Introduction
The development of sustainable aquaculture relies on diversifying production with low-trophic-level species whose nutritional needs do not depend on fishmeal (FM) and fish oil (FO). Given that feed costs represent approximately 60% of aquaculture production expenses (APROMAR, 2022), farming species with reduced FM and FO diets offers both environmental and economic advantages. Flathead grey mullet (Mugil cephalus) is a promising candidate due to its natural lack of dependency on FM and FO (Gisbert et al., 2016; Wassef et al., 2002). However, despite extensive knowledge of its natural feeding habits (Brusle, 1981), data on its nutritional requirements and response to formulated feeds remain limited. Moreover, the effects of these low FM and FO diets on the polyunsaturated fatty acids (PUFA) content of the fish must be taken into account, as this is an important factor in the quality of the final product (Sprague et al., 2016). Scientific research is therefore essential to know the appropriate composition of the specific diet for this species and the identification of the best culture methods.
Material and Methods
Two feeding trials were conducted using juvenile flathead grey mullet hatched and reared at IRTA. In Trial 1, 495 fish (11.77 ± 2.17 g) were randomly distributed into nine 500 L tanks (IRTAmar® water recirculation system) to test three diets (triplicate groups): Control (20% FM), FM-0 (0% FM), and FM-5 (5% FM). After 72 days of ad-libitum-feeding trial, a final biometric sampling was performed, and samples of plasma, liver, and muscle were collected. Somatic and zootechnical indices were calculated, and samples from each tissue were analysed (biochemical profile in plasma and circulating cortisol levels; proximate composition and fatty acid profile in liver and muscle; and gene expression in liver). Trial 2 required 720 fish (16.39 ± 4.07 g) distributed across 18 tanks (200 L) in two salinity conditions (36 and 8 ppt) at 20 °C with natural photoperiod. Three diets (Control: 10% FO; FO-0: 0% FO; FO-5: 5% FO) were tested in triplicate per salinity. After 83 days, samples of plasma, liver, anterior intestine, gill, and muscle were taken for the same analyses, with gene expression additionally assessed in the intestine and gill. Statistical analyses were performed using one-way ANOVA followed by Tukey’s post hoc test or Welch’s ANOVA when variance homogeneity assumptions were not met (Trial 1) and two-way ANOVA followed by Tukey’s post hoc test (Trial 2).
Results and Discussion
In Trial 1, final body weight (BWf) was lower in the FM-5 group (17.15 ± 4.66 g) compared to the Control (18.42 ± 4.71 g) and FM-0 (18.56 ± 4.16 g) groups, with a significant difference between the FM-0 and FM-5 groups (p < 0.05; Welch’s ANOVA). Feed conversion ratio (FCR) and specific growth rate (SGR) were similar between the Control (FCR: 8.39 ± 1.46; SGR: 0.62 ± 0.08) and FM-0 (FCR: 7.66 ± 1.64; SGR: 0.61 ± 0.12) groups, while FM-5 exhibited a higher FCR (13.11 ± 6.93) and a lower SGR (0.55 ± 0.29). Viscerosomatic index (VSI) and hepatosomatic index (HSI) values showed no significant differences among treatments. No differences were found in plasma glucose and cortisol levels; however, lactate levels were significantly lower in the FM-0 group compared to the Control group (p < 0.05; one-way ANOVA). This study indicated that the total replacement of FM with plant protein sources (FM-0 diet) did not significantly affect growth performance, feed efficiency, or somatic indices in juvenile flathead grey mullet compared to the Control diet. These findings align with the natural feeding behavior of mullet, a species well adapted to herbivorous and detritivorous diets (Brusle, 1981; Wassef et al., 2002). In contrast, partial replacement (FM-5 diet) led to a significant reduction in BWf compared to the FM-0 group, suggesting that intermediate FM levels combined with plant-based ingredients may have negatively impacted diet palatability (Gisbert et al., 2016). Regarding plasma biochemical parameters, such as lower plasma lactate concentrations were recorded in fish fed the FM-0 diet compared to the control group, potentially reflecting a better metabolic adaptation to the plant-based formulation.
In Trial 2, BWf did not differ significantly among dietary treatments at either salinity level. Mean values were: Control, 28.96 ± 7.332 g (8 ppt) and 30.79 ± 8.964 g (36 ppt); FO-0, 27.45 ± 10.11 g (8 ppt) and 28.84 ± 11.53 g (36 ppt); FO-5, 28.80 ± 8.847 g (8 ppt) and 28.98 ± 9.920 g (36 ppt). Similarly, no significant differences were observed in FCR or SGR. At 36 ppt, VSI was significantly higher in fish fed the Control diet (9.22 ± 0.91%) compared to those fed FO-0 (7.76 ± 0.55%) and FO-5 (7.55 ± 0.54%) diets (p < 0.05). HIS was significantly higher in FO-5 fed fish at 8 ppt (0.80 ± 0.15%) than at 36 ppt (0.64 ± 0.13%) (p < 0.05). Plasma glucose levels were significantly higher in FM-5 fed fish at 36 ppt compared to the Control group (p < 0.05). A salinity effect was also evident in FO-5 fed fish, with significantly higher glucose concentrations at 36 ppt than at 8 ppt (p < 0.05). Plasma lactate levels showed a similar trend. Finally, circulating cortisol was significantly higher in control-fed fish reared at 8 ppt compared to those at 36 ppt (p < 0.05). This trial indicated that the total replacement of FO with vegetable lipids did not affect growth performance and feed efficiency in mullet, consistent with their omnivorous feeding habits (Wassef et al., 2002). This is highly positive from a sustainability perspective, supporting the reduction of marine resource dependency. However, metabolic responses varied with diet and salinity, highlighting the need to consider environmental factors in feed formulation.
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Acknowledgments
This study was funded by Spanish Ministry of Science and Innovation , Project RTA2021-126070OR-100. The experiments were carried out at the Institute of Agrifood Research and Technology (IRTA) facilities and the diets were prepared at Skretting and Sparos.