Introduction
The transport of live fish is a critical welfare bottleneck in aquaculture, exposing fish to multiple interacting stressors, including handling, high stocking densities, feed deprivation, and deteriorating water quality (e.g., oxygen depletion and accumulation of CO2 and ammonia). These factors can disrupt physiological homeostasis, impair immune function, and increase mortality (Portz et al., 2006), leading to welfare and economic concerns. While the stress-reducing effects of dietary tryptophan in fish are well documented (H��glund et al., 2005; Le Floc'h & Seve, 2007; Basic et al., 2013; Sahu et al., 2020; Bampidis et al., 2024), its application as a water-soluble additive during live transport has not yet been investigated. A recent study in hybrid groupers indicates that exogenous tryptophan can modulate antioxidant status, inflammatory markers, and stress hormones under simulated transport conditions (Cao et al., 2024). However, these findings cannot be transferred directly to European freshwater species, such as rainbow trout. Furthermore, there is a need to investigate effects under real transport conditions, elucidate dose–response relationships, and consider regulatory safety aspects. Tryptophan, an essential amino acid and serotonin precursor, plays a central role in modulating the stress axis (H��glund et al., 2005; Le Floc'h & Seve, 2007; Bampidis et al., 2024; Cao et al., 2024). However, dietary approaches are limited during transport, as fish are typically fasted and tryptophan is rapidly metabolized, resulting in low bioavailability under acute stress. To address this limitation, this study investigates the direct application of tryptophan in transport water to enable gill uptake and continuous exposure. This approach offers a non-invasive, residue-free strategy to improve fish welfare and a potential alternative to conventional anesthetics.
Materials and Methods
The efficacy of a water-soluble tryptophan formulation was evaluated in rainbow trout (Oncorhynchus mykiss) using a two-phase experimental design.
In Phase I, transport simulation experiments were conducted under controlled laboratory conditions (up to 24 h, 30 L tanks, continuous oxygenation). Fish (500–700 g) were stocked at transport-relevant biomass densities. Treatments included an untreated control and multiple tryptophan concentrations (30, 50, 70 mg L-1), each with three replicates. Based on initial results, additional trials at higher tryptophan concentrations (100, 200, and 300 mg L-1) were conducted to better characterize dose–response relationships and identify effective concentration ranges.
In Phase II, the optimal concentration was validated at a commercial trout farm (Zordel, Germany), using transport tanks and systems representative of industry practice (up to 8 h transport duration and continuous oxygenation).
Across all experiments, physiological stress indicators (cortisol, glucose, lactate, hematocrit), antioxidant responses (superoxide dismutase), behavioral parameters, and water quality variables were assessed.
Results
Preliminary results indicate that waterborne tryptophan can reduce physiological stress responses, with trends toward lower lactate levels in treated groups. However, at lower concentrations (30–70 mg L-1), the effects were moderate and not statistically significant, likely due to relatively low overall stress levels and biomass-dependent differences in exposure. High-dose experiments (100–300 mg L-1) established a clear dose–response relationship and determined effective concentrations for industry use.
These findings highlight a key limitation of both low-dose water application and dietary supplementation: insufficient effective exposure under realistic transport conditions. This supports the hypothesis that feed-based strategies are not optimal during transport due to fasting and rapid tryptophan turnover, whereas waterborne application ensures immediate and continuous availability. Importantly, no negative effects on fish behavior or water quality were observed across treatments, confirming the safety and practical applicability of the approach.
Discussion
This study provides the first evidence that acute exposure to waterborne tryptophan can mitigate transport-induced stress in fish, highlighting its role as a regulator of stress response under fish transport conditions. By enabling immediate and continuous uptake, this approach addresses key limitations of conventional strategies, particularly the reduced effectiveness of feed-based additives during fasting periods prior to transport. The results demonstrate the potential of waterborne tryptophan as an innovative, welfare-oriented tool to improve fish resilience during transport, with no observed negative effects on behavior or water quality. In this context, it represents a promising alternative or complement to conventional anesthetics, which may show inconsistent efficacy and are often subject to regulatory constraints.
Beyond its immediate application, the approach offers a pathway to sustainable, residue-free stress mitigation strategies. In particular, the use of natural tryptophan sources, such as macroalgae (e.g., Ulva spp.), may further support the development of environmentally friendly solutions for aquaculture, aligning improved animal welfare with industry sustainability goals.
Acknowledgements
This research project, "TranQuil-Fish," was supported and funded by the Alfred Wegener Institute Helmholtz Center for Polar and Marine Research (AWI) Innovation Fund.
References
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