Study reveals the link between hypoxia and the sense of smell in gilthead seabream

The sense of smell is one of the most vital faculties for teleost fish, as it enables them to detect chemical signals in the water to locate food, identify suitable habitats, communicate with other fish, find mates, and avoid pretators. For gilthead sea bream (Sparus aurata), a cornerstone species in aquaculture, these abilities are critical to their wellbeing, especially in controlled environments where water quality management directly affects their behavior and performance.

However, a recent study has shown that this sense can be significantly compromised by low oxygen levels in water, even under moderate hypoxic conditions. The findings indicate that the olfactory sensitivity of gilthead seabream decreases notably when exposed to compounds like L-cystieine under hypoxia, reducing their ability to detect critical stimuli.

This is particularly problematic in low-visibility environments, such as turbid waters in ponds systems or floating farms, where the sense of smell plays a compensatory role when vision is limited.

To investigate the connection between hypoxia and olfaction, researchers from the University of Toronto and the University of Algarve conducted an experiment using electro-olfactograms, a technique that measures electrical responses from sensory neurons in the olfactory epithelium when exposed to specific chemical stimuli.

The tested juvenile seabream weighing 200-300 grams under different levels of dissolved oxygen: normoxia (20 kPa O₂): standard oxygen conditions; Mild hypoxia (12.5 kPa O₂): 60% oxygen saturation; Moderate hypoxia (5.7 kPa O₂): 35% oxygen saturation.

To assess their olfactory capacity, the fish were exposed to three aminoacid commonly associated with odour detection: L-cysteine, L-arginine, and L-leucine, in five different concentrations.

The results revealed a significant decrease in olfactory sensitivity to L-cysteine and L-arginine under mild and moderate hypoxia, but not detectable effect on L-leucine. For L-cysteine, the response decreased by 16% under mild hypoxia and by 17% under moderate hypoxia at the highest concentrations tested.

Encouragingly, the olfactory sensitivity of the fish fully recovered once normoxic conditions were restored, indicating that the effects of hypoxia are reversible if corrected in time.

The study highlights that even mild levels of hypoxia can impact critical functions for the survival of gilthead seabream, such as feeding and reproduction. This underscores the importance of maintaining optimal levels of dissolved oxygen in aquaculture systems to avoid issues related to stress, behavior, and productivity.

In the context of aquaculture, these findings have direct implications for designing effective environmental management strategies. Ensuring a constant oxygen supply can improve the wellbeing of gilthead seabream and enhance the sustainability of their production, benefiting both farmers and the ecosystems in which they operate.

This research also paves the way for future studies to explore methods to mitigate these effects and optimize farming practices in controlled environments.