MODELLING HYPOXIA IN E. SEABASS Dicentrarchus labrax: A BIOENERGETIC APPROACH

Introduction

Oxygen is an important environmental parameter affecting fish growth and health, and is expected to play an increasingly vital role in fish farming in the light of a warming ocean. Furthermore, the transition to a digital future where the industry scales up the adoption of automation and precision, mandates the development of tools that can enhance our predictive capacity regarding the availability of oxygen, or lack thereof, and its effect on the fish, particularly in scenarios of rising temperatures, low water circulation, and local hypoxic events (Wu et al., 2024). In this study we propose a modelling approach based on the Dynamic Energy Budget (DEB) theory, that quantify the potential hypoxic effects on the metabolism of the European seabass (Dicentrarchus labrax), explicity incorporating environmental oxygen as input.

Materials and methods

A previously parametrized and validated DEB model for E. seabass (Stavrakidis-Zachou et al., 2019) was used as the basis for incorporating the effects of hypoxia, with the environmental dissolved oxygen acting as an explicit model input. By adopting the framework of toxicodynamics-toxicokinets, oxygen deficit was viewed as a stressor via the production of reactive species which act as toxicants and follow certain tolerance thresholds and elimination dynamics. By using literature data on growth and feed consumption under various DO levels, the model was parametrized and a stress factor was applied and evaluated in regards to several modes of actions including effects on feeding, maintenance costs and costs for structural growth. Finally, simulations were performed using in-situ measurements of temperature and DO at the site of the HCMR pillot scale fish farm in Souda bay (Figure 1, left) to showcase model performance along with a scenario of unlimited oxygen supply (DO = 7.5mg l^-1) for comparison.

Results

While the development of the oxygen module is ongoing, provisional results offer promising insights. The proposed approach offers realistic outputs as evident by acceptable goodness of fit obtained on the datasets used for parametrization. Evaluation of the different modes of action indicates that effects on feeding, and therefore assimilation, can sufficiently explain hypoxic effects on the metabolism while the inclusion of additional modes of action only marginally improves model fits at the expense of added complexity. Growth simulations based on observed seasonal measurements of DO and temperature at a farm site demonstrate realistic empirical outputs for E. seabass growth (Figure 1). Moreover, comparisons with a hypothetical simulation under unlimited oxygen availability demonstrate observably better growth performance for the latter group suggesting that even small oxygen deficits throughout the production cycle may amount to a cumulative negative impacts on fish growth performance (Figure 1, right).

Figure 1. Seasonal changes in temperature and dissolved oxygen (in-situ measurements) at the HCMR farm (left) with the corresponding growth simulations for E. seabass (right) under the actual field DO values (dotted line) and a scenario of unlimited oxygen supply (DO = 7.5mg l^-1, solid line)

Conclusions

This study used a mechanistic framework to explicitly model the effect of environmental oxygen availability on the metabolism of E. seabass. Preliminary results from model parametrization are characterized by realism while simulations under field and scenario conditions yield interesting insights. Namely, the deviations from the theoretically maximum growth performance simulated under field observations, point to detectable hypoxic effects. Especially under climate change, these highlight the crucial role for close DO monitoring, potential mitigation strategies or adjustment of existing production practices.

References

Stavrakidis-Zachou, O., Papandroulakis, N., Lika, K., 2019. A DEB model for European sea bass (Dicentrarchus labrax): Parameterisation and application in aquaculture. Journal of Sea Research, Ecosystem based management and the biosphere: a new phase in DEB research 143, 262–271

Wu, S., Huang, J., Li, Y., Zhao, L., 2024. Comparative transcriptomics combined with physiological and functional analysis reveals the regulatory mechanism of rainbow trout (Onchorynchus mykiss) under acute hypoxia stress. Ecotoxicology and Environmental Safety, 278, 116347. https://doi.org/10.1016/j.ecoenv.2024.116347