Appropriateness Evaluation of Releasing Area for Four Marine Organisms in Stock Enhancement: A Fatty Acid Approach

Abstract

In light of the ongoing depletion of global fishery resources, there has been a growing trend towards increasing the scope of stock enhancement activities. The objective of these efforts is to replenish the diminishing fishery resources and restore the ecological balance within marine biological communities. Nevertheless, the efficacy of the stock enhancement project has been hindered by the differential growth and environmental adaptability of released species, which can be attributed to the influence of abundant food resources. As a consequence, the project has not yielded the anticipated outcomes. One useful strategy for enhancing the efficacy of stock enhancement is the deliberate allocation of appropriate habitats for diverse released organisms. Fatty acids were extracted by the Folch method and the composition and content of muscle fatty acids were determined by gas chromatography mass spectrometry. This study examines the fatty acid composition of four commercially important species, namely, little yellow croaker (Larimichthys polyactis), red sea bream (Pagrus major), swimming crab (Portunus trituberculatus), and ridgetail white prawn (Exopalaemon carinicauda). The sum of available docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) is employed as an indicator to assess the appropriateness of the marine environment for releasing these species (DE). The red sea bream exhibited the lowest DE value of 13.69% in the northern coastal water of the Bohai Sea, and the little yellow croaker displayed the lowest DE value of 10.91% in the southern coastal water of the Shandong Peninsula. Conversely, the DE values of other seas were comparable, averaging 20.16%. The range of the DE value of the swimming crab across various coastal waters was observed to be between 33.59% and 45.21%. The ridgetail white prawn had a DE value of 21.10% in the coastal water of Yancheng City, Jiangsu Province, as well as the southern coastal water of the Shandong Peninsula, and a DE value of 31.75% in the southern and central coastal waters of Zhejiang Province. The findings of the study indicate that the Bohai Sea and the northern region of the Yellow Sea are the appropriate stock enhancement areas for red sea bream. Similarly, the Yellow Sea and the northern part of the East China Sea are identified as suitable habitats for the little yellow croaker. The swimming crab, on the other hand, is adapted to be released in the Yellow Sea, the Bohai Sea, and the East China Sea. The north of the central part of the East China Sea is an appropriate release area for the ridgetail white prawn.

1. Introduction

Since the onset of the 21st century, there has been a discernible decline in global fishery resources due to a confluence of factors including overfishing, climate change, environmental degradation, and habitat destruction [1]. Stock enhancement is a strategy employed to replenish fishery resources and rehabilitate stocks and community structures by releasing artificially cultivated larvae or adults of marine organisms into their native aquatic habitats, which is one of the most essential and efficacious methods for facilitating the restoration of fishery resources [2,3,4]. Despite the global expansion of stock enhancement initiatives, numerous efforts within this domain exhibit little efficacy in terms of fishery resources recovery, and the associated economic, social, and ecological advantages are often insufficient, resulting in a failure to meet anticipated outcomes [5]. In the practice of stock enhancement, the growth and environmental adaptability of organisms released in different regions within the same marine area varies due to changes in food availability. Consequently, it is crucial to strategically designate suitable areas for the release of different organisms [6,7], which serves to effectively restore marine ecosystems, foster the sustainable development of fishery resources, and enhance the economic prospects of coastal fishermen.

Conducting large-scale experiments to assess the effectiveness of stock enhancement has proven challenging due to the extensive duration of study and the substantial financial resources required [8]. The existing research pertaining to the evaluation of marine environments for stock enhancement purposes primarily centers around the utilization of environmental modeling techniques to gauge the potential effects of these locations on the creatures that are released into them. Sharma et al. (2005) conducted a study to examine the impact of habitat and hatchery parameters on the abundance of silver salmon (Oncorhynchus kisutch) and employed numerical modeling techniques to evaluate the efficacy of silver salmon stock enhancement in the northwestern region of the United States [9]. In order to determine whether or not certain places are suitable for the cultivation of mussels and fish, Vaz et al. developed a habitat suitability index model that takes into account hydrodynamic and water quality factors [10]. Yu et al. employed four environmental variables, namely, sea surface temperature, sea bottom temperature, sea surface salinity, and sea bottom salinity, to develop a model for evaluating the prospective aquaculture sites for Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss) in the Yellow Sea of China [11]. Nevertheless, few previous studies have sufficiently taken into account the impact of food availability in the surrounding aquatic environment on the growth of the marine organisms that are released into these waters. In marine organisms, fatty acids have a chemical structure that is stable, and they also tend to be reasonably stable when altered by the food they consume, their own metabolism, and the environment; as a result, fatty acids may effectively reflect the food composition of the organism over time [12]. The inclusion of essential fatty acids, particularly those that are crucial for maintaining normal metabolic function, is imperative for marine organisms. However, it is important to note that the organism is unable to generate these fatty acids on its own and, hence, must acquire them from dietary intake [13]. The variation in the levels of eicosapentaenoic acid (DHA) and docosahexaenoic acid (EPA), for example, can be a valid indicator for determining the survival of a species within a particular sea area [14,15].

This study utilized the combined quantities of DHA and EPA present in the muscles of marine organisms as a metric to evaluate the appropriateness of four economically valuable species for stock enhancement initiatives in various coastal regions of China. The objective was to establish a systematic approach for the identification and designation of suitable sea areas for organism release, thereby increasing the efficacy of stock enhancement efforts.

2. Materials and Methods

2.1. Experimental Materials

The quality and nutritional composition of marine organisms, such as DHA and EPA, are not only related to the species but also depend on the nutritional composition of the food, habitat, temperature, seasonality, environmental conditions, age, and sex; among which, the season can be especially important because it affects the composition of the food [16,17]. Differences in the chemical composition of fish are closely related to feed intake, and the presence of cues typically associated with the winter season suggests a scarcity of food supplies [18,19], making it important to assess the nutritional status of fish in the spring. It is important to note that April marks the beginning of stock enhancement for marine organisms in China. Therefore, a study was conducted in the southern coastal water of Zhejiang Province to investigate the capture of marine species in April 2019.

A total of 31 species were collected from 16 sampling sites. Four important releasing species are the focus of this study: little yellow croaker (Larimichthys polyactis), red sea bream (Pagrus major), swimming crab (Portunus trituberculatus), and ridgetail white prawn (Exopalaemon carinicauda); three biological replicates were used for each species. The specific locations where the samples were obtained are depicted in Figure 1, while

Table 1. Basic information on the four stock enhancement species.

Common Name	Species	Biological Classification	Feeding Habits	Body Length and Cephalothoracic Length/cm	Weight/g	Sampling Parts
Little yellow croaker	Larimichthys polyactis	Perciformes, Sciaenidae, and Larimichthys	Benthic feeder, mainly feeding on planktonic crustaceans, but also on decapods and other juvenile fish	14.43 ± 1.88	33.83 ± 5.28	The white muscle near the first dorsal fin
Red sea bream	Pagrus major	Perciformes, Sparidae, and Pagrus	Benthic feeder, mainly feeding on benthic crustaceans, mollusks, prawns, and algae	11.13 ± 0.52	47.89 ± 8.18	The white muscle near the first dorsal fin
Swimming crab	Portunus trituberculatus	Decapoda, Portunidae, and Portunus	Benthic feeders, feeding on benthic algae, shellfish, telopods, etc.	7.63 ± 0.09	57.13 ± 3.64	The crab claw or abdominal muscles
Ridgetail white prawn	Exopalaemon carinicauda	Decapoda, Palaemonidae, and Exopalaemon	Planktonic feeders, feeding on phytoplankton, organic detritus, etc.	9.92 ± 0.21/5.33 ± 0.33	5.76 ± 0.89	The abdominal muscles

provides essential details about the collected specimens. The cryopreservation process was employed to preserve the samples, which were subsequently sent to a laboratory for analysis. The samples underwent a thawing process, following which the muscles were extracted and subsequently cleaned with ultrapure water. Freeze-drying was then performed in a freeze-dryer operating at a temperature of −55 °C for a duration exceeding 24 h. The resulting freeze-dried material was further processed into a powdered form using a hybrid ball mill, facilitating the subsequent extraction of fatty acids. It has been observed that there exists a variation in the fatty acid composition between different body parts of the same marine organism [20]. In order to mitigate the potential inaccuracy arising from this variation, uniformly picked muscle samples were selected for fatty acid composition analysis, which was conducted three times for each species.

2.2. Fatty Acid Analysis

The lipid extraction procedure was conducted following the modified Folch method [21]. Specifically, 0.2 g of powdered sample was placed in a centrifuge tube, and then 15 mL of a trichloromethane-methanol solution (2:1) was added. The mixture was aggressively agitated and thereafter left to soak for 20 h. The centrifugation process was conducted on the sample at a speed of 3000 revolutions per minute (r/min) for a duration of 10 min. Following centrifugation, the liquid portion above the sediment, known as the supernatant, was carefully separated. Following that, a 4 mL amount of a solution containing 0.9% sodium chloride (NaCl) was introduced into the sample and allowed to remain undisturbed for roughly 2 h. The crude lipids were extracted by evaporating the lower layer in a round-bottomed flask placed in a water bath, resulting in the formation of crude lipids.

The process of the esterification of fatty acids using the boron trifluoride-methanol technique was conducted as described by Metcalfe et al. (1966) [22]. A round-bottomed flask was utilized for the experimental procedure. Initially, 4 mL of a sodium hydroxide-methanol solution with a concentration of 0.5 mol/L was introduced into the flask. The resultant mixture was subsequently exposed to reflux for 8 min using a water bath. A 4 mL solution of boron trifluoride-methanol, which had a concentration of 14%, was added to the flask and subjected to reflux for 25 min. Subsequently, a volume of 4 mL of hexane was added to the flask and underwent an extraction procedure via reflux for 2 min. After cooling, 10 mL of a saturated sodium chloride solution was added to the mixture. The resulting mixture was stirred and left undisturbed for 2 h. The upper layer of the methyl azelate, namely the n-hexane layer, was carefully transferred into a chromatographic vial. The transferred solution was evaporated with a stream of nitrogen gas. A methyl nineteenthanoate (C19:0) internal standard with a concentration of 50 mg/L was introduced in a 1:1 proportion for the purpose of quantification.

The fatty acids were analyzed by gas chromatography-mass spectrometry (GC-MS) using an Agilent 7890B/5977A instrument (Agilent Technologies, Santa Clara, CA, USA). The analysis was performed on ant HP-88 capillary column (60 m × 0.25 mm × 0.2 µm, Agilent Technologies). A high-purity helium gas was used as the carrier gas, and the split ratio was set at 10:1. The intake temperature for the analysis was maintained at 250 °C.

2.3. Data Preprocessing

The quantification of the fatty acid composition in the samples was performed utilizing the internal standard method, and the equation is shown below.

$$X_i=F_i\times \frac{A_i}{A_{19}}\times \frac{m_{19}}{m}\times 100\%$$

(1)

where X_i is the content of component i in the sample under examination, F_i is the ratio of the correction factor of group i to that of the internal standard, A_i is the peak area of component i, A₁₉ is the peak area of the internal standard added to the sample being tested, m₁₉ is the mass of methyl nineteen-alkanoate of the internal standard, and m is the mass of the sample.

The appropriateness evaluation index (DE, %) for the stock enhancement areas was determined by the combined levels of DHA and EPA.

$$DE=X_{DHA}+X_{EPA}$$

(2)

where X_DHA is the concentration of DHA in the muscles of the organisms (%), and X_EPA is the concentration of EPA in the same muscles (%). The determination of the DE values in other coastal waters was conducted by utilizing the essential fatty acid composition data of the same species as reported in the existing literature.

3. Results and Discussion

3.1. Fatty Acid Composition of Four Marine Organisms in the Southern Zhejiang Coastal Water

Figure 2 displays the primary fatty acid compositions of little yellow croaker, red sea bream, swimming crab, and ridgetail white prawn in the coastal waters of southern Zhejiang Province. The saturated fatty acids (SFA) of the four marine organisms ranged from 23.61% to 37.65% of the total fatty acids, with higher concentrations observed in the little yellow croaker and red sea bream. Monounsaturated fatty acids (MUFA) accounted for 12.97% to 24.36% of the total fatty acids, with swimming crab and ridgetail white prawn exhibiting higher levels. Polyunsaturated fatty acids (PUFA) constituted 12.97% to 24.36% of the total fatty acids in the little yellow croaker and red sea bream, with swimming crab and ridgetail white prawn having higher proportions. PUFA was found to be the predominant fatty acid in all four species, with percentages ranging from 43.99% to 53.47%. The proportion of C22:6n3 (DHA) was consistently higher than that of C20:5n3 (EPA) in all species. Marine organisms acquire long-chain n-3 fatty acids, specifically EPA and DHA, predominantly through their feeding behavior [12]. This reliance on external sources of these fatty acids can be attributed to evolutionary adaptations that have developed in response to the naturally abundant presence of PUFA in marine habitats [23]. Organisms that exhibit a significant prevalence of dinoflagellates in their dietary intake are commonly associated with a notable proportion of DHA in their fatty acid composition, whereas diatom-derived diets are characterized by a substantial proportion of EPA [24]. This study revealed that the little yellow croaker exhibited approximately 34% DHA and 6% EPA. The red sea bream had a somewhat lower content of the two crucial fatty acids compared to the little yellow croaker, with DHA and EPA levels measuring around 32% and 4%, respectively. This observation implies that the inclusion of dinoflagellate food has a greater impact on the dietary composition of both fish species. The percentage of DHA content in swimming crab and ridgetail white prawn showed a similarity, ca. 17%. Additionally, the percentage of EPA content in the swimming crab was approximately 16%, while in ridgetail white prawn, it was approximately 15%. No statistical variations were found in the levels of DHA and EPA between the two crustaceans, indicating that both crustaceans absorbed similar quantities of food derived from diatom and dinoflagellate sources. The DHA content percentage of the two marine fish species was found to be greater in comparison to that of the two crustaceans. This disparity can be attributed to the variance in the trophic levels between the two groups [25,26,27].

3.2. Analysis of DHA and EPA Levels in Four Marine Organisms from Different Coastal Waters

The levels of DHA and EPA present in four distinct marine species originating from various coastal waters are shown in Table 2. The little yellow croaker and red seabream species exhibited the highest levels of DHA content, measuring 22.75% and 31.72%, respectively, in the southern coastal water of Zhejiang Province. Conversely, the lowest DHA content was observed in the southern coastal water of the Shandong Peninsula (6.84%) and the northern coastal water of the Bohai Sea (9.14%). The EPA content had minimal variations across different areas. The coastal waters of Yancheng City in Jiangsu Province and southern Fujian Province displayed the highest EPA content for little yellow croaker and red seabream, respectively, while the southern coastal water of the Shandong Peninsula had the lowest EPA content. Both the little yellow croaker and red seabream exhibited significant variations in DHA levels while displaying minor oscillations in EPA levels across different sea areas. The observed phenomenon may be attributed to variations in food availability across different regions. In instances of limited food supply, both the small yellow croaker and red seabream exhibit a preference for consuming EPA in order to meet their energy demands, implying that DHA holds greater importance in the physiological processes of marine fish [28].

The southern coastal water of the Bohai Sea exhibited the highest DHA content in the swimming crab, measuring 21.96%. Similarly, the southern coastal water of Zhejiang Province displayed the highest DHA content in the ridgetail white prawn, which was 17.24%. Conversely, the northern coastal water of Zhejiang Province and the coastal water of Yancheng City, Jiangsu Province had the lowest DHA contents of 16.16% and 10.2%, respectively. In terms of EPA composition, the swimming crab showed a peak concentration of 23.25% in the southern coastal water of the Bohai Sea, whereas the lowest concentration of 14.75% was observed in the southern coastal water of Fujian Province. Conversely, the ridgetail white prawn demonstrated a maximum EPA content of 18.42% in the central coastal water of Zhejiang Province, while a minimum concentration of 9.2% was recorded in the coastal water of Yancheng City. The levels of DHA and EPA in the swimming crab and ridgetail white prawn, found in various sea regions, exhibited a relatively consistent and lesser degree of variability compared to the two fish species. This observation may be attributed to the broader dietary preferences of swimming crab and ridgetail white prawn, as well as the relatively abundant food sources available in each sea region, generating a more stable accumulation of DHA and EPA in their organisms [29].

Table 2. DHA and EPA levels of four marine organisms in coastal waters.

Sampling Locations	DHA Content/(%)				EPA Content/(%)				References
	Little Yellow Croaker	Red Sea Bream	Ridgetail White Prawn	Swimming Crab	Little Yellow Croaker	Red Sea Bream	Ridgetail White Prawn	Swimming Crab
Southern coastal water of Zhejiang Province	22.75 ± 1.86	31.72 ± 5.47	17.24 ± 0.47	16.48 ± 1.03	5.02 ± 0.21	3.59 ± 0.5	15.5 ± 0.14	15.83 ± 50	This study, [30,31]
Southern coastal water of the Shandong Peninsula	6.84 ± 0.02	12.86	11.56	/	4.07 ± 0.03	2.7	11.25	/	[31,32,33]
Central coastal water of Zhejiang Province	11.29	/	12.33	18.55 ± 0.14	5.56	/	18.42	17.26 ± 0.13	[30,31,34]
Northern coastal water of Zhejiang Province	17.41 ± 2.47	/	11.4 ± 0.2	16.16 ± 0.34	4.42 ± 1.72	/	17.2 ± 0.23	17.45 ± 0.28	[25,26,27,28,29,30,31,32,33,34,35,36,37,38]
Coastal water of Yancheng City	17.38 ± 0.06	/	10.2 ± 1.5	/	6.05	/	9.2 ± 1.3	/	[39]
Coastal water of Nantong city	20.2 ± 0.02	/	15.61 ± 0.13	/	5.67 ± 0.42	/	13.13 ± 0.13	/	[34,40,41]
Northern coastal water of Fujian Province	8.64	/	/	/	5.85	/	/	/	[34]
Southern coastal water of Fujian Province	/	17.1 ± 0.32	/	21.69 ± 2.01	/	5.2 ± 0.07	/	14.75 ± 0.79	[36,42]
Eastern coast of the Kii Peninsula	/	25.5	/	/	/	4.9	/	/	[43]
Northern coastal water of the Bohai Sea	/	9.14	/	/	/	4.55	/	/	[44]
Western coastal water of the Liaodong Peninsula	/	/	/	17.69 ± 1.71	/	/	/	15.2 ± 1.67	[45]
Southern coastal water of the Bohai Sea	/	/	/	21.96 ± 1.48	/	/	/	23.25 ± 1.29	[36]
Coastal water of Lianyungang city	/	/	/	18.85 ± 3.92	/	/	/	19.99 ± 2.82	[36]
Coastal water of Zhanjiang city				18.55 ± 1.87				17.26 ± 1.78	[36]

Table 2. DHA and EPA levels of four marine organisms in coastal waters.

Sampling Locations	DHA Content/(%)				EPA Content/(%)				References
	Little Yellow Croaker	Red Sea Bream	Ridgetail White Prawn	Swimming Crab	Little Yellow Croaker	Red Sea Bream	Ridgetail White Prawn	Swimming Crab
Southern coastal water of Zhejiang Province	22.75 ± 1.86	31.72 ± 5.47	17.24 ± 0.47	16.48 ± 1.03	5.02 ± 0.21	3.59 ± 0.5	15.5 ± 0.14	15.83 ± 50	This study, [30,31]
Southern coastal water of the Shandong Peninsula	6.84 ± 0.02	12.86	11.56	/	4.07 ± 0.03	2.7	11.25	/	[31,32,33]
Central coastal water of Zhejiang Province	11.29	/	12.33	18.55 ± 0.14	5.56	/	18.42	17.26 ± 0.13	[30,31,34]
Northern coastal water of Zhejiang Province	17.41 ± 2.47	/	11.4 ± 0.2	16.16 ± 0.34	4.42 ± 1.72	/	17.2 ± 0.23	17.45 ± 0.28	[25,26,27,28,29,30,31,32,33,34,35,36,37,38]
Coastal water of Yancheng City	17.38 ± 0.06	/	10.2 ± 1.5	/	6.05	/	9.2 ± 1.3	/	[39]
Coastal water of Nantong city	20.2 ± 0.02	/	15.61 ± 0.13	/	5.67 ± 0.42	/	13.13 ± 0.13	/	[34,40,41]
Northern coastal water of Fujian Province	8.64	/	/	/	5.85	/	/	/	[34]
Southern coastal water of Fujian Province	/	17.1 ± 0.32	/	21.69 ± 2.01	/	5.2 ± 0.07	/	14.75 ± 0.79	[36,42]
Eastern coast of the Kii Peninsula	/	25.5	/	/	/	4.9	/	/	[43]
Northern coastal water of the Bohai Sea	/	9.14	/	/	/	4.55	/	/	[44]
Western coastal water of the Liaodong Peninsula	/	/	/	17.69 ± 1.71	/	/	/	15.2 ± 1.67	[45]
Southern coastal water of the Bohai Sea	/	/	/	21.96 ± 1.48	/	/	/	23.25 ± 1.29	[36]
Coastal water of Lianyungang city	/	/	/	18.85 ± 3.92	/	/	/	19.99 ± 2.82	[36]
Coastal water of Zhanjiang city				18.55 ± 1.87				17.26 ± 1.78	[36]

3.3. Evaluation of the Appropriateness of the Stock Enhancement Area for Four Marine Organisms Based on DE Indicator

DHA and EPA play a crucial role in the proper development and growth of marine species. However, it is important to note that the availability of dietary resources might significantly differ across various coastal waters [46,47]. Starvation can result in inadequate energy provision for marine organisms. To sustain normal survival, these organisms must metabolize fatty acids within their bodies to maintain required energy levels. It is worth noting that DHA and EPA, which are crucial components of biological membranes, nerves, and the visual system, are not easily broken down for energy supply [48,49,50]. As a result, the DE value, which serves as an indicator to assess the appropriateness of a marine area for promoting the growth of marine organisms, can to some extent effectively reflect the survival of the organisms released in that particular sea area. The muscular tissues of channel catfish (Ictalurus punctatus), turbot (Scophthalmus maximus), and Eurasian perch (Perca fluviatilis) exhibited a noteworthy augmentation in their DE values when subjected to extended periods of hunger. The insufficient availability of food resources in the marine environment to fulfill the dietary requirements of released organisms in terms of necessary polyunsaturated fatty acids might have detrimental effects on their growth and survival [51,52,53]. Consequently, this can lead to suboptimal outcomes in terms of stock enhancement efforts.

According to the data presented in Figure 3, the red seabream in the northern coastal water of the Bohai Sea exhibited a DE value of 13.69%, which was lower than the average DE level of 25.43% observed in the cultured red seabream [43]. This suggests that the feeding bait provided to the red seabream in the northern coastal water of the Bohai Sea was abundant and appropriate for their growth and reproductive processes. Additionally, it is possible that the stocking effect of red seabream in the Bohai Sea and the northern part of the Yellow Sea could be enhanced. The southern coastal water of Zhejiang Province had the highest DE value of 35.31%, indicating potential limitations for the large-scale release of red seabream in this area. The red seabream is classified as a species of coastal warm-water fish that lives mainly on the seabed and has a wide distribution in the Yellow Sea, Bohai Sea, East China Sea, and South China Sea [54]. OTAKI found that the spawning and nursery grounds of red sea bream are in the Yellow and Bohai Seas, mainly in Haizhou Bay and Bohai Bay, and their main population is growing in the East China Sea [55], which agrees well with our findings. The southern shore of the Shandong Peninsula had the lowest recorded value of DE in little yellow croaker, measuring at 10.91%. In comparison, the average DE value in other places was 20.16%, which was lower than the DE value of 24.07% observed in little yellow croaker under artificial culture environments. The natural habitat of the little yellow croaker extends from the Bohai Sea, Yellow Sea to the East China Sea, and the main population is in the Yellow Sea [55] as well as the northern of the East China Sea where the little yellow croaker population is experiencing faster growth [56]. Hence, the regions encompassing the Yellow Sea and the northern portion of the East China Sea, including the southern coastal water of the Shandong Peninsula, the central coastal water of Zhejiang Province, and the northern coastal water of Fujian Province, present more favorable conditions as feeding grounds for little yellow croaker. These areas are deemed suitable for the stock enhancement of little yellow croaker and promote the equitable accumulation of diverse fatty acids within the species.

In contrast to the two aforementioned fish species, the swimming crab exhibited varying DE values ranging from 33.59% to 45.21% across different aquatic environments. Notably, this range closely approximated the DE value of 36.91% observed in the swimming crab under controlled artificial culture conditions [37]. This finding suggests that the swimming crab exhibits adaptability to diverse marine environments and variations in bait conditions across different sea regions by regularly ingesting necessary fatty acids as a means to sustain its growth and facilitate its developmental processes, which aligns with the findings reported by Dong et al. [36]. In a recent study conducted by Liu et al. [57], the researchers employed a modeling approach to assess the present prospective habitat of swimming crabs. Their findings indicate that these crabs exhibit a broad distribution over the Bohai Sea, the Yellow Sea, and the East China Sea. Notably, the highest level of appropriateness for their habitat was observed in the East China Sea. Similarly, based on our findings, the optimal marine regions for the extensive cultivation and subsequent release of the swimming crab might be located not only in the East China Sea but also in the southern Yellow Sea. In the Yellow Sea, the coastal water of Yancheng City, Jiangsu Province, and the southern coastal water of Shandong Peninsula showed lower average DE values (21.10%) in the ridgetail white prawn, which is also a crustacean species. These values were found to be lower than the DE values (30.30%) observed in ridgetail white prawns under controlled culture conditions [30]. Conversely, the DE values of samples collected from the southern and central coastal water of Zhejiang Province (31.75% on average) were similar to those obtained from the northern coastal water of the central East China Sea. The ridgetail white prawn is naturally widespread along the entire coast waters of mainland China, and the Bohai and Yellow Seas exhibited the highest levels of abundance [58]. Our study also indicates that the release of these ridgetail white prawns is appropriate from the middle region of the East China Sea to the northern area, and the northern part of the Yellow Sea is better suited for large-scale stock enhancement.

Lipid and fatty acid analyses have been employed as biological markers and indicators of dietary patterns in the field of marine ecology, serving as a valuable tool in the investigation of food webs [59]. The presence of sufficient quantities of food sources containing abundant long-chain polyunsaturated fatty acids, such as DHA and EPA which are major sources of metabolic energy and influence individual and population growth rates and reproduction, is crucial for the well-being of ecosystems [23,60]. In addition, the supply of DHA and EPA from basal food sources is a valuable indicator of ecosystem health [61]. The DE index is based on the levels of DHA and EPA proposed in this study and has the potential to serve as a surrogate marker for determining the appropriateness of releasing areas for stocked organisms. Nevertheless, variations in metabolism, as well as both primary productivity and the diversity of benthic communities varying significantly between habitats, can have an impact on ecological tracers [62,63,64], leading to distortions of the DE index derived from short-time scale results. Future studies should prioritize the investigation of additional potential factors that may affect the accurate assessment of the appropriateness of linkages between marine organisms and their release areas in stock enhancement activities.

4. Conclusions

The sufficient ingestion of essential fatty acids plays a crucial role in ensuring the survival, growth, and development of marine species throughout their larval stage [65,66,67]. If the larvae of marine species are unable to acquire sufficient nourishment and accumulate critical fatty acids in the waters where they are released, it will result in a decline in their development and survival rate. Consequently, stock enhancement projects will be unsuccessful.

In this study, we analyzed the content of the essential fatty acids DHA and EPA in the bodies of four common economic stock enhancement organisms and evaluated suitable waters for their release based on DE values. Both the Bohai Sea and the northern section of the Yellow Sea are suitable environments for the stock enhancement of red sea bream. The Yellow Sea and the northern portion of the East China Sea are both suitable areas for the stocking and release of little yellow croaker. The Yellow Sea, the Bohai Sea, and the East China Sea are all suitable waters for the stock enhancement of the swimming crab. Both the central region of the East China Sea and the northern region of the East China Sea are suitable waters for the stocking and release of ridge trail white prawn. The findings of this study can, to some extent, serve as a scientific basis for identifying appropriate marine areas for the stock enhancement of economically valuable marine organisms. This, in turn, can enhance the survival and growth rates of released organisms, thereby playing a crucial role in the restoration of coastal marine ecological resources. Nevertheless, this study is considered preliminary as it has not yet accounted for parameters such as sample size and time span, season, and benthic characteristics, which may limit the direct application of our proposed methodology in stock enhancement practice. In light of the fact that the composition of fatty acids changes with season and age, and that sex differences also have some bearing on fatty acid composition [68,69,70,71], it will be possible in the future to conduct in-depth research on the differences in the composition of fatty acids found in organisms that have been released at different ages, sexes, and seasons with the intention of further perfecting the stock enhancement strategy.