Abstract
Reducing water salinity towards iso-osmotic conditions is a common practice applied in euryhaline fish farming to limit osmoregulation costs and enhance growth. In this respect, the present study investigated the timing of salinity reduction in an abrupt manner during European eel (Anguilla anguilla) larval culture by examining associated impacts on morphological and molecular levels. Larvae from 3 different parental combinations (families) were reared at constant 36 psu for 6 days (control) or subjected to a direct reduction to 18 psu on 1, 2, or 3 days post-hatch. Overall, salinity reduction enhanced growth and survival, resulting from more efficient energy resource utilization. In the control group, expression of growth-related igf2 remained constant, demonstrating a steady growth progression, while igf1 expression increased over time only for the salinity reduced treatments, potentially qualifying as a useful biomarker for growth performance. Even though each parental combination seems to have a different capacity to cope with salinity alterations, as observed by family-driven water-transport-related aquaporin (aqp1, aqp3) gene expression, it could be inferred that the abrupt salinity change is generally not stressful, based on non-upregulated heat shock proteins (hsp70, hsp90). However, the applied salinity reduction (irrespective of timing) induced the development of pericardial edema. As such, we conclude that despite the positive effect of salinity reduction on early growth and survival, the long-term benefit for eel larval culture lies in establishing a protocol for salinity reduction, at a precise developmental time point, without causing pericardial malformations.
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Data availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.
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Acknowledgements
We would like to thank Paraskevas Koumpiadis and Annika Toth for broodstock husbandry and provision of gametes for the experiment, Eftychia Maria Goniou for embryonic rearing, and Eugenia Capatina, Joanna Miest, and Adrian Loh for assistance in molecular work.
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This study received funding from the Innovation Fund Denmark under grant agreement no. 7076-00125B (ITS-EEL) and from “ENV-Fonden.”
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JT, SP, SS, and IB provided funding, while JT, SP, and SS designed the study. JT established the assisted broodstock protocols providing gametes for the experiment. SS constructed and tailored the larval systems and rearing tanks. ES, SP, and EB conducted the experiment and collected samples. SP and EB carried out the gene expression analysis. IB performed the statistical analysis. SP and SS made illustrations. ES wrote original manuscript draft. All the authors contributed to data interpretation and manuscript revision.
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All fish were handled according to the European Union regulations concerning the protection of experimental animals (Dir 86/609/EEC). The experimental protocol for the present study was approved by the Danish Ministry of Food, Agriculture and Fisheries (permit number: 2015–15-0201–00696). In particular, broodstook used were anesthetized with ethyl p-aminobenzoate (benzocaine) prior to any handling, while efforts were made to minimize animal handling stress.
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Syropoulou, E., Benini, E., Sørensen, S.R. et al. Early and abrupt salinity reduction impacts European eel larval culture. Fish Physiol Biochem 48, 355–366 (2022). https://doi.org/10.1007/s10695-022-01056-6
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DOI: https://doi.org/10.1007/s10695-022-01056-6