The development of non-invasive methods for fish welfare and contaminant monitoring is of high relevance for the production of high quality and safe farmed fish. We have recently shown the suitability of the Shannon entropy (SE) in a commercially relevant fish system (European seabass, Dicentrarchus labrax) to differentiate methylmercury (MeHg) treated fish from non-treated fish. The present work examines the evolution of the SE of a European seabass system during an 11 days recuperation period immediately after a 2 weeks treatment with 4 µg MeHg/L (Case C2) and compares it to that of a control group not treated with MeHg (Case 1). While the SE of the C1 increased during the recuperation period, that of C2 showed erratic responses with a very modest decreasing trend. Keywords— Entropy, farmed fish, fish welfare, fish behaviour, environmental monitoring, contaminant detection, biological system, biological warning systems, system perturbation.