The Shuikou Reservoir is a narrow and deep riverine reservoir located in the middle reaches of the main stream of the Minjiang River. It has a total capacity of 2.6×10⁹ m³, making it the largest reservoir in Fujian Province. Following the water storage in March 1993, C. zillii was once farmed on a large scale around the reservoir. Escape of redbelly tilapia into the reservoir occurs from time to time due to poor culture management. In recent years, tilapia farming has ceased in the vicinity of the reservoir. However, the population of C. zillii within the reservoir has exhibited a notable increase year by year. In certain areas of the reservoir, C. zillii have become a dominant taxon (Cai 2021).

The sampling site (26°40'N, 118°49'E) is located in Zhanghu Town, Yanping District, Nanping City, Fujian Province (Figure 1). The research was conducted with the permission of the Fisheries Department of the Bureau of Agriculture and Rural Affairs of Nanping City, Fujian Province. During the study period, which spanned from March 2023 to February 2024, 40–200 live specimens of C. zillii were collected from the reservoir in the middle of each month, resulting in a total of 1,041 samples. The sampling gear employed was a three-layer drift gillnet, with a length of 30 m, a height of 1.5 m, and a mesh size of 3 cm, 5 cm, and 7 cm, respectively.

Figure 1. 

Map of the location of Shuikou Reservoir (c) in Fujian Province (b) of China (a). The sampling site for Coptodon zillii was indicated with a red dot.

Immediately following capture, the samples were anaesthetised with MS-222 (100 mg/L), after which routine biological measurements were conducted on site. These included standard length (accurate to 0.1 cm) and weight (accurate to 0.01 g). Gender of specimens were distinguished using the morphology of the genital papilla and macroscopic examination of gonads. Gonadal development was divided into six stages (I–VI) according to the criteria of Brown-Peterson et al. (2011), which were based on the anatomical morphology, colour, volume, and histology of the gonads. The gonad weight (ovary, spermatheca) (accurate to 0.01 g) and eviscerated weight (accurate to 0.01 g) were determined. The classification of oocyte maturity was conducted in accordance with the criteria established by Coward and Bromage (2000). The ovarian maturity stage was determined by the most advanced oocyte present in the ovary. The ovaries at stages IV and V were subsequently preserved in a 5% formaldehyde solution to obtain absolute and relative fecundity.

The scales of specimens were employed as the material for age identification. Five to ten scales of each specimen were taken from between the lateral line and first dorsal fin. They were cleaned, fixed between the two glass slides, soaked with drops of alcohol, dried and preserved. And the total scale radius and the radius of each annulus were measured under a dissection microscope, with annuli identified using criteria of Carbonara and Follesa (2019).

The length-weight relationship was established using the equation W = aL^b (Froese 2006), where W is the measured weight (g), L is the measured standard length (cm), and both a and b are constants. Deviation from isometry (b = 3) for the calculated b value was ascertained by the student’s t-test. In the absence of a statistically significant deviation, the fish was deemed to exhibit isometric growth. Conversely, when a statistically significant deviation was observed, the fish was classified as anisometric (with positive anisometric growth when b > 3 and negative anisometric growth when b < 3). The von Bertalanffy growth function (VBGF) was employed to delineate the growth characteristics of C. zillii (Coulson and Wakefield 2022), with the following expression: L_t = L_∞[1 - e^{-k (t-t0)}]; W_t = W_∞[1 - e^{-k (t-t0)}]^b. The growth rate of standard length and body weight were described using the following equations: dL/dt = L_∞ke^{-k (t-t0)}, dW/dt = bW_∞ke^{- k (t-t0)}[1 - e^{-k (t-t0)}]^b-1. The acceleration of growth in standard length and body weight were described using the following equations: d²L/dt² = -L_∞k²e^{-k (t-t0)}, d²W/dt² = bW_∞k2e^{- k (t-t0)}[1 - e^{-k (t-t0)}]^{b- 2}[be^{-k (t-t0)}-1]. Age at inflexion point was calculated using the following formula, t_i = lnb/k+t₀. t, L_t, and W_t represent the age, mean standard length at age t, and mean body weight at age t, respectively. L_∞ and W_∞ denote the asymptotic standard length and asymptotic body weight, respectively. k and t₀ represent the growth coefficient and the theoretical onset age point, respectively. The value of k is typically employed to reflect the growth rate of the population. Branstetter (1987) classified the growth coefficients into three intervals (0.05 to 0.1,0.1 to 0.2,0.2 to 0.5), which represent the slow-growth type, the uniform-growth type, and the fast-growth type, respectively.

The monthly gonado-somatic index (GSI) was used to reflect the degree of gonadal development, and the intensity of reproductive activity was assessed based on the monthly trend of the GSI, estimated by the formula GSI (%) = 100×(W_g/W_E), where W_g and W_E are the gonadal weight (g) and the eviscerated weight (g), respectively. The fecundity of C. zillii was estimated by the standard gravimetric method. We obtained and weighed three systematic subsamples (5–20% of total ovary weight) from the anterior, middle, and posterior sections of the ovary. And the fecundity was calculated according to the following formulas: F = nW_O/w, F_w = F/W_E, F_L = F/L. The variables are defined as follows: F is absolute fecundity; n is the number of eggs counted; W_O is the weight of the ovary (g); w is the weight of the subsamples of the ovary (g); F_w is the relative fecundity to body weight; W_E is the eviscerated weight (g); F_L is the relative fecundity to standard length; and L is the standard length (cm). Ten ovaries on stage IV were selected for analysis of the diameter of the eggs. Eggs were extracted from the anterior, middle, and posterior sections of the ovaries, which had been fixed in a 5% formaldehyde solution. The eggs were observed and photographed under a dissection microscope. The diameters of eggs were measured under a stereomicroscope using imaging software, with an accuracy of 0.01 mm.

Statistical analysis and graphical representation of the data were conducted in Microsoft Excel 2019, SPSS 27.0.1 and ORIGIN 2021, respectively. The chi-square test was utilised to ascertain whether the sex ratio of females to males was in accordance with the expected 1:1 ratio. Significance of difference was tested using independent samples t-test and one-way analysis of variance, with a significance level α of 0.05.

The entire population consists of individuals from age 1 to age 6, with the highest percentage (78.29%) of younger individuals at 1 years old. Individuals of age 2 constitute the second largest age group, which accounted for 16.81% of the total number of samples. The maximum ages of the female and male samples were age 4 and 6, respectively (Suppl. material 1). The standard length of females ranged from 8.7 cm to 18.7 cm, with a mean standard length of 11.51 cm. The body weight of females ranged from 23.1 g to 308.86 g, with a mean body weight of 66.34 g. The standard length of males ranged from 9.0 cm to 23.0 cm, with a mean standard length of 11.84 cm. The body weight of males ranged from 21.45 g to 463.5 g, with a mean body weight of 68.86 g. The difference in mean standard length between females and males was not statistically significant (P = 0.174, t = 4.327, df = 1039), nor was the difference in mean body weight (P = 0.75, t = 2.992, df = 1039). The male-to-female sex ratio of C. zillii was 1.05:1, which did not deviate significantly from the theoretical value of 1:1 (χ² = 0.617, P = 0.432, df = 1).

A total of 533 males and 508 females were collected for analysis in this study. The standard length and body weight of all samples ranged from 7.5 to 23.0 cm and 15.49 to 463.15 g, respectively. The length-weight relationship for all samples was determined to be W = 0.048*L^2.938(R²= 0.903) (Figure 2a). The length-weight relationship for female and male fish are as follows: W = 0.104*L^2.609 (R² = 0.718) (Figure 2b) and W = 0.053*L^2.861 (R² = 0.837) (Figure 2c). The value of the constant ‘‘b’’ in the length-weight relationship were found to show negative allometry for all samples, females and males, respectively. This is consistent with the negative allometric growth type. The b-value of male samples was found to be significantly greater than that of female samples (t = 3.143, P = 0.00177, df = 506).

Figure 2. 

Relationship between standard length and body weight of Coptodon zillii for overall samples (a), female samples (b), and male samples (c) in Shuikou Reservoir. W and L denote the expected weight and standard length, respectively.

The estimated asymptotic length (L_∞) of C. zillii population (all samples) was determined to be 32.937 cm, while the asymptotic body weight (W_∞) was found to be 1381.010 g (Suppl. material 2; Figure 3a). Among them, the L_∞ of females and males were 31.041 cm and 34.934 cm, respectively (Suppl. material 2; Figure 3b, c). While the W_∞ of females and males were 811.883 g and 1378.825 g, respectively (Suppl. material 2; Figure 3b, c).

Figure 3. 

Theoretical standard length and body weight of Coptodon zillii for overall samples (a), female samples (b), and male samples (c) in Shuikou Reservoir.

The growth rate and growth acceleration equations for standard length and body weight of C. zillii are presented in Suppl. material 2. The inflection age of the C. zillii population was 1.05 years, corresponding to a standard length and body weight of 10.89 cm and 57.58 g, respectively (Figure 4a, d). The inflection age of females was 1.04 years, corresponding to a standard length and body weight of 10.80 cm and 55.34 g (Figure 4b, e). The inflection age of the males was 1.21 years, corresponding to a standard length and body weight of 11.44 cm and 56.33 g, respectively (Figure 4c, f). The growth rate of standard length was consistently positive and exhibited a gradual decline with age. Conversely, the growth acceleration of standard length was consistently negative and demonstrated a gradual increase with age.

Figure 4. 

Growth rate and acceleration of standard length and body weight of Coptodon zillii for overall samples (a, d), female samples (b, e) and male samples (c, f) in Shuikou Reservoir.

Of the 1041 specimens examined, 158 had reached sexual maturity (gonads at stage IV or V) (Figure 5a), of which 54.43% were females. The standard length of the smallest sexually mature female was 8.7 cm, with a body weight of 23.097 g. The standard length of the smallest sexually mature male was 9.0 cm, with a body weight of 28.19 g.

The gonadal weights of females ranged from 0.7 to 15.62 g, with a mean gonadal weight of 1.14 g. The gonadal weights of males ranged from 0.03 to 6.48 g, with a mean gonadal weight of 0.21 g (Suppl. material 1). The gonadal development of C. zillii was clearly cyclic (Figure 5b), and the changes in GSI were all in the form of a single peak (Figure 5b). There was a general consistency between the GSI and the changes in water temperature (Figure 5b). In March, 33.33% of the ovaries were in Stage III and 27.78% were in Stage IV (Figure 5a), with the GSI beginning to rise. In May, 30.20% of the ovaries were in Stage V (Figure 5a), while in June, the percentage of Stage V ovaries reached its highest point (58.62%) (Figure 5a), with a corresponding peak in the GSI. From July to October, there was a gradual decline in both the percentage of stage V ovarian and GSI. In November, the frequency of ovaries in stage V was reduced to zero, although some ovaries remained in stage IV (Figure 5a). From December to February, the majority of the ovaries were in stage I or II (Figure 5a), and the GSI reached its lowest point of the year. The breeding period of C. zillii in Shuikou Reservoir was estimated to span the months of March to November, with a peak observed in June.

Figure 5. 

Annual changes in gonadal developmental stages of female samples (a), gonado-somatic index (GSI) (b) and distribution of egg diameter in stage IV ovaries (c) of Coptodon zillii in Shuikou Reservoir. Temp denotes water temperature. The symbol ♀ and ♂ denote GSI of female and male samples, respectively. Error bars denote standard error.

The relative fecundity to standard length of females (n = 86) ranged from 85.48 to 833.09 eggs/cm, with a mean value of 298.54 eggs/cm (Table 1). The relative fecundity to body weight ranged from 18.69 to 168.22 eggs/g, with a mean value of 59.89 eggs/g (Table 1). And the absolute fecundity ranged from 982 to 13413 eggs, with the mean value of 3854.38 eggs (Table 1). The relative fecundity to body weight was negatively correlated with age as in formula F_W = 68.554 t^-0.369, R² = 0.070 (P>0.05) (Figure 6b), while the relative fecundity to standard length and absolute fecundity were positively correlated with age as in formula F_L = 253.977t^0.373, R² = 0.089 (P>0.05) (Figure 6a) and F = 2685.846t^0.742, R² = 0.278 (P>0.05) (Figure 6c), respectively.

Figure 6. 

Relationship between fecundity [Relative fecundity to standard length (a), Relative fecundity to body weight (b), Absolute fecundity (c)] and age of Coptodon zillii in Shuikou Reservoir.

The mean egg diameter was 1.52±0.45 mm, with a bimodal distribution of egg diameter (Figure 5c), indicating the presence of two types of oocytes in the ovaries of C. zillii. The first type of oocyte makes up 36.57% of the total number of eggs and its diameter varied between 0.25 and 0.70 mm. The second type of oocyte makes up 63.43% of the total number of eggs and its diameter ranged from 1.20 to 2.20 mm. It is hypothesized that this population of C. zillii is characterised by batch spawning.

The age structure of fish populations is an important ecological characteristic, and changes in age structure reflect, to a certain extent, the adjustment of life-history strategies of fish populations in the face of ecological changes (Ma et al. 2024). In this study, the age structure of C. zillii population in Shuikou Reservoir was simple. Individuals of age 1 were the most numerous, accounting for 78.17% of the entire population, and the maximum age was age 6. This is similar to the age structure of C. zillii in Haebaru Reservoir, Okinawa, Japan (another invasion site), where the highest number of individuals was also found at age 1 (accounting for 52.38% of the entire population) and the maximum age was 7 (Ishikawa and Tachihara 2008). However, in one of the sites in its native range, Lake Nozha Hydrodrome, Egypt, individuals of age 1 accounted for only 3.96% of the entire population, while age 2 and 3 were the dominant age groups, with the number of individuals accounting for 67.84% of the entire population (Mahmoud et al. 2013). It can be seen that the population in the invasion site showed obvious rejuvenation compared with the native site, which may be attributed to the fact that the invasive population is still in the initial stage of population spread, the age structure is still not intact. It should be noted that a relatively under-aged population structure indicates that the population of C. zillii in Shuikou Reservoir is in a period of rapid growth, reflecting the strong potential for expansion of the population.

Plasticity in growth traits has been demonstrated in bleak (Alburnus alburnus) (Latorre et al. 2018), pumpkinseed sunfish (Lepomis gibbosus) (Fox et al. 2007), Amur sleeper (Perccottus glenii) (Joanna et al. 2011), and many other invasive fish species during the invasion process. In the present study, the growth coefficient (k value) of the C. zillii population in Shuikou Reservoir was 0.131, which was lower than those in both the native and invasive populations reported in the literature (see Table 2). The wide range of variation in k values (from 0.131 to 0.680) (Table 2) among different populations indicated a large inter-population variability in the growth rate of C. zillii. This is speculated to be the plasticity of growth performance in populations of C. zillii to adapt to different habitats. In addition, the two invasive populations (China, Iraq) all belonged to the medium-rate growth type, whereas the four native populations all belonged to the fast-growth type, which may reflect resource limitation and density-dependent effects acting upon the invasive populations. Similar results have been observed in invasive bighead carp (Hypophthalmichthys nobilis) (Coulter et al. 2018) and Indo-Pacific red lion-fish (Pterois volitans) (Dahl et al. 2019). At the same time, we also suggest that this is a resource allocation strategy of the invasive populations of C. zillii to shift energy inputs towards other physiological functions (e.g. reproduction) by sacrificing growth performance in response to the selection of abiotic or biotic factors in the new environments.

Successful reproductive strategies and their plasticity are crucial during the invasion process for fishes (Garcia-Berthou 2007). Typically, this plasticity involves higher reproductive investment (e.g. higher absolute fecundity, longer spawning seasons, earlier age at sexual maturity, etc.) compared to native populations (Bøhn et al. 2004; Britton et al. 2008; Joanna et al. 2011; Gutowsky and Fox 2012).

The onset of fish spawning and the duration of the reproductive period are usually dependent on temperature and photoperiod. However, some studies have shown that reproductive events in the same water body are not always directly related to these environmental parameters (Gertzen et al. 2016). Some invasive fish species spawn for extended periods of time in invaded water bodies to produce more offspring, thus rapidly expanding population sizes. For example, round goby (Neogobius melanostomus), native to the Caspian and Black Seas, spawns between May and September in their native habitat (Hôrková and Kovác 2014), but spawning time is extended in some invasive populations (Tomczak and Sopota 2006). Valiente et al. (2010) found that plasticity in spawning time gives brown trout (Salmo trutta) a competitive advantage when invading aquatic ecosystems in South America. In the present study, we observed that the spawning period of C. zillii in Shuikou Reservoir extends from March to November, exceeding the spawning periods of native populations from the Damietta tributary of the Nile River (El-Kasheif et al. 2013), the Lake Timsah in Egypt (Mahomoud et al. 2011), and Lake Kinneret in Israel (Ben-Tuvia 1978) in terms of spawning duration. While the spawning periods of C. zillii are highly variable in other invasion sites, for the sake of discussion, we have tabulated the spawning periods of C. zillii populations in the literature (Table 3). Differences in spawning time between population in the Shuikou Reservoir and other populations may depend on different environmental characteristics, such as water temperature, habitat, and food resources. However, we believe that a more important reason may be the plasticity of the reproductive characteristics of the populations. This is because the spawning time of C. zillii populations in two reservoirs (Shuikou Reservoir and Shanmei Reservoir), which have very similar environmental characteristics, also differed significantly. In addition, the average water temperature in Shuikou Reservoir was below 20°C (only 15–18°C) in March when spawning has just begun, and thus the water temperature at the time of the first spawning of females was below the range of water temperatures (22–28°C) reported in most literature (Li et al. 2023). We hypothesised that the invasive population in Shuikou Reservoir exhibits plasticity in the water temperature that is suitable for breeding. It has been reported in the literature (Coward and Bromage 1999) that C. zillii spawn on average every 26 days under favourable environmental conditions and with abundant food. So based on a spawning season of 270 days per year (from March to November), females in Shuikou Reservoir could theoretically lay an average of about 10 batches of eggs per year. The extended spawning period maximises the reproductive potential of C. zillii in the Shuikou Reservoir and rapidly expands the population size.

Life-history theory (Stearns and Koella 1986) predicts that individuals from invasive populations will allocate more resources to reproduction than to growth (Ylikarjula et al. 1999), resulting in maturing at a much younger age compared to individuals from their native range (Philippart and Ruwet 1982). In a study conducted by Russell et al. (2012), it was observed that the age and body size at first sexual maturity of the invasive species of Oreochromis mossambicus in the Kewarra Beach drain in north-eastern Australia was significantly smaller than that of the native population. Fox (1994) conducted a comparative analysis of the life-history characteristics of native and non-native pumpkinseed (Lepomis gibbosus) populations, and observed that the non-native population exhibited earlier maturation and a smaller size than the native population. In the present study, the standard lengths of the smallest sexually mature female and male in Shuikou reservoir were 8.7 cm and 9.0 cm, respectively, and the minimum ages at sexual maturity were 0.44 and 0.41 years old, respectively. In their native range, the standard lengths at first sexual maturity of the female and male in Abu Qir Bay, Egypt, were 9.7 cm and 8.7 cm, respectively (Moharram and Akel 2007). The standard length at first sexual maturity of the female and male in Cross River, Nigeria, were 11.0 cm and 9.8 cm, respectively (Uneke and Nwani 2014). In other invaded sites, for example, in the AL-Swaib Marsh, Iraq, the standard lengths at first sexual maturity of the male and female were 13.2 cm and 12.4 cm, respectively (Qadoory 2012). In the Dongjiang River, China, the standard lengths at first sexual maturity of the male and female were 9.7 cm and 10.0 cm, respectively. The results of the above studies indicate that the standard lengths at first sexual maturity of the invasive populations are generally smaller than those of the native populations. Meanwhile, the standard lengths of the smallest sexually mature female and male in Shuikou Reservoir were observed to be smaller than those in the native and other invasion sites. The reasons for this may be attributed, on the one hand, to differences in genetic and environmental conditions between populations. However, on the other hand, it is more probable that this is an adjustment of reproductive strategies of invasive populations in Shuikou Reservoir to adapt to the environment of their new habitats. This may be a common life-history strategy in invasive fishes, where a higher investment in fecundity in invasive populations is accompanied by a decrease in age and size at first sexual maturity compared to indigenous populations (Amundsen et al. 2012; Gutowsky and Fox 2012; Masson et al. 2016; Masson et al. 2018).

Fish fecundity is a widely used indicator of reproductive capacity in breeding populations, serving as an adaptive compensation to external environmental factors (Roff 1992). The fecundity of many invasive species is regarded as a plastic trait, with females exhibiting trade-offs between the number and size of eggs spawned in response to environmental factors (Marshall and Uller 2007; Allen et al. 2008). The bighead carp (Aristichthys nobilis) and silver carp (Hypophthalmichthys molitrix) that have invaded the Upper Mississippi River in the United States have been observed to exhibit plasticity in fecundity (Lenaerts et al. 2023). The plasticity in fecundity of Cichla kelberi may be a principal factor responsible for its successful colonisation and invasion in the Lajes Reservoir in southeastern Brazil (Soares et al. 2021). The present study revealed that the population of C. zillii in Shuikou Reservoir exhibited a high level of fecundity, with a range of absolute fecundity from 991 to 13,413 eggs and a mean value of 3,809 eggs. For purposes of comparison and analysis, the results of previous studies on the fecundity of C. zillii in the native and invasive sites are summarized in Table 3. C. zillii demonstrated varying degrees of fecundity in different environmental settings, from its native tropical riverine habitat to tropical and subtropical rivers and reservoirs within its invasive range. The variation in fecundity observed among the populations in their native and invasive habitats may be attributed to differences in climatic conditions (e.g., temperature, photoperiod, etc.), age and nutritional status of the individuals, etc. However, this also reflects the ability of C. zillii to adapt to a variety of habitat environments and adjust the level of fecundity, indicating its strong plasticity in reproductive traits. Concurrently, our findings revealed that the absolute fecundity of populations in the invasive sites were superior to those of the populations in the native sites. When these observations were considered alongside the altered patterns of the length at the first sexual maturity (shorter in the invasive site than in the native site) and the time of spawning (longer in the invasive site than in the native site), we postulated that the C. zillii population in Shuikou Reservoir exhibited a high degree of plasticity in reproductive characteristics. To rapidly establish a population in the new environment of the invasive site, it is probable that they will allocate more energy required for early growth and development to gonadal development, thus maintaining a high fecundity of the population. This phenomenon has been observed in a variety of invasive fish species, including the round goby (Neogobius melanostomus) (Hôrková and Kovác 2014) and the bluegill sunfish (L. macrochirus) (Ma et al. 2024).

The life-histories of fish are more flexible than those of thermostats, and often adjust in response to changes in environmental stressors in order to maximise fitness (Nelson et al. 2016). Equilibrium life-history strategies provide a natural advantage to fish as they invade new habitats, but the ability to shift to opportunistic life-history strategies may facilitate the expansion of invasive fish populations (Vila-Gisper et al. 2005; Olden et al. 2006). For example, opportunistic life-history strategies facilitated the successful invasion of the pumpkinseed (L. gibbosus) in Europe (Fox and Copp 2014). A combination of opportunistic and equilibrium life-history strategies was identified as a key factor in the successful invasion of the Shimofuri goby (T. bifasciatus) in Nansi Lake, China (Qin et al. 2020). In this study, population of C. zillii in Shuikou Reservoir exhibited early maturity, a prolonged reproductive period, and high fecundity, which are hallmarks of opportunistic life-history strategies. Additionally, they demonstrated parental care behaviors in an equilibrium life-history strategy. The entire population displayed high reproductive potential and recovery ability, which facilitates a rapid expansion of the population.

Given the extent of the expansion of C. zillii in Shuikou Reservoir, eradication is no longer a viable option. In light of the findings of this study, the following recommendations are put forth as a means of effectively curbing the rapid spread of this species. The first step is to conduct tracking and monitoring of the invasive populations of C. zillii. Advanced technologies such as environmental DNA (eDNA) meta-barcoding are recommended to monitor low-density populations and combined with acoustic telemetry to track dispersal patterns. Long-term monitoring must assess seasonal abundance shifts, particularly in thermally favorable zones (>24°C), where spawning aggregations occur. The second step is to use a variety of measures to effectively control the invasive populations of C. zillii. Regular and intensive fishing and removal of the redbelly tilapia by local people mobilised by the fisheries authorities should be carried out, so as to reduce the size of its population. At the same time, during the breeding season (March to November) of the C. zillii, the fishing intensity is increased in the waters where it nests and spawns to control the number of juveniles. It is recommended that the protection and restoration of native piscivores (e.g., Culter alburnus, Siniperca chuatsi, Siniperca scherzeri) in Shuikou Reservoir be strengthened. Scientific stocking of native piscivore in this reservoir should be put into practice based on a comprehensive resource assessment. This will inhibit the number of eggs and fry of C. zillii and reduce the density of its juveniles through predation and competitive effects. Breeding and releasing of indigenous fish (e.g. Channa maculates) with similar life-history characteristics to C. zillii should be considered. Furthermore, the stocking of large-size indigenous fish into the habitat of C. zillii may result in the reduction of the population size of C. zillii due to interspecific competition for resources. The third step is habitat restoration in waters invaded by C. zillii. A comprehensive assessment of the invaded ecosystem, including water quality analyses, biodiversity surveys, and mapping of C. zillii distribution, must be conducted to determine the extent of the invasion and the specific habitats degraded. Displaced or threatened native species should be reintroduced or protected through breeding programmes and habitat enhancement measures. Ecosystem stability should be maintained through restoration of aquatic vegetation at the site of invasion.