Olive flounder (P. olivaceus) belonging to the order Pleuronectiformes is considered the most economically valuable marine water fish in the Republic of Korea (KOSIS, 2023). Farming olive flounder is becoming popular among fish farmers because of its good flavor and taste, environmental tolerance, disease resistance, and high market demand worldwide (Dar et al., 2021; Stieglitz et al., 2021). The total yield of this fish from aquaculture was shown to be 45,801 metric tons (MT), occupying about 50.6% out of total aquaculture production of 90,545 MT in Korea in 2022 (KOSIS, 2023). Due to the carnivorous feeding nature of olive flounder, it requires a high protein diet (40–65% protein depending on life stage) of which about 50–70% is fish meal (FM) (Tacon and Metian, 2008; Hamidoghli et al., 2020). FM is very expensive in the global market due to its scarcity (Oliva-Teles et al., 2015) and increased processing and transportation cost (Tacon and Metian, 2008), which is ultimately increasing feed cost. FM price will continue to rise because of the high demand for global scale expansion of aquaculture and scarcity of wild sources of fish generally used for producing FM (Kim et al., 2021). Therefore, many efforts are required to look for an appropriate substitute for high-cost FM to achieve sustainable aquaculture production of olive flounder.

Tuna by-product meal (TBM) primarily made of bone, fin, skin, head, blood, viscera, and left over of tuna after the tuna-canning process is a popular replacer for FM in marine fish species in Korea (Kim et al., 2018, 2021, 2022). There is no precise statistical information available on annual production of TBM, however, about 8.2 million metric tons of different tuna species were caught globally in 2019 (FAO, 2022). The amount of solid waste produced by the tuna-canning industry is 70% in relation to the raw material (Guerard et al., 2002). TBM as a replacer for FM not only ensures the use of fish by-product, but also minimizes fishery industry waste (Kim et al., 2022). In olive flounder diet, dietary optimum replacement levels for FM were estimated to be 47.1 and 46.3% for the maximum weight gain and feed consumption, respectively (Kim et al., 2022).

Reduced feed consumption and deteriorated feed palatability are the common concerns caused when FM is substituted by various other protein sources in the fish diet (Cho et al., 2005; Uyan et al., 2006; Oncul et al., 2019; Ha et al., 2021). Therefore, incorporation of feed ingredients having a feed stimulant effect can be an effective approach to resolve the issue. Feed attractants or stimulants are low molecular weight free amino acids (AA), nucleotides, nucleosides, organic acids, and quaternary ammonium (NH₄) bases (Morais, 2017; Kim and Cho, 2019), commonly added into fish feed to reduce feeding time and improve acceptability by fish (Peng et al., 2022). Feeding response of fish depends on three chemoreception channel, olfaction and vision, which facilitates smell and location, and gustation, which facilitates taste or consumption (McCrickerd and Forde, 2016; Senzui et al., 2020; Yu et al., 2021). In aquatic surroundings, fish use highly developed chemosensory (olfactory and gustatory) pathways to detect food at distance (Hara, 1994; Morais, 2017). Addition of attractants in feed allows fish to quick access feed, and creates condition for fast ingestion, in other words, minimizes leaching-out ratio of nutrients and deterioration of water quality (Morais, 2017; Jeong et al., 2020). Moreover, attractants also provide supplemental nutrients for protein and energy metabolism (Biswas et al., 2018). Along with the optimum nutrition-balance, palatability is also considered crucial in the development of an artificial diet (Takakuwa et al., 2019). Artificial diets must contain the modest quantity of chemostimulants to be chemically appealing to reveal their location, increase ingestion rates, and elevate feed efficiency (Nunes et al., 2006). Feed stimulants, therefore, have received considerable attention to lower feed waste, enhance feed consumption and growth of fish, and lower pollution source to surrounding water (Li et al., 2019; Yuan et al., 2021; Peng et al., 2022).

A variety of feed ingredients, such as fish products including FM (Singh et al., 2006), fish soluble (Kader et al., 2012), mollusc meals including blue mussel meal (Nagel et al., 2014) and squid meal (Novriadi et al., 2017), crustacean meals including shrimp meal (Kim and Cho, 2019) and krill meal (Smith et al., 2005), animal by-product meals including blood meal (Tusche et al., 2011), plant by-products (Liew et al., 2020), and the synthetic chemicals (Biswas et al., 2018) have been tested to evaluate their attractiveness to different aquatic animals. Especially, jack mackerel (Trachurus japonicus) meal (JMM) has been reported to be the prominent stimulants and attractants in various fish species, such as olive flounder (Jeong et al., 2020, 2022), rockfish (Sebastes schlegeli) (Kim and Cho, 2019; Kim et al., 2019; Baek et al., 2021), and yellowtail (Seirola quinqueradiata) (Hosokawa et al., 2001). AA in the extracts of jack mackerel, especially, histidine has remarkable feeding stimulant activity on olive flounder (Ikeda et al., 2012). Likewise, Takakuwa et al. (2019) proved that muscle extracts of jack mackerel is rich in nucleotide, particularly inosine monophosphate (IMP), which has feed stimulatory effect on greater amberjack (Seriola dumerili). In addition, Kim et al. (2019) unveiled that the most potent feed attractant activity of rockfish was found in JMM among 16 crude feed ingredients. Kim and Cho (2019) reported that the greatest improvement in weight gain and highest feed consumption were attained in rockfish fed a diet supplemented with 5% JMM when rockfish were fed with a 55% anchovy meal-based diet or diets supplemented with 5% JMM, pollock meal, sardine meal, squid meal or shrimp meal at the expense of anchovy meal. Therefore, JMM is not only a rich source of protein for fish, but also exhibits a strong feed attractant and stimulant effect. However, the inclusion effect of a specific ingredient relies profoundly on species of fish, feeding habit of fish, and type and doses of the ingredient (Miyasaki and Harada, 2002; Hancz, 2020; Jeong et al., 2020).

Thus, the primary goal of this study was to elucidate inclusion effect of graded levels of JMM in the olive flounder diets replacing 50% FM by TBM on growth, feed availability, proximate composition, blood chemistry, and economic efficiency.

In aquaculture production, feed is the most expensive component in operating aquaculture systems due to increased price of FM, which is generally utilized as a primary supply of protein in fish feed (Zhao et al., 2021). A variety of studies have been performed to elucidate substitutability of FM in fish diets. Kim et al. (2022) emphasized that the greatest growth performance was achieved in olive flounder fed a diet replacing 40% FM by TBM when olive flounder were fed with a 65% FM-basal diet or diets substituting the graded levels (10–50%) of FM by TBM for 56 days, but growth was slightly lowered in olive flounder fed a diet substituting 50% FM with TBM. Kim et al. (2022) also estimated dietary optimum TBM replacement levels of 47.1, and 46.3% for the maximum weight gain, and feed consumption, respectively. However, in this experiment, all TJ diets substituting 50% FM with TBM supplemented with JMM inclusion, except for the TJ10 diet achieved better weight gain and higher feed consumption by olive flounder compared to fish fed with a 60% FM-based diet, proving that further amount (50%) of FM in the olive flounder diet could be substituted by TBM when JMM was incorporated. Especially, inclusion of 30–50% JMM at the expense of FM in low FM diets substituting 50% FM by TBM brought about superior growth and feed consumption of olive flounder compared to fish fed a 60% FM-basal diet, but not significantly different from those of fish fed the TJ20 diet. Superior growth performance of olive flounder fed the TJ30, TJ40, and TJ50 diets compared to olive flounder fed the Con and TJ10 diets, but comparable to olive flounder fed the TJ20 diet in this experiment were closely associated with higher feed consumption, causing no distinctive variation in feed utilization (FE, PER, and PR). Similarly, elevated growth performance of rockfish, walleye pollock (Gadus chalcogrammus), and olive flounder resulted from elevated feed consumption led to no distinctive difference in feed utilization when 10–60% JMM was included in diets (Choi et al., 2020; Baek et al., 2021; Jeong et al., 2022).

Significantly or slightly, but not significantly, improved feed consumption by olive flounder fed the TJ20, TJ30, TJ40, and TJ50 diets compared to that of fish fed the Con diet in this experiment could be demonstrated by the fact that smell (olfactory cue) and taste (gustatory cue) induced by the incorporation of JMM could have elevated feed consumption and weight gain of rockfish and olive flounder (Kim et al., 2019; Jeong et al., 2020). Similarly, AA, such as histidine, glycine, alanine and proline, and nucleotides, such as inosine, inosine monophosphate, adenosine monophosphate, adenosine diphosphate, and adenosine triphosphate in jack mackerel muscle have shown feeding stimulatory effects on yellowtail, olive flounder, and greater amberjack (Hosokawa et al., 2001; Ikeda et al., 2012; Takakuwa et al., 2019). Abundant EAA and NEAA, except for cysteine and proline in JMM ( Table 2 ) may indicate that JMM has a feed enhancing effect along with being a good quality source of protein in the olive flounder feeds. Likewise, AA and nucleotides having strong chemosensory ability are responsible for flavor and taste in feeds (Nunes et al., 2006; Chotikachinda et al., 2013).

In orthogonal polynomial contrast, linear relationships between weight gain, SGR, and feed intake of fish versus dietary JMM inclusion levels were the most suitable model in this study, however, the quadratic model was the best adjusted model between weight gain, SGR, and feed intake and dietary JMM inclusion levels in regression analysis. The inclusion levels of JMM to induce the maximum weight gain and SGR, and feed consumption were calculated to be 42.1, 42.2, and 40.3% in diets substituting 50% FM by TBM, respectively, according to the regression analysis. As JMM is one of the highest price FM sources in fish feeds, it is recommendable to use JMM inclusion at a minimum level in diets without causing deterioration in feed consumption and growth performance. No distinctive changes in weight gain, SGR, and feed consumption of olive flounder fed the TJ20, TJ30, TJ40, and TJ50 diets in this experiment suggested that 20% of JMM inclusion at the expense of FM was the most recommendable dietary treatment when 50% FM were substituted with TBM in the olive flounder diets. In considering ECR and EPI of the study, however, the TJ30 diet seemed to be the most recommendable to farmer because of higher economic return. Dietary replacement of 50% FM by TBM with all JMM inclusion levels appeared to be economically feasible as prices of all TJ diets were relatively lower than that of the Con diet. Moreover, the lower ECR and higher EPI in all TJ diets, except for EPI of the TJ10 diet led to a greater economic return to the farmers in the grow-out stage (≤ ca. 100 g) of olive flounder (Table 9).

The EAA are important for growth (Elmada et al., 2016), feed efficiency (Tang et al., 2009), and immunity modulation of fish (Pan et al., 2016). Arginine, lysine, and threonine requirements (2.04–2.10%, 1.55–1.97%, and 1.03% of diet, respectively) in all experiment diets were fulfilled for olive flounder (Forster and Ogata, 1998; Alam et al., 2002), but methionine content seemed to be little bit lower than dietary requirement (1.44–1.49%) in the presence of 0.06% cysteine (Alam et al., 2000). However, cysteine could spare 42, 50, and 60% of methionine in the diets of rainbow trout (Oncorhyncus mykiss), yellow perch (Perca flavescens), and channel catfish (Ictalurus punctatus), respectively (Harding et al., 1977; Kim et al., 1992; Twibell et al., 2000). Therefore, EAA requirements known in the olive flounder feeds seemed to be all met in the current study. All TJ diets achieved comparable or superior growth performance compared to olive flounder fed the Con diet in this study because of comparable or superior feed consumption of olive flounder fed the former. Although FA was also very important for many physiological functions of fish, there would be still a big knowledge gap in FA requirement of olive flounder. Dietary ∑n-3 HUFA requirement (8.0–10.0 of total FA) for olive flounder (Kim and Lee, 2004) was also satisfied in all experimental diets in this study. Farmed fish need to supply long chain n-3 HUFA through diets because they have limited capacity to develop them in their body (Lee et al., 2020).

The biological indicators, such as K, VSI, and HSI are widely utilized as they’re representatives of growth, nutritional status, energy reserve, and physiological condition of fish because they are fast and easily ascertainable (Chellappa et al., 1995; Ng et al., 2000). K is an indicator of energy reserve in fish and variation in K signals changes in health and nutritional state, while VSI and HSI are associated with lipid content of viscera and liver (Lee et al., 2000; Wang et al., 2016). In this study, no distinctive variation in K and VSI of olive flounder was observed among dietary treatments, while HSI of olive flounder fed the TJ40 and TJ50 diets was higher than that of fish fed the TJ10 and TJ30 diets. Likewise, dietary inclusion of commercially available stickwater (by-product of fish meal processing), squid paste, and their mixture as feed attractants altered K and HSI in Chinese perch (Siniperca chuatsi), but did not change VSI (Peng et al., 2022). Incorporated feed stimulants in diets may enhance the ingestion, fat metabolism, and nutrient absorption of fish. Unlike this study, however, dietary supplementation of 10% fish soluble, 5% squid meal, 5% krill meal, and their mixture as feed attractants had no distinctive impact on K, VSI, and HSI of red sea bream (Pagrus major) (Kader et al., 2012). This disparity in impact on morphologic indices of fish are possibly due to the type and dose of feed stimulants and attractants (Peng et al., 2022).

No distinctive variation in proximate composition and AA profiles of whole-body olive flounder was found in this experiment, agreeing with other investigations (Jeong et al., 2022; Jeong and Cho, 2023) demonstrating that the proximate composition and AA profile of the whole-body fish were not affected by dietary inclusion of JMM. This could be explained by the fact that body proteins are synthesized based on the genetic information from DNA, and that is why AA profiles of the whole-body protein is the same although there were differences in treatment groups (Yamamoto et al., 2000). Likewise, chemical composition of red sea bream and turbot (Psetta maxima) was not altered by supplementation of fish soluble, squid meal, krill meal, shrimp protein hydrolysate, and mussel meal as feed attractants in low-FM diets (Kader et al., 2012; Nagel et al., 2014; Gunathilaka et al., 2021). Kim et al. (2021) also demonstrated that there was no marked variation in chemical composition and AA profiles of olive flounder when fish were fed with diets substituting 30% FM with different animal protein sources. Higher ∑SFA and ∑MUFA content, but lower ∑n-3 HUFA and EPA content in the whole-body olive flounder fed all TJ diets compared to those of fish fed the Con diet resulted from the FA profiles of the experimental feeds in the current study. This is consistent with other research (Kim and Lee, 2004; Lee et al., 2020; Ha et al., 2021; Kim et al., 2021) that revealed dietary FA profiles had a significant effect on whole-body FA profile of olive flounder.

Plasma parameters of fish are commonly used non-lethal way of monitoring physio-logical condition of farmed fish. In relation to nutritional changes of fish, they can also be used to evaluate the state of fish health (Fazio, 2019). All plasma parameters of olive flounder in this experiment showed no distinctive difference among dietary treatments. This might indicate that the nutritional and physiological condition of olive flounder were not deteriorated when fish were fed with diets substituting 50% FM by TBM supplemented with various levels of JMM. Khosravi et al. (2015) also demonstrated that incorporated krill hydrolysate concentrate or tuna hydrolysate as feed attractants in the 50% and 52% FM-basal diets did not alter blood chemistry of olive flounder and red sea bream, respectively. Supplementation of various (krill, shrimp, and tilapia) protein hydrolysates as feed attractants in low FM diets substituting 50% FM by soy protein concentrate did not alter haematological parameters of olive flounder, except for ALT (Khosravi et al., 2018).

Serum lysozyme activity and SOD are popular biomarkers for innate immune response and oxidative response of fish, respectively (Lange et al., 2001; Ha et al., 2021). No distinctive difference in serum lysozyme activity and SOD in this experiment might indicate the diets substituting 50% FM by TBM supplemented with JMM did not cause any negative impact on serum parameters of olive flounder. Likewise, Jeong et al. (2022) highlighted that lysozyme activity and SOD of olive flounder were not influenced by dietary inclusion of JMM. Unlike this study, however, inclusion of krill and tuna hydrolysate for FM increased nitro blue tetrazolium (NBT) activity and lysozyme activity, and NBT activity, lysozyme activity, and SOD in red sea bream, and olive flounder, respectively (Khosravi et al., 2015). NBT, total immunoglobulin, lysozyme activity, and SOD of olive flounder were increased by low FM diets supplemented with different protein hydrolysates (Khosravi et al., 2018). This contradiction is perhaps because bioactive peptides are produced by enzymatic hydrolysis of marine originated proteins that have immunomodulatory properties (Kang et al., 2019; Muley et al., 2021).

Olive flounder fed the TJ30 diet obtained the greatest weight gain, SGR and feed consumption. Nevertheless, feed utilization, biological indices (K and VSI), proximate composition, AA profile, and plasma and serum measurements of olive flounder were unaffected by dietary treatments. Furthermore, the TJ30 diet seemed to be the most recommendable in considering both ECR and EPI. A commercial-scale investigation is required to evaluate the long-term effect and feasibility of using JMM in commercial production system.

The original contributions presented in the study are included in the article/supplementary material. Further inquiries can be directed to the corresponding author.

The animal study was approved by Korea Maritime and Ocean University. The study was conducted in accordance with the local legislation and institutional requirements.

MI: Data curation, Investigation, Writing – original draft. SC: Conceptualization, Funding acquisition, Methodology, Project administration, Supervision, Writing – review & editing. TK: Funding acquisition, Writing – review & editing.