* By Aquaculture Magazine Editorial Team
In a groundbreaking 95-day study, researchers explored the potential of replacing traditional fishmeal with enzymatic hydrolysate of poultry by-product meal (EHPB) in the diets of juvenile largemouth bass (Micropterus salmoides). The experiment tested five different diets, gradually reducing fishmeal content while increasing EHPB. The study highlights the potential for EHPB to enhance aquaculture nutrition and sustainability without compromising fish growth and health.
The global demand for highquality animal protein is soaring due to population growth, rising incomes in denations, and changing diets in developed countries, driving the rapid expansion of the aquaculture industry. Fishmeal, known for its excellent nutritional value, palatability, and digestibility, has traditionally been the primary protein source in aquatic feed.
However, relying on fishmeal for aquaculture is becoming increasingly unsustainable due to supply challenges and rising costs. Therefore, developing novel, highquality, low-cost protein sources is essential for enhancing the ecological and economic sustainability of aquaculture.
Over the past decades, various alternative protein sources have been explored for aquaculture. However, many have significant drawbacks, including anti-nutritional factors, unbalanced amino acid composition, and poor palatability, digestibility, and absorption, which can hinder the growth and health of aquatic animals.
Enhancing these protein sources’ quality through processes like heating, cooking, fermentation, and enzymatic digestion has shown promise. Enzymatic hydrolysis, in particular, stands out due to its high biosafety, mild reaction conditions, and environmental friendliness.
Poultry by-products have emerged as a potential fishmeal substitute in aquatic feed. Studies have shown that enzymatic digestion can improve the nutritional value and utilization of poultry by-products. For instance, poultry by-product hydrolysate has been shown to enhance growth in juvenile red drum, and partially replacing fishmeal with poultry by-products improved growth performance and health in various aquatic species. However, the impact of enzymatic hydrolysate of poultry by-product meal (EHPB) on largemouth bass (Micropterus salmoides) has not been comprehensively studied.
Largemouth bass is valued for its fine texture, lack of interstitial spines, and high nutritional value, making it a promising candidate for aquaculture. This carnivorous freshwater species requires a high fishmeal content in its diet, about 40-55% dry matter.
Previous research on EHPB in largemouth bass has primarily focused on muscle quality, leaving gaps in understanding its effects on hepatic antioxidant and immunological properties. This study aims to fill gaps and evaluate the comprehensive impacts of replacing fishmeal with EHPB on largemouth bass.
Materials and methods
The experimental diet formulations and ingredients used were referenced from Yi’s study (Yi et al., 2023) from the lab’s previous research (Table 1).
The juvenile largemouth bass feeding trial was conducted at the breeding farm of the Charoen Pokphand Group (Huanggang, China). Before the onset of the experiment, the fish were allowed to acclimate to the feeding environment for two weeks and given commercial feed.
Then 600 active fish similar in size were evenly distributed among 15 cages (2.5 m × 2.5 m × 1.5 m) at a density of 40 per cage. The cages were then set in outdoor ponds and organized into five groups with three replicates per group.
The initial fish weights and numbers were recorded. The fish were fed to apparent satiety at 6:30 and 18:30 daily. They were weighed and counted every two weeks and the feeding amount was adjusted accordingly. The duration of the feeding trial was 95 days. The fish were fasted for 24 hours before being sampled. They were weighed, enumerated, and collected to measure their growth indices.
Results
Growth performance and morphometry data are listed in Table 2. The fish survival rate (SR) was above 85% for all groups, and none of the experimental diets influenced this metric.
The trends in specific growth rate (SGR) and weight gain rate (WGR) were consistent with that of the final body weight (FBW). Furthermore, the values of these parameters obtained for each EHPB treatment group did not significantly differ from those obtained for the control group (EHPB0). However, FBW, SGR, and WGR decreased with increasing EHPB content.
In terms of whole-body composition, the fish in EHPB0 had the lowest moisture content (p < 0.05). In contrast, the crude protein was highest under EHPB0 (p < 0.05). The crude lipid and ash content did not significantly differ among treatments (p > 0.05).
While the glucose (GLU) levels under all EHPB diets except EHPB3 were higher than those under EHPB0. GLU was highest under EHPB12 (p < 0.05). Lactate dehydrogenase (LDH), total cholesterol (TC), and triglyceride (TG) were significantly lower under all EHPB except EHPB9 and EHPB12 compared with the control. For the latter two treatments, LDH, TC, and TG had increased relative to EHPB0 (p < 0.05).
The hepatic antioxidant-related activity and content parameters are graphically represented in Figure 1.
EHPB0 had the highest GSH content (p < 0.05) (Figure 1a). The same was true for GPX except that its activity did not significantly differ between EHPB9 and EHPB0 (p < 0.05) (Figure 1c).
Discussion
This trial demonstrated that substituting up to 12.4% enzymatic hydrolysate of poultry by-product meal (EHPB) in the diet of juvenile largemouth bass can replace 35.56% of fishmeal without negatively affecting growth parameters like final body weight (FBW), specific growth rate (SGR), and weight gain rate (WGR). Similar results were reported in other species, suggesting EHPB as an effective fishmeal substitute.
EHPB’s effectiveness lies in enzymatic digestion, which liberates small peptides and free amino acids that enhance appetite, improve feed conversion, and promote growth. Unliked the increased feed conversion ratio (FCR) seen in Atlantic salmon with altered feed palatability, EHPB did not affect the FCR in largemouth bass, even at maximum dosages. However, growth performance decreased with higher EHPB content, with significant differences between the EHPB3 and EHPB12 groups, suggesting an optimal replacement level.
Hepatosomatic index (HSI) and condition factor (CF) analyses indicated no adverse effects from EHPB supplementation. The results showed higher CF values in EHPB6 and EHPB9 compared to the control, suggesting better overall nutritional status. Although crude protein and moisture levels varied, these findings align with previous studies, indicating species-specific responses to EHPB.
Biochemical indices like total protein (TP) and albumin (ALB) were unaffected by EHPB addition, indicating maintained nutrient intake and liver function. Enzymatic activity analysis showed lower levels of glutathione peroxidase (GPX), superoxide dismutase (SOD), and catalase (CAT) in EHPB treatments, suggesting slight negative effects on antioxidant mechanisms. However, no significant changes in malondialdehyde (MDA) content indicated no toxic effects.
Inflammatory and immune response gene expression analyses revealed overexpression of pro-inflammatory cytokines tnf-α and il-8 in EHPB6, suggesting an inflammatory response at this EHPB level. Conversely, anti-inflammatory il-10 was also elevated, indicating a balanced immune response. Overall, competitive inhibition of amino acid transport likely occurs at higher EHPB levels, affecting metabolism and growth. In summary, EHPB can effectively replace a significant portion of fishmeal in largemouth bass diets without compromising growth or health, with an optimal replacement level of around 3.1% EHPB.
Conclusion
Diets containing a basal fishmeal level of 29% (12.4% EHPB) did not affect the growth performance of juvenile largemouth bass. However, in terms of the expression of genes related to antioxidant performance and immune function, the optimal level of EHPB added to largemouth bass diets was 3.1% (41% fishmeal). The EHPB6 treatment activated the immune system, alleviated oxidative stress, and attenuated inflammation via the MAPK/Nrf2/NF-κB signalling pathways.
This is a summarized version developed by the editorial team of Aquaculture Magazine based on the review article titled “EFFECTS OF PARTIAL SUBSTITUTION OF ENZYMATIC HYDROLYSATE OF POULTRY BY-PRODUCT MEAL FOR FISHMEAL ON THE GROWTH PERFORMANCE, HEPATIC HEALTH, ANTIOXIDANT CAPACITY, AND IMMUNITY OF JUVENILE LARGEMOUTH BASS (MICROPTERUSSALMOIDES)” developed by: GU, J. and ZHANG, Q. – Nanjing Agricultural University and Chinese Academy of Fishery Sciences; HUANG, D. – Chinese Academy of Fishery Sciences; ZHANG, L., CHEN,X. and WANG Y. – Tongwei Agricultural Development Co., LTD.; LIANG, H. – Nanjing Agricultural University and Chinese Academy of Fishery Sciences; REN, M. – Nanjing Agricultural University. The original article was published, including tables and figures, on FEBRUARY, 2024, through AQUACULTURE REPORTS. The full version can be accessed online through this DOI: https://doi.org/10.1016/j.aqrep.2024.101990
