. The removal of the net enclosure temporarily disrupted the substrate, which had an impact on the water quality and reduced their populations [
40,
41]. The Pelteobagrus genus is a native fish of Gehu Lake, its natural production was initially low, and the IRI of both
P. fulvidraco and
P. nitidus declined when the net enclosures were removed because of the disappearance of the cluster effect.
P. eupogon, which is categorized as vulnerable on the Red List of Chinese species [
42], originally had a tiny population in Gehu Lake, and the population fell when the net enclosure was removed, most likely owing to increased competitive pressure for food and greater vulnerability to predation by natural predators [
43], which made it difficult to catch.
It is worth noting that 37
E. bambusa were captured in this survey, yet none of which were collected before removal in 2017–2018. Compared with other fish species, the average weight of
E. bambusa was higher, resulting in an IRI greater than 100 (IRI = 109.35). Additionally, this species was captured in 11 of the 15 sample sites, indicating the wide distribution in Gehu Lake. Hence, one can see that, to some extent, the removal of the net enclosure contributes to the existence and growth of the
E. bambusa population.
E. bambusa, also known as “water tiger”, is a big fierce fish species that feed on other fish. In the past, it was referred to as a pest in aquaculture and needed to mainly eliminate in natural fish fry production [
44]. However, as a fierce carnivorous fish,
E. bambusa occupies a high trophic ecological level in the lake, and benefits in eliminating the sick and weak individuals and inhibiting the growth of small fish species within the range of moderate quantity, which is instrumental in maintaining the stability of the water ecosystem [
45]. Nevertheless, when exceeding a certain amount, it may do harm to the fish community structure and fishery resources in the lake, which necessitates continuous investigation and reasonable control.
4.3. Community Structure and Stability
The cluster analysis from 2021 to 2022, which revealed that sites S14, S15, S16, and S22 formed a more aggregated area with higher similarity in fish community composition while the other areas had a lower similarity, explained that the structure of the fish community in this aggregated area differed from other lake areas. At the same time, the ABC curve data suggest that the region was little disturbed in both survey periods, indicating that this area was less disturbed by humans and the overall condition is more stable. According to the relevant information, the National Ministry of Agriculture designated this region as the Culter national aquatic germplasm resources protection zone of Guhu Lake in order to accomplish the conservation and wise use of Gehu Lake fish genetic resources and their survival habitat. The main species to be protected are
C. alburnus,
C. dabryi, and
C. mongolicus. Because of their early establishment in 2009 and the commencement of the retreat policy in 2018, they has been allowed a longer natural recovery period. The three species of Culter are all carnivorous fish.
C. dabryi and
C. alburnus mainly feed on
C. nasus, while
C. mongolocus feeds on
P. parva,
H. leucisculus, and
T. swinhonis. The removal of seine nets has improved the living environment of Culter populations to a certain extent and expanded their feeding range; the growth of “predator” populations has led to a reduction in “prey” fish populations (
Figure 4). However, the decline in the number of
C. alburnus and
C. erythropterus may be related to the large increase in the number of
E. bambusa, which also prey on other small fish [
20]. Additionally, compared to other parts of Gehu Lake, the northern area of Gehu Lake’s shoreline is mostly made up of urban and natural parks, which not only have the capability to enhance water quality and ecological control, but also provide a lesser risk of external pollutant intrusion [
46]. Compared to the study in 2017–2018, H′-index grew in the northern reserve, indicating that the structure of the fish community has become more complicated [
23]. The fish community structure of the northern section of the Gehu Lake reserve exhibits a better improvement condition and its structure is more stable through a series of fisheries management actions, such as the removal of net enclosure and retreat.
The number of small fish in the pen culture area in the southern region of Gehu Lake decreased significantly after the removal of the seine, which is inconsistent with the results of Gu et al. [
13] in East Taihu Lake, which may be related to the number of carnivorous fish. The southern seine area of Gehu Lake in 2017–2018 (S1, S2, S3, S4, and S5) focused on the culture of
H. molitrix and
A. nobilis, so they occupy a larger spatial niche. Carnivorous fish such as
C. dabryi,
C. alburnus, and
C. erythropterus have reached the level of important species. After the removal of the seine and before
H. molitrix and
A. nobilis that occupied the spatial niche were released [
47], the predation range of carnivorous fish expanded and the number of fish species that can be predated increased, In addition, the original seine area is rich in bait residue, which attracts omnivorous and carnivorous fish to congregate here. Thus, the number and IRI of fish such as
C. erythropterus,
C. dabryi,
C. mongolocus,
C. carpio, and
E. bambusa increased. With the growth of the carnivorous fish population such as
T. swinhonis,
P. simoni,
H. bleekeri, and
H. leucisculus, which should have increased in number, have declined because they were predated. Gu et al. mentioned that the bait residues in the original seine culture area attracted omnivorous fish to gather, and it so happened that the dominant species composition of fish in the area was
C. auratus,
H. leucisculus,
Acheilognathus macropterus,
P. fulvidraco,
Pseudorasbora parva,
A. nobilis, and
C. nasus, while the number of carnivorous fish was poor.
Table 6 shows that the biodiversity, in general, decreased after the removal of the seine, which may be related to the fluctuation of water quality due to the removal of the seine [
48] and also to the competition among fish populations for new space [
49,
50].
After the removal of the seine in 2019, the local government in the central area of Gehu Lake (S11, S12, S13, and S21) established “the fish-controlled algae“ ecological restoration area with floating net interception facilities. The southeast wind in the summer will make cyanobacteria blow to the northern part of Gehu Lake. Thus, the central lake area will establish an ecological restoration area, mainly by non-baiting farming of
H. molitrix and
A. nobilis, to control the number of cyanobacteria, reduce the degree of eutrophication of the water body, and achieve the purpose of repairing and improving water quality.
H. molitrix and
A. nobilis from the southern seine culture area are now transferred to the central ecological restoration area of the lake, and the spatial niche of the former seine culture area ecosystem in the south is released, while the new
H. molitrix and
A. nobilis populations seize the spatial niche of the central ecological restoration area, making the spatial niche of other fish populations in the area shrink. The proliferation of
H. molitrix and
A. nobilis in the seine may have an impact on the overall composition of the fish community. Zhao et al. used stable isotope techniques to conclude that the excretion of
H. molitrix and
A. nobilis in the Meiliang Bay seine in Taihu Lake caused significant changes in sediment composition and that the fish community inside the net may have higher stability than outside the net [
36]. The stocking of
H. molitrix and
A. nobilis will certainly have some impact on the ecosystem of the Gehu Lake ecological restoration area, so more in-depth research is subsequently needed.
According to the ABC curve for the entire lake, the fish community structure is in a moderate anthropogenic disturbance condition in 2021–2022, and is highly disturbed and less stable compared to 2017–2018. When the fish community structure is disrupted at a larger level, the composition of fish species will eventually be dominated by small, fast-growing species [
38]. As shown in
Table 3, the increase in IRI of
C. nasus and other small fish indicates that the small fish have exhibited a growing trend in the lake in recent years, while their H′ lowers, implicitly indicating that the lake is now in a weak stable condition. The decrease in both the W value of the ABC curve and the value of the H′-index for the remaining lakes revealed that the lakes were not as stable in 2021–2022 as they were prior to the entire removal of the net enclosures in 2017–2018. The lakes went from a more stable to a weakly stable condition after the net enclosures were completely removed. Lakes can achieve a certain degree of stability owing to human management involvement during pen culture, but the removal of net enclosure facilities influences the living environment and spatial distribution of fish, eliminating human management to enable natural recovery, and thus a weak stable state recovery phase is normal.
After removal, changes in fish habitat led to changes in biodiversity, while related studies show that changes in habitat will have an impact on biodiversity. According to Edge et al., changes in habitat were to blame for the decline in the α-diversity of fish populations in rivers [
51]. Dam building, similar to the net enclosure, can obstruct the passage of fish disrupt the environment of fish populations, resulting in a decline in biodiversity [
52]. Subsequent ecological monitoring of Gehu Lake will continue, as well as strengthening fish habitat protection, thus maintaining the balance of the ecological system of the lake.