Voluntary timing of food intake increases weight gain and reduces basal plasma cortisol levels in common carp (Cyprinus carpio L.)

•Demand-feeding carp grow four to five times faster than pair-fed controls.•Plasma in particular cortisol concentrations vary with feed regime.•The expression of leptin-a1 and -a2 mRNA in carp differs markedly between experimental groups and time points.

We investigated the effect of timing of food intake on growth in common carp (Cyprinus carpio L.). Juvenile carp were demand-fed for 22 days using a computerized pendulum feeder that registered meal requests. Controls were pair-fed at 10:00 h, both groups were kept at 12L:12D (lights on at 06:30 h). Demand-fed fish displayed highest food intake at 22:00 h, and the lowest at 10:00. After 22 days, demand-fed fish had grown by 20% of their initial body weight, compared to 4% of the pair-fed control. Plasma cortisol levels in demand-fed fish were remarkably low and stable, whereas in the control group levels had increased 60-fold at 10:00 h compared to 22:00 h. Hepatic mRNA expression of leptin-a1 and leptin-a2 also differed markedly between groups and time points, with leptin-a2 expression being lowest in the demand-fed group at the time point of lowest food intake. We conclude that timing of food intake is an important determinant of endocrine status, growth and welfare.