OPTIMIZING LOW-SALINITY ACCLIMATION PROTOCOLS IN THE GREY MULLET Chelon labrosus: OSMOREGULATORY AND LIPID METABOLISM RESPONSES

Introduction

Mullets are catadromous teleost that migrate to coastal environments during their juvenile stage (Whitfield & Pattrick, 2015) and are considered promising candidates for sustainable aquaculture within the context of blue growth due to their omnivorous feeding habits and tolerance to a wide range of salinities (Krause et al., 2022). Among these species, the thick-lipped grey mullet, Chelon labrosus, has demonstrated a remarkable ability to convert 18-C fatty acids into long-chain polyunsaturated fatty acids (LC-PUFAs), particularly under near-isoosmotic conditions (~16 ppt) (Quirós-Pozo et al., 2025). However, despite its tolerance to low salinities, optimal growth has been reported at intermediate salinities (10–16 ppt). Reduced survival has been observed at lower salinities, indicating that salinity plays a key role in its physiological performance and highlights the need to better understand fish responses to acclimation and physiological limits as salinity management could therefore enhance fish welfare, lipid metabolism, and the overall efficiency of aquaculture systems. The aim of the present work was to understand the effects of different salinity gradients (36 to 0 ppt) on Chelon labrosus.

Materials and Methods

A total of 126 juveniles of Chelon labrosus were distributed across nine 500 L tanks, within a recirculating aquaculture system equipped with independent mechanical and biological filtration units. Three experimental treatments were established (triplicate tanks each): (C) control (constant 36 ppt), (S2) fast gradient salinity reduction (36 to 0 ppt over 18 days), and (S3) slow gradient salinity reduction (36 to 0 ppt over 24 days). Water quality parameters were monitored daily, as sampling was conducted at key salinity points according to review (36,16, 5, and 0 ppt) throughout the one-month trial. At each sampling point, fish were collected and blood samples were obtained in heparinized tubes, centrifuged to isolate plasma for the analysis of cortisol, glucose, lactate, and triglycerides using commercial assay kits. Data was analyzed using ANOVA.

Results

No mortality was observed under any of the applied salinity gradients throughout the experimental period. Plasma cortisol levels in Chelon labrosus were significantly affected by salinity and acclimation rate. At 16 ppt, both fast (S2, 7days) and slow (S3, 14days) acclimation treatments exhibited significantly higher cortisol levels compared to the control (36 ppt), indicating an acute stress response associated with salinity reduction. At 5 ppt, significant differences were detected among treatments (Tukey's post hoc test, p < 0.05), with fish under fast acclimation (S2) showing significantly higher cortisol levels than both the control (C) and slow acclimation (S3) treatments, which displayed similar values. At the final salinity (0 ppt), cortisol levels decreased across treatments, with the lowest values recorded in the slow gradient treatment. Cortisol levels exhibited a treatment-dependent pattern, with an initial increase and a subsequent decline at the end (Figure 1). Regarding metabolic responses, plasma lactate levels at 16 ppt showed significant differences among treatments (Tukey's post hoc test, p < 0.05), with higher values observed in S2 compared to the control, while S3 displayed intermediate values. Fatty acids (FAMEs) and lipid profile are currently being processed to further evaluate metabolic adjustments associated with salinity acclimation.

Figure 1. Effect of salinity acclimation rate on plasma cortisol (µg/dL) in Chelon labrosus, in all the treatments C (Control), S2 (Fast) and S3 (Slow) and the salinities 36, 16, 5 and 0 ppt.

Figure 1. Effect of salinity acclimation rate on plasma cortisol (µg/dL) in Chelon labrosus, in all the treatments C (Control), S2 (Fast) and S3 (Slow) and the salinities 36, 16, 5 and 0 ppt.

Discussion

The increase in plasma cortisol at intermediate salinity (16 ppt) confirms that salinity reduction induces a physiological stress response in Chelon labrosus, consistent with previous reports on euryhaline teleosts exposed to osmotic fluctuations. Differences between acclimation strategies indicate that the rate of salinity change is a key factor modulating endocrine stress responses. Fast gradient (S2) resulted in sustained cortisol elevation at low salinity, whereas slow gradient (S3) promoted a faster return toward baseline levels. These findings are consistent with the concept of stress preconditioning, whereby exposure to sublethal environmental change can enhance physiological resilience to subsequent abiotic stress through conserved cellular stress response mechanisms in fish (Evans & Kültz, 2020; Quirós-Pozo et al., 2025). Overall, the results support the importance of controlled acclimation strategies to optimise physiological stability and welfare in Chelon labrosus under low-salinity aquaculture conditions.

Acknowledgment

This work was partially supported by the project "Climate Resilience and Adaptation in Aquaculture. Testing innovative technologies and ecosystem-based solutions for the resilience of Atlantic aquaculture to climate change (CLIRAQUA), co-financed by the Interreg Atlantic Area Programme through the EU Regional Development Fund (EAPA_0103/2024), and the project "Contributing to the recycling of industrial and agricultural waste through the cultivation of algae and the use of their polysaccharides in biomedical and environmental products that have a positive impact on society (CALYPSO), Interreg MAC 2021-2027 (1/MAC/1/1.1/0088).

References

Evans, T. G., & Kültz, D. (2020). The cellular stress response in fish exposed to salinity fluctuations. Journal of Experimental Zoology Part A: Ecological and Integrative Physiology, 333(6), 421-435.

Krause, G., Le Vay, L., Buck, B. H., Costa-Pierce, B. A., Dewhurst, T., Heasman, K. G., ... & Strand, A. (2022). Prospects of low trophic marine aquaculture contributing to food security in a net zerocarbon world. Frontiers in Sustainable Food Systems, 6, 875509.

Quirós-Pozo, R., Roo, J., Izquierdo, M., Koven, W., Ramírez-Bolaños, S., Ventura-Castellano, A., ... & Robaina, L. (2025). Effective complete replacement of fish oil by linseed oil in diets for thick-lipped grey mullet (Chelon labrosus) juveniles reared at three environmental salinities. Aquaculture, 594, 741425.

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