Introduction
Meagre (Argyrosomus regius) is an emerging Mediterranean aquaculture species characterised by rapid growth, favourable feed conversion and desirable flesh quality; however, species-specific functional feeds are needed to support its sustainable production (G��roy et al., 2024). Macroalgae, particularly Ulva sp., are promising functional ingredients because they provide nutrients, minerals, vitamins and bioactive compounds capable of supporting fish growth, immunity and antioxidant capacity (Wan et al., 2019). However, their nutritional effectiveness may be constrained by variable composition and nutrient accessibility, while biological or enzymatic processing may improve digestibility and functional value (Wan et al., 2019; Wiszniewski et al., 2019; Qudratullah et al., 2025). Therefore, this study compared raw, yeast-fermented, and bromelain-processed Ulva sp. products at three dietary inclusion levels in juvenile meagre.
Materials and Methods
Juvenile meagre with an initial body weight of approximately 5 g were randomly distributed into 400-L tanks at 15 fish per tank, with three replicates per treatment. In addition to a control diet, nine experimental diets were formulated with raw, yeast-fermented or bromelain-processed Ulva sp. products at inclusion levels of 2, 4 and 8 g kg-1. Fish were fed to apparent satiation three times daily for 12 weeks under a 12 h light:12 h dark photoperiod and were weighed every two weeks following 24 h fasting and mild clove-oil sedation. Growth and feed-utilisation parameters, somatic indices, whole-body composition, haematological parameters, innate immune responses, and digestive and antioxidant enzyme activities were evaluated. Three fish were randomly sampled from each tank. Data were analysed by one-way analysis of variance followed by Tukey's multiple-comparison test, with significance accepted at P<0.05.
Results
Diets containing bromelain-processed Ulva sp. significantly improved growth and feed-utilisation performance (P<0.05). The highest final body weight (38.10 g), specific growth rate (2.41% day-1) and protein efficiency ratio (1.99), together with the lowest feed conversion ratio (1.04), were recorded in fish fed 8 g kg-1 bromelain-processed Ulva sp. The viscerosomatic index was reduced in the bromelain-processed groups and in the 4 g kg-1 yeast-fermented group, while the mesenteric fat index was significantly lower at 4 and 8 g kg-1 bromelain-processed Ulva sp. than in the control. Condition factor and hepatosomatic index were unaffected.
The highest red blood cell count, haemoglobin and haematocrit values were observed in fish fed 8 g kg-1 bromelain-processed Ulva sp. This treatment also produced the highest protease, amylase and lipase activities. In contrast, the 2 g kg-1 inclusion level of bromelain-processed Ulva sp. elicited the strongest responses in most innate immune indicators, including extracellular oxidative radical production, lysozyme, alternative complement pathway activity, bactericidal activity and phagocytic activity.
Discussion
The superior growth, feed utilisation and digestive enzyme activities observed with bromelain-processed Ulva sp. suggest that enzymatic pre-treatment enhanced nutrient accessibility and utilisation, consistent with improved digestive capacity and feed efficiency reported following bromelain supplementation (Wiszniewski et al., 2019; Qudratullah et al., 2025). The comparatively weaker responses to raw Ulva sp. indicate that restricted nutrient accessibility may limit its nutritional effectiveness, whereas processing can improve the digestibility and functional properties of macroalgal biomass (Wan et al., 2019). The stronger innate immune responses recorded at the 2 g kg-1 inclusion level indicate a dose-dependent functional effect, consistent with evidence that seaweeds and their bioactive compounds can enhance fish immunity, although responses vary among species and dietary inclusion levels (Th��pot et al., 2021). Overall, bromelain-processed Ulva sp. appears to be a promising functional ingredient for juvenile meagre, with the 8 g kg-1 inclusion level favouring growth and digestive performance and the 2 g kg-1 inclusion level eliciting the strongest innate immune responses.
Acknowledgment
This study was supported by the Scientific and Technological Research Council of T��rkiye (T��B��TAK) within the scope of the 1004 Centre of Excellence Support Programme, under Project No. 22AG066, entitled "Tr.Aqua: Innovative and Sustainable Applications in Aquaculture within the Framework of Food Supply Security." The authors gratefully acknowledge Tarkan Cemalettin Erkut, Caner Berk and Mustafa Pekcan for their valuable support.
References
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