PARENTAL SELENIUM NUTRITION FROM TUNA BY-PRODUCTS AND DIETARY MERCURY EXPOSURE: IMPLICATIONS FOR OFFSPRING PERFORMANCE IN RAINBOW TROUT Oncorhynchus mykiss

Introduction

Broodstock nutrition is a key determinant of offspring quality in salmonids. Tuna by-products (TBP) represent a promising alternative to fishmeal due to their high selenium (Se) content. However, their potential contamination with mercury (Hg) raises concerns regarding possible transgenerational effects (Berntssen et al., 2004; Donnarumma et al., 2021). This study aimed to evaluate the respective effects of parental dietary Se supplied by TBP and dietary methylmercury (MeHg) exposure in offspring on growth performance in rainbow trout (Oncorhynchus mykiss) fry. In addition, the regulation of selenoproteins in response to both Se supply and Hg exposure was investigated using immunohistochemical approaches.

Materials and methods

Five broodstock diets were tested: a plant-based control (VEG), a conventional fishmeal-based diet (FM60), and three diets including TBP at 15% (TBP15), 30% (TBP30), and 60% (TBP60), providing Se levels ranging from 0.3 to 8 mg Se kg^-1 diet. Groups of 20 females and 10 males were reared at 7 ± 1 °C and fed the experimental diets for 6 months prior to spawning. Oocytes from each female were fertilized with milt from males within the same dietary group.

The resulting fry were reared at 17 ± 1 °C and fed for 3 weeks either a plant-based control (FCO) or MeHg contaminated diet (FHG, 2 mg Hg kg^-1 feed). At the end of each feeding period, zootechnical performance was assessed in broodstock (growth, reproductive success) and offspring (growth, feed efficiency, survival). Data were analysed by ANOVA and expressed as means ± SD.

Results

No significant differences in growth performance were observed among broodstock dietary groups. No significant interaction between parental diet and offspring diet was detected for the measured parameters.

In contrast, offspring fed the MeHg-contaminated diet (FHG) showed significantly lower specific growth rate (SGR) and higher feed conversion ratio (FCR) compared to those fed the control diet (FCO), while survival was unaffected (Fig. 1).

Parental diet influenced offspring performance: fry originating from VEG and TBP30 groups exhibited lower SGR compared to FM60 and TBP15 (p < 0.05). FCR was lowest in FM60 and highest in VEG (p < 0.05). Survival was higher in TBP30 and FM60 compared to VEG.

Fig. 1. A) Specific growth rate (% day^-1), B) Feed conversion ratio (FCR; g feed·g^-1 body mass), and C) Survival (%) as a function of parental diet and offspring diet. Bars represent mean values for each parental diet group (VEG, FM60, TBP15, TBP30, TBP60). Colours indicate parental diets while patterns distinguish offspring diets (FCO: solid; FHG: striped). Error bars represent standard deviation. Different letters indicate significant differences among parental dietary groups (p < 0.05).

Discussion

No interaction between parental diet and offspring feeding regime was detected, suggesting limited influence of parental Se transfer on early-life Hg exposure under the present conditions. Dietary MeHg exposure impaired growth performance, as reflected by reduced SGR and increased FCR. Parental diet nevertheless modulated offspring performance, with TBP-based diets showing comparable or slightly lower performance than FM60, and no clear effect of dietary Se level on growth at this stage, possibly reflecting the specific chemical form of Se provided by tuna by-products, predominantly selenoneine, whose metabolism and transgenerational transfer remain largely unexplored.

Ongoing investigations aim to decipher the mechanistic basis of Se-Hg interactions, with a particular focus on selenoprotein regulation, tissue distribution, and oxidative stress pathways, providing key insights into the nutritional modulation of mercury toxicity during early development. These results will help refine the use of tuna by-products as a sustainable ingredient in aquafeeds.

Acknowledgment

This work was supported by the French National Research Agency (ANR) under grant ANR-24-CE34-0451 (MOONFISH, 2025-2029). The authors thank Marine Biotechnology Products (Port-Louis, Mauritius) for providing the tuna by-product meal.

References

Berntssen, M.H.G., Hylland, K., Julshamn, K., Lundebye, A.-K., Et., A., 2004. Maximum limits of organic and inorganic mercury in fish feed. Aquac. Nutr. https://doi.org/10.1046/j.1365-2095.2003.00282.x

Donnarumma, D., La Tella, R., Vento, F., Salerno, T.M.G., Micalizzi, G., Rigano, F., Mondello, L., 2021. Evaluation of the level of toxic contaminants and essential molecules in the context of the Re-use of tuna fishery industry by-products. Food Anal. Methods 14, 2161–2174. https://doi.org/10.1007/s12161-021-02045-w