Liver and Plasma Fatty Acid Characterization in Cultured Brown Trout at Distinct Reproductive Stages

SIMPLE SUMMARY: Lipids, namely fatty acids, are crucial energy sources in fish development, reproduction, and migration, among other processes. In this study, we are interested in identifying the most relevant fatty acids in the liver and plasma throughout the reproductive cycle of farmed brown trout in both sexes. Four reproductive stages along the cycle were selected: spawning capable (December), regressing (March), regenerating (July), and developing (November) stages. The most abundant fatty acids were palmitic acid (16:0), stearic acid (18:0), oleic acid (18:1 n-9), arachidonic acid (20:4 n-6), eicosapentaenoic acid (EPA, 20:5 n-3), and docosahexaenoic acid (DHA, 22:6 n-3). Sex differences were found for some fatty acids, but seasonal changes stood out more. In the liver, polyunsaturated fatty acid (PUFA) levels were significantly higher in December than in March and July for both males and females. Changes between the reproductive stages in male and female plasma were mostly related to SFA and PUFA, and PUFA and ∑n-3 PUFA, respectively. This evidences that PUFAs are crucial for reproduction in both sexes. Further, in female plasma samples, the most representative fatty acids distinguished the four reproductive stages, which indicates that those fatty acids should be further investigated as biomarkers of female maturity statuses. ABSTRACT: Fatty acids are energy sources, and their profiles are used as biomarkers of metabolic status and physiological changes in fish. Within this context, the main aim of this study was to identify the fatty acids that best discriminate the reproductive status of male and female farmed brown trout. The fatty acid composition in liver and plasma samples from the adults of both sexes was monitored along four distinct reproductive stages, namely the spawning capable (December), regressing (March), regenerating (July), and developing (November) stages. Irrespective of the sex and stage, the most representative fatty acids were palmitic acid (16:0), stearic acid (18:0), oleic acid (18:1 n-9), arachidonic acid (20:4 n-6), eicosapentaenoic acid (EPA, 20:5 n-3), and docosahexaenoic acid (DHA, 22:6 n-3). There were no significant sex differences in fatty acid classes in the liver and plasma. Despite this, there were several changes in individual fatty acid levels between the sexes. In the liver, both males and females showed high monounsaturated fatty acid and low polyunsaturated fatty acid (PUFA) levels during the regressing and regenerating stages. At spawning capable and developing stages, a reverse profile was noted. The plasma profiles were mainly influenced by changes in saturated fatty acids and PUFAs in males and by PUFA in females. Based on the most representative fatty acids, four patterns were established for female plasma samples, one for each reproductive stage. This scenario suggests that female plasma samples are promising for the discrimination of gonadal reproductive status, and this potential can be further explored in aquaculture and environmental monitoring studies.