Introduction

Human activities such as overfishing, water pollution, and industrial development have led to a severe decline in marine fishery resources in most countries. Marine stock enhancement is a powerful tool for the short-term recovery of fishery resources. Large-scale stock enhancement began in the late 19th century. By the end of the 20th century, the rapid development of marine aquaculture, labeling technologies, and genetic technologies promoted the large-scale development of marine stock enhancement. Local species with mature cultivation techniques, low cultivation costs, fast growth rates, high economic value, and short food chains are preferred as stock enhancement species. However, most of them-about 200 species- have failed to achieve the expected enhancement outcomes, because these factors focus more on the suitability of the population itself1. Two main factors are hindering the development of marine stock enhancement. One is that many marine species fail to survive beyond the larval stage, so their numbers cannot meet the requirements for stock enhancement; the other is a lack of understanding of the population dynamics of released species, as well as relevant qualitative assessment tools, which leads to poor results2. In 2010, the Mariculture Conference proposed 15 guidelines for responsible stock enhancement, including biological, economic, social, and institutional attributes3. This conference played a vital role in raising awareness of responsible stock enhancement among various countries. The fourth principle among these 15 guidelines emphasizes prioritizing target species and populations for stock enhancement programs. Furthermore, the list of 14 recommended professional disciplines and technical skills includes both population ecology and community ecology. Due to the lack of universal standards and quantitative assessment tools, how to guide the selection of species for stock enhancement from the perspectives of population ecology, community ecology, and resource ecology remains unclear. Currently, ecological elements for stock enhancement species selection are mostly based on two aspects: interspecific feeding interactions of released stocks, and intraspecific fitness(e.g., genetic degradation) and stocking density of wild and cultured stocks.

The long-term interactions of organisms with one another and their environment give rise to adaptive and coevolutionary ecological relationships—relationships that play a critical role in sustaining the ecological structure and function of aquatic ecosystems. Ecosystem succession is closely linked to the composition of species within a community and their dynamic changes4,5,6,7. Changes in community structure are widely used to assess ecosystem shifts, and this structure is also pivotal for ecosystem restoration8,9,10,11. Notably, metrics of fish community structural stability are more effective indicators of ecosystem degradation than population-level measures, making them a stronger basis for selecting stock enhancement species. This claim is supported by findings that changes in aquatic communities-particularly fish assemblages-directly reflect the degree of ecosystem degradation12,13. Ultimately, when selecting species for ecological stock enhancement, frameworks based on community structure stability (e.g., analyzed via ABC curves) may be more reliable than traditional approaches focused solely on populations.

The abundance-biomass comparison curves (ABC curves) method was originally developed to monitor the impact of disturbance on benthic invertebrates, as these organisms have limited mobility and occupy stable habitats14‌. ABC curves provide the theoretical basis for ecological r-selection and k-selection, and this method enables comparative analysis of how nekton communities respond to different historical fishing conditions and disturbances-provided that all organisms are exposed to the same disturbances and human impacts. In Laoshan Bay, the annual fishing moratorium runs from June 1st to August 31st. Fishing activities had a partial impact on nekton structures‌ in May during the breeding season15. The August survey was conducted after juveniles had matured, which eliminated both interference from fishing activities and biases associated with small-sized fish in the interpretation of ABC curves. Therefore, this method can be used to determine the stability level of the fishery in Laoshan Bay and identify the key organism species that play a role in stabilizing the nekton structure. Because the ABC curve method was used for the first time in stock enhancement, we analyzed the rationality of the selected stock enhancement species based on ABC curves in the discussion. This analysis considered multiple aspects of the ecosystem, including feeding interactions, energy cycling, and the ecological roles of benthic organisms.

Material and method

Data

Laoshan Bay is located to the east of Laoshan District, Qingdao City, Shandong Province, China, facing southeast. It is surrounded by large natural mountains, with a terrain that slopes downward from west to east and from south to north. The bay connects to the Yellow Sea and is a natural cape bay. Its entrance is approximately 11 km wide and 13 km deep, covering a total area of 164.02 km2. The average water depth is about 4 m, with a maximum depth of 13 m. The total shoreline length is 64.59 km.

The data are sourced from fishery trawl surveys conducted in spring (May) and summer (August) between 2013 and 2015. The survey area ranged from 36°N to 36.6°N and from 120.6°E to 121°E (Fig. 1). For the fishery resource assessment survey, a 14.7 kW single-boat trawler equipped with a bottom trawl was used. The trawl had an opening width of 3.75 m, a height of 1.65 m, and a beam trawl mesh size of 13 mm. Each survey station involved one hour of trawling at a speed of 3.0 knots. All samples were processed, with each organism identified to the species level. All experiments were performed in accordance with China’s Specification for Oceanographic Surveys.

Fig. 1
figure 1

The sampling station in Laoshan Bay. The map was generated using R (Version 4.4.1; Available at: https://www.r-project.org/).

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Analytical method

The Sweep-Area Method was adopted to estimate fisheries resources, and ABC Curves and W statistical values (computed using Microsoft Excel) were applied to determine the degree of interference with the nekton16. The calculation formulae were as follows:

$$D = C/\left( {q \times A} \right)$$
(1)
$$W=\sum\limits_{i = 1}^s {\frac{{{B_i} - {A_i}}}{{50(s - 1)}}}$$
(2)

 

In Formula (1), D is the fishery resource assessment value based on the Swept-Area Method, C is the biomass or abundance of each organism caught by the trawler per hour; q is the fishing net capture rate (0.5 for benthic fishes, 0.3 for pelagic fishes, and 0.8 for benthic invertebrates); and A is the area swept by the fishing net per hour. In Formula (2), W is the community structural stability index, Bi and Ai are the cumulative percentages of biomass (W%) and abundance (N%) corresponding to the species count in the ABC Curve, and s is the number of species caught. W% refers to the proportion of a species’ biomass to the total biomass of the community, while N% refers to the proportion of a species’ individual count to the total number of individuals in the community. The x-axis of the ABC Curves indicates the species count, and the y-axis shows the cumulative percentage of species biomass or abundance. If the biomass curve lies above the abundance curve, it indicates the community structure is stable and less disturbed, with a positive W -statistic. If the biomass curve intersects the abundance curve, the community structure is relatively stable. If the biomass curve lies below the abundance curve, the community structure is severely disturbed, with a negative W-statistic. The temporal stability of fish and nekton (primarily including fish, crustaceans, and cephalopods, excluding other rare and occasional taxa whose biomass contribution accounts for less than 3% of the total biomass) was analyzed for springs (May) and summers (August) from 2013 to 2015. Species with higher biomass percentages exhibit stronger resource acquisition capabilities and rapid regeneration, which could accelerate the community’s recovery from disturbances17. In mature ecosystems, this distribution pattern is characterized by larger-bodied species contributing disproportionately to biomass, while smaller species dominate in abundance. By comparing the temporal stability of fish and nekton communities in Laoshan Bay during springs (May) and summers (August) from 2013 to 2015, we selected candidate species that could stabilize the nekton community structure and had relatively high or the highest biomass percentages.

Each species was identified according to its morphological characteristics. Organisms surveyed in Laoshan Bay were divided into pelagic, near-benthic, and benthic organisms based on their habitat in the water layer, to identify which species contribute to the stability of the community structure. The swimming crab (Portunus trituberculatus) is abundant in the study area and, according to historical fishery resource data, acts as a keystone species. It has consistently been a target species for stock enhancement in the study area each May18,19. As a benthic organism, if the swimming crab is selected as a stock enhancement species based on the ABC curve in this study, we can verify the rationality of this selection through its trophic relationships. The predator-prey relationships between the swimming crab and other organisms were derived from studies in the Yellow Sea and the Bohai Sea20,21,22,23,24,25.

Results

Community structure

From 2013 to 2015, a total of 86 species of nekton were caught, including 52 species of fish (belonging to 32 families and 47 genera), 28 species of benthic invertebrates (belonging to 17 families and 25 genera), and 6 species of molluscs (belonging to 5 families and 5 genera). In 2013, the total catch included 37 species of fish (belonging to 25 families and 34 genera), 26 species of benthic invertebrates (belonging to 15 families and 23 genera), and 4 species of molluscs (belonging to 3 families and 3 genera).

In 2013, the number of pelagic organisms, near-benthic organisms, and benthic organisms was 6, 13 and 34 in May, respectively, and 7, 15 and 35 in August. The biomass percentage of pelagic organisms, near-benthic organisms and benthic organisms relative to the total nekton biomass was 6.78%, 26.63%, and 66.59% in May, and 16.00%, 14.76%, and 69.24% in August, respectively. In 2014, the total catch included 39 species of fish (belonging to 27 families and 37 genera), 20 species of benthic invertebrates (belonging to 13 families and 18 genera), and 5 species of molluscs (belonging to 4 families and 4 genera). The number of pelagic organisms, near-benthic organisms, and benthic organisms was 6, 15, and 32 in May, respectively, and 7, 17, and 28 in August, respectively. The biomass percentage of these three groups relative to the total nekton biomass was 7.61%, 17.02%, and 75.37% in May, and 17.51%, 30.96%, and 51.53% in August, respectively. In 2015, the total catch included 45 species of fish (belonging to 29 families and 42 genera), 15 species of benthic invertebrates (belonging to 9 families and 13 genera), and 5 species of molluscs (belonging to 4 families and 4 genera). The number of pelagic organisms, near-benthic organisms, and benthic organisms was 6, 15, and 23 in May, respectively, and 10, 19, and 24 in August, respectively. The biomass percentage of these three groups relative to the total nekton biomass was 5.25%, 10.46%, and 84.29% in May, and 13.07%, 27.71%, and 29.22% in August, respectively.

ABC curves of nekton and fish communities in springs

The nekton community structures in Laoshan Bay in spring from 2013 to 2015 were relatively stable and experienced little disturbance; the W-statistics in ABC curves of these three years were 0.02, 0.18, and ‒0.01, respectively. In 2013 and 2014, the abundance curves of the nekton communities were initially higher than the biomass curves, while the biomass curves became higher than the abundance curves immediately after the second species (Fig. 2D, 2E). In spring 2013, Charybdis bimaculata ranked first in terms of biomass percentage, followed by the mantis shrimp (Oratosquilla oratoria); the biomass percentages of these two species were 22.11% and 21.75%, respectively (Fig. 2D). In spring 2014, however, their biomass percentage ranks were reversed, and their biomass percentages had increased compared to 2013, reaching 35.24% and 31.08%, respectively. In 2015, the swimming crab and mantis shrimp dominated in terms of biomass percentages (Fig. 2F), with values of 35.89% and 21.72%, respectively. For the 2015 nekton community, the biomass curves were higher than the abundance curves initially; the abundance curves only became higher than the biomass curves after the fourth species.

Fig. 2
figure 2

The spring ABC curves of fish and nekton community structure in Laoshan Bay in May (three species were added beside the abundance and biomass curves, respectively, which were the top 3 organisms in percentile ranking.).

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While the fish community structures in Laoshan Bay in spring from 2013 to 2015 were in disturbed states, the W-statistics in ABC curves of these three years were −0.52, −0.16, and −0.43, respectively. In 2013 and 2014, the abundance curves of the fish communities were above the biomass curves (Fig. 2A and B); however, the stability of the community structure in 2014 increased compared with that in 2013. In 2014, the biomass percentage of relatively large-bodied fish in Laoshan Bay-including Belanger’s croaker (Johnius belangerii), red tonguesole (Cynoglossus joyneri), dotted gizzard shad (Konosirus punctatus), yellow goosefish (Lophius litulon), and silvery pomfret (Pampus argenteus)-was higher than that in 2013, while the biomass percentage of small-bodied gobies was drastically reduced. In 2015, the biomass curve of the fish communities was above the abundance curve initially (Fig. 2C). However, as the number of fish species increased, the biomass curve gradually declined below the abundance curve. The biomass percentage of red tonguesole reached its peak in spring 2015, causing the biomass curve to rise above the abundance curve. In addition, small-bodied species such as Chaeturichthys stigmatias, Ctenotrypauchen chinensis, and Odontamblyopus rubicundus exhibited significant growth in abundance percentage, which led the abundance curve to rise above the biomass curve after the second fish species. For this reason, we concluded that the nekton structure in Laoshan Bay underwent relatively large changes in spring.

ABC curves of nekton and fish communities in summers

The nekton community structures in Laoshan Bay in summer from 2013 to 2015 were either slightly disturbed or relatively stable. In 2013, the abundance curve of the nekton community was far higher than the biomass curve (Fig. 3D), with a W-statistic of −0.78; thus, the community structure was disturbed. In 2014, the abundance curve of the first nekton species rose above the biomass curve, and at the second nekton species, these two curves started to overlap. From the sixth nekton species onward, the biomass curve began to rise above the abundance curve (Fig. 3E). The W-statistic was 0.08, indicating a relatively stable community structure. In summer 2015, the biomass curve of nekton community in Laoshan Bay was far higher than the abundance curve; from the 12th nekton species onward, these two curves started to overlap (Fig. 3F). The W-statistic was 0.77, and the community structure was relatively stable. In summer from 2013 to 2015, the species with the highest biomass percentages were the following combinations, respectively, the mantis shrimp and Metapenaeopsis dalei (2013), swimming crab and mantis shrimp (2014), and mantis shrimp and swimming crab (2015) (Fig. 3D, E and F). The total biomass percentages of these two-species combinations were 38.27%, 44.38%, and 53.26%, respectively.

Fig. 3
figure 3

The summer ABC curves of fish and nekton community structure in Laoshan Bay in August (three species were added beside the abundance and biomass curves, respectively, which were the top 3 organisms in percentile ranking.).

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The fish community structures in Laoshan Bay in summer from 2013 to 2015 were either disturbed or relatively stable. In 2013, the abundance curve of the fish communities was higher than the biomass curve (Fig. 3A), with a W-statistic of −1.16; thus, the community structures were disturbed. In 2014, the abundance curve of the fish community was higher than the biomass curve initially, but after the third fish species, the biomass curve rose above the abundance curve. Later, these two curves overlapped (Fig. 3B). The W-statistic was 0.13, indicating a relatively stable community structure. In this year, the abundance and biomass percentages of the small yellow croaker (Larimichthys polyactis) were higher than those of other organisms (Fig. 3B), ranking first with percentages of 45.58% and 35.30%, respectively. Its average individual mass was 12.81 g. Due to the large number of small yellow croaker, the overall fish biomass curve remained lower than the abundance curve. In 2015, the biomass curve of the fish community rose above the abundance curve initially, but after the second fish species, the biomass curve fell below the abundance curve. These two curves then overlapped (Fig. 3C), and the community structure was disturbed, with a W-statistic of −0.50. In this year, the biomass percentage of the small yellow croaker increased drastically, reaching 40.55%. Its average individual mass was 14.26 g, which was an 11.30% increase compared with that in summer 2014; this caused the biomass curve to rise above the abundance curve initially. The biomass percentage of the small-bodied kammal thryssa (Thryssa kammalensis) ranked second after the small yellow croaker (Fig. 3C). For this reason, the biomass curve of the fish community quickly fell below the abundance curve.

Enhancement candidate species based ABC curves

Based on ABC curves and W-statistic values, the stability of the nekton community structure in Laoshan Bay during spring and summer from 2013 to 2015 increased significantly compared to that of the fish community structure. For the fish community in spring, the species ranking first and second in biomass percentage (based on biomass curve data) over the three-year period were as follows: Johnius belengenii and Cryptocentus filifer (2013), J. belengenii and C. joyeria (2014), and C. joyeria and J.belangenii (2015). For the nekton community in spring, the top two species in biomass percentage over the three-year period were crustacean nekton species; however, the aforementioned fish species still ranked first and second among all fish species in biomass percentage. For the fish community in summer, the species ranking first and second in biomass percentage (based on biomass curve data) over the three-year period were Chelidonichthys spinosus and C. joyeria (2013), Larimichthys polyactis and Konosirus punctatus (2014), and L. polyactis and Thryssa kammalensis (2015). For the nekton community in summer, the top two species in biomass percentage over the three-year period were crustacean nekton species; similarly, the aforementioned fish species still ranked first and second among all fish species in biomass percentage.

Specifically, for the spring nekton community, Charybdis bimaculata and Oratosquilla oratoria ranked first and second in the biomass percentage (based on biomass curve data) over the three-year period. For the summer nekton community, the top two species in biomass percentage (based on biomass curve data) were: O. oratoria and Metapenaeopsis dalei (2013), the swimming crab and O. oratoria (2014), and O. oratoria and the swimming crab (2015). From the biomass percentage ranking derived from biomass curve data, O. oratoria and the swimming crab could be selected as candidate species for stock enhancement. ‌However, artificial seed rearing of O. oratoria was not well-developed, whereas that of swimming crab was highly mature. Thus, the swimming crab was selected as the stock enhancement species.

Food relationship history of swimming crab

Based on the ABC curves and stability factors of the nekton structure in spring and summer from 2013 to 2015, we concluded that benthic organisms-especially the swimming crab-play an important role in maintaining the stability of the nekton structure. As a benthic species in the Bohai Sea and the Yellow Sea, the swimming crab mainly feeds on bivalves, secondarily on crustaceans, and only on small quantities of echinoderms, juvenile fish, cephalopods, and polychaetes. It also occasionally feeds on gastropods, organic debris, algae debris, and sand particles in the Yellow Sea and the Bohai Sea23,24,25. Meanwhile, the swimming crab is a food source for nearly 20 species of pelagic, near-benthic, and benthic organisms, including fish such as the pike eel (Muraenesox cinereus), the Pacific chub mackerel (Scomber japonicus), the Pacific cod (Gadus macrocephalus), the black rockfish(Sebastes schlegelii), the silver croaker(Pennahia argentata), the yellow drum (Nibea albiflora), the Callionymus spp., the stone flounder(Platichthys bicoloratus), the spotted seabass (Lateolabrax maculatus), the tongue sole(Cynoglossus semilaevis), the greenling (Hexagrammos otakii), the goby fish(Chaemrichthys spp), filefish(Thamnaconus spp), and the skate (Raja porosa)20,21,22.

Discussion

Research time

Temporal change is a fundamental characteristic of the ecosystem structure. Both natural and anthropogenic disturbances can disrupt ecosystem stability, put the ecosystem in a transitional status, and impair its structure and function26,27,28. The time scale for ecosystem succession ranges from short-term (days) to long-term (years)21,27,28,29. The short-term time scale refers to daily and seasonal time frames; research on ecosystems at this scale focuses on factors such as water quality, substrate, biology, habitat, and biological residual hazards. The medium-term time scale refers to periods of one to several years; research on ecosystems at this scale focuses on factors including coastal zone conditions, sea areas, landscapes, rainfall, annual runoff, and freshwater input30. For example, the food web structure in the upwelling areas along the coast of Washington and Oregon, USA, changed dramatically over a four-year period (1981–1984)31. The research time span of this study was three years, and it covers diachronic and synchronic comparisons, which can reflect changes in the fishery resource community structure in Laoshan Bay.

Stock enhancement of swimming crab

Stable biological communities include abundant and diverse species, whereas disturbed biological communities are typically characterized by fewer species and a simpler structure6,7,8,32. When comparing the spatial and temporal patterns of the fish and nekton communities in Laoshan Bay-using ABC curves, W-statistics and a species with a higher biomass percentage-it was found that the annual stability of the nekton community structure was significantly greater than that of the fish community. Only in 2014 was the W-statistic of the summer fish community higher than that of the nekton community; however, both communities were stable. These large-bodied shrimps and crabs, which had the highest biomass, may locally alter or reduce the predation pressure exerted by fish within the community, thereby making the fishery resource communities more balanced and stable.

Based on initial economic considerations and many years of stock enhancement experience, China has conducted stock enhancement since the 1980s, releasing the indigenous species, the swimming crab. Currently, this species has a high recapture rate and population supplement rate in the Yellow Sea and Bohai Sea19,33. Through the adjustment of stock enhancement strategies, informed by years of accumulated experience, the swimming crab is now a perennial stock enhancement species in China-and in 2021, it even became a joint stock enhancement species between China and South Korea in the Yellow Sea. Currently, the main species released in Laoshan Bay, China, are the swimming crab and the Chinese shrimp. A 2012 fishery resource investigation in six bays of the Shandong Peninsula (including Laoshan Bay) showed that after the release of the swimming crab, the community structures of the six bays shifted from severely disturbed to mildly disturbed18. Additionally, the community evenness index and biodiversity index in Laoshan Bay had a substantial increase. These studies demonstrated that the stock enhancement of the swimming crab not only achieves high recapture and population supplement rates but also contributes to ecosystem improvement. However, these studies had not linked the species’ stock enhancement to the selection of stock enhancement species. In this study, the stock enhancement species selected based on the ecological element of community structure stability was exactly the swimming crab-an indigenous species that has been used for stock enhancement in China for many years. This finding shows that it is feasible to use the ecological element of nekton structure stability to select stock enhancement species.

Benthic organisms and ecosystem succession

Changes in ecosystems are always measured by changes in their community structures9,10. Many studies on ecosystem retrogressive succession have shown that the percentages of benthic organisms and carnivores in slightly disturbed or relatively stable ecosystems are much higher than those in severely disturbed ecosystems. As ecosystem degradation intensifies, high trophic-level carnivores in the food web disappear first, followed by organisms that prey on benthos, planktons, and omnivores9,13,34. A seven-year study on ecosystem restoration was conducted in the Mar de Las Calmas Marine Reserve (Spain) following a large-scale organism die-off due to a volcanic eruption. This study showed that carnivorous fish and benthic organisms that prey on large invertebrates require longer recovery periods, and these organisms exhibit higher spillover effects on adjacent areas35. Both retrogressive and progressive successions of ecosystems demonstrate that benthic organisms-especially large invertebrates-have important effects on nekton structures.

In ecosystems, the basic energy in middle and upper water bodies mainly relies on primary production, while bottom water bodies typically feature four basic energy sources: sediment, primary production, under-ice production (in high-altitude sea areas), and mixed organic matter transported by advection36,37. A study found that the production of Atlantic salmon (Salmo salar) in Baltic cage aquaculture and natural aquaculture in marine ranches both heavily depend on the basic energy of natural ecosystems38. Food availability and predation probability are the main factors influencing population dynamics and ecosystem succession39. Fishery resources data from May and August (2013 to 2015) in Laoshan Bay showed that the biomass of benthic organisms in spring can affect the biomass of pelagic and near-benthic organisms in summer; if the biomass of benthic organisms is high in spring, the biomass of pelagic and near-benthic organisms tends to be high in summer. Ten years of fishery resources survey data in the Bohai Sea also confirmed this pattern: high spring benthic organisms biomass correlates with high summer pelagic and near-benthic organisms biomass (Yellow Sea Fisheries Research Institute, unpublished data). The activities of benthic organisms-such as foraging, crawling, predator avoidance, burrowing, and habitat construction-are collectively termed bioturbation40. Bioturbation can promote the release of nitrogen and phosphorus from sediments, enhance the metabolic activity of microorganisms, replenish the phosphorus nutrients required for diatom growth, and increase the biomass of microorganisms41,42,43. The diverse basic energy sources in bottom water bodies and bioturbation by benthic organisms ensure that benthic organisms-especially small-bodied benthic invertebrates-have sufficient energy for growth36,44. Through the food chain, this basic energy is transferred from benthic diatoms and sediment to small-bodied benthic invertebrates, then to medium- and large-bodied benthic invertebrates, followed by organisms that prey on medium-and large-bodied invertebrates, and finally to high-trophic-level organisms. This energy flow helps maintain the stability of community structures and ecosystems. In the Laoshan Bay ecosystem, bioturbation by swimming crabs and other benthic organisms has contributed to providing more basic energy for high-trophic-level organisms via bottom-up effects. Additionally, predation on swimming crabs by high-trophic-level organisms has transferred energy from bottom water bodies to upper water bodies via top-down effects. The combination of bottom-up and top-down effects has been maintaining the stability of the ecosystem’s community structure and has had positive impacts on biodiversity and species interactions.

Multiple ecological factors for selecting enhancement species

The bioturbation caused by the benthic swimming crab can accelerate the flow of sediments-a key basic energy source-into the ecosystem. This not only provides more benthic prey for the ecosystem but also promotes the flow and circulation of energy within it. Additionally, the swimming crab has complex trophic relationships in the Yellow Sea and Bohai Sea.

As a long-term stock enhancement species in Laoshan Bay, the swimming crab has exerted a positive effect on biodiversity and species interactions. In this study, the swimming crab was specifically selected as an enhancement species based on the stability of nekton structure, using the ABC Curve Method. This result may be a coincidence, or it may reflect the validity of the method itself. Since this was the first time the ABC Curve Method had been applied to select enhancement species, further evidence from studies in other ecosystems is needed to verify its universality-especially in ecosystems with different species compositions.

Currently, the criteria for selecting enhancement species mainly focus on elements such as (1) the indigenous species status, (2) the high economic value of the species, and (3) consideration of the species and biomass of the enhancement species’ predators and prey. However, most enhancement species have failed to achieve the expected ecological and economic benefits. This may be due to the lack of consideration of integrating the enhancement species with the nekton structure in the marine ecosystem targeted for enhancement.

In this study, we attempted to use the ABC Curve Method to select enhancement species in Laoshan Bay, China, based on the stability indicator of the nekton structure. This approach has a certain ecological basis and is worth exploring further.

Based on the results of the ABC Curves Method, it appears that several dominant species-including the swimming crab-that rank highly in a biomass curve over multiple years can be identified as potential targets for stock enhancement. However, in practical applications, not all dominant species with high biomass rankings may be suitable as stock enhancement species. In this study, the swimming crab was selected as a stock enhancement species from several candidates not only for its demonstrated role in maintaining nekton community structure stability (as a long-term released species) but also for its contribution to energy flow (as a benthic organism) and its high recapture rates as a released species. Ecosystems are complex, and the stability of the nekton community structure is only one ecological factor in selecting stock enhancement species; other ecological factors are also required to supplement this criterion. Given the intertwined nature of the structural stability of fish and nekton communities, species with high biomass rankings can first be selected as potential stock enhancement candidates. Subsequent steps should involve evaluating their distinct ecological functions to identify: (1) which species have complex trophic relationships, (2) which can promote ecosystem energy flow and circulation, and (3) which can accelerate the transfer of excess energy from a specific trophic level into energy-deficient trophic levels within the ecosystem. The optimal stock enhancement species should be carefully selected to align as closely as possible with all ecological criteria. In some cases, multiple species may be selected simultaneously as stock enhancement species.

Since the early 1980s, China has been conducting nationwide aquatic organism enhancement and stock releasing, and has accumulated extensive experience in this field. Despite the primary productivity being very high in Jiaozhou Bay, China, large-scale shellfish enhancement has reduced the contribution of primary productivity to fishery resources, resulting in lower biomass of high-trophic-level organisms45. However, the simultaneous stock enhancement of false kelpfish (Sebastiscus marmoratus) and small yellow croaker in the Ma’an marine ranching of China has negatively affected the stability of the local ecosystem46. In contrast, the simultaneous release of large yellow croaker(L. crocea) and swimming crab in Hangzhou Bay, China, has effectively increased biodiversity and improved the structure of nekton communities47. From these examples, it is evident that when selecting species for stock enhancement, it is necessary to comprehensively consider multiple ecological factors.

Conclusions

The study selected stock enhancement species using the ABC Curve Method, based on the stability of the nekton structure. However, the stability of community structure is only one of many ecological factors. Ideally, candidate enhancement species should not only possess the population-related advantages identified by existing methods but also have community-related benefits-such as complex feeding relationships with their predators and prey, promoting the flow and circulation of ecosystem energy, and particularly accelerating the transfer of surplus energy from a specific trophic level to energy-deficient trophic levels within the ecosystem.

The ABC Curve Method assesses the degree of community disturbance using cumulative percentage curves, but it has inherent shortcomings when used to select stock enhancement species. Specifically, the absolute W-statistic values and ABC curves of fishery resources depend on biomass data and the taxonomic level (e.g., species, genus, or family). Its uncertainties primarily stem from biomass data collection, taxonomic classification, and dynamic environmental changes. To address these limitations, several optimization measures can be implemented. During sampling, the operation of survey vessels and nets should be standardized to improve sampling accuracy, and Monte Carlo simulation can be integrated to quantify the impact of outliers on cumulative percentages. During taxonomic classification, the taxonomic category should remain consistent across all surveys. Additionally, to eliminate ABC curve shape fluctuations caused by seasonal or short-term environmental changes, curves from the same season, month, and date across different years should be compared-this is critical because W-statistic values and ABC curves from a single year cannot be directly used to determine community structure stability or ecosystem disturbance levels48. As far as possible, long-term data trend analysis should be conducted to distinguish short-term fluctuations from long-term trends. Nevertheless, if temporal W-statistic values and ABC curves of the ecosystem’s community structure are obtained over consecutive time periods-especially from the ecosystem’s original state to its overdeveloped stage-and the taxonomic category remains consistent, the resulting W-statistic values and ABC curves of fishery resources will have a stronger scientific basis and greater practical applicability. These data can then be used to assess ecosystem disturbance levels and develop an ecological element for responsible stock enhancement guidelines.