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The present study investigates the suitability of a machine vision-based method to detect deviations in the Shannon entropy (SE) of a European seabass (Dicentrarchus labrax) biological system fed with different selenium: mercury (Se:Hg) molar ratios. Four groups of fish were fed during 14 days with commercial feed (control) and with the same feed spiked with 0.5, 5 and 10 mg of MeHg per kg, giving Se: Hg molar ratios of 29.5 (control-C-1); 6.6, 0.8 and 0.4 (C-2, C-3 and C-4). The basal SE of C-1 and C-2 (Se:Hg > 1) tended to increase during the experimental period, while that of C-3 and C-4 (Se:Hg < 1) tended to decrease. In addition, the differences in the SE of the four systems in response to a stochastic event minus that of the respective basal states were less pronounced in the systems fed with Se: Hg molar ratios lower than one (C-3 and C-4). These results indicate that the SE may be a suitable indicator for the prediction of seafood safety and fish health (i.e., the Se: Hg molar ratio and not the Hg concentration alone) prior to the displaying of pathological symptoms. We hope that this work can serve as a first step for further investigations to confirm and validate the present results prior to their potential implementation in practical settings. We wish to thank Grupo Tinamenor (Cantabria, Spain) for providing the European seabass. Jose Antonio Carrero, of the Department of Analytical Chemistry of the University of the Basque Country, UPV-EHU, measured the concentrations of Hg and Se in the feeds by ICP-MS. The work was supported by grants from the Spanish MINECO (RTC-2014-2837-2-"SELATUN: Minimizacion de la problematica del mercurio del atun y valorizacion del atun como alimento saludable, Programa Retos-Colaboracion 2014" and CTM2012-40203-C02-01-"BMW: Biomarcadores estandar de base cientifica en mejillon para diagnosticar y monitorizar los efectos biologicos de la polucion en el Golfo de Bizkaia: implementacion de la DEME"), Euskampus Fundazioa-Campus of International Excellence (307615SAA2) and from the Basque Government Elkartek Grant KK-2016/00057-"MAROMEGA: Nuevas alternativas para la produccion de omega-3 a partir derecursos marinos". The funding sources had no involvement in the preparation of this manuscript. We would like to express our gratitude to the three anonymous reviewers for their useful comments in order to improve this manuscript.

Methylmercury (MeHg) is an environmental contaminant of increasing relevance as a seafood safety hazard that affects the health and welfare of fish. Non-invasive, on-line methodologies to monitor and evaluate the behavior of a fish system in aquaculture may make the identification of altered systems feasible-for example, due to the presence of agents that compromise their welfare and wholesomeness-and find a place in the implementation of Hazard Analysis and Critical Control Points and Fish Welfare Assurance Systems. The Shannon entropy (SE) of a European seabass (Dicentrarchus labrax) system has been shown to differentiate MeHg-treated from non-treated fish, the former displaying a lower SE value than the latter. However, little is known about the initial evolution of the system after removal of the toxicant. To help to cover this gap, the present work aims at providing information about the evolution of the SE of a European seabass system during a recuperation period of 11 days following a two-week treatment with 4 mu g center dot MeHg/L. The results indicate that the SE of the system did not show a recovery trend during the examined period, displaying erratic responses with daily fluctuations and lacking a tendency to reach the initial SE values. We wish to thank Grupo Tinamenor (Cantabria, Spain) for providing the European sea bass, Urtzi Izagirre for his contribution to the design of the experimental treatments, and Xabier Lekube and Gregor Bwye for technical assistance. The work received financial support from the Spanish Ministry of Economy and Competitiveness Project number: CTM2012-40203-C02-01, Towards science-based standard biomarker methods, suitable to diagnose and monitor pollution biological effects in the Bay of Biscay for the purpose of implementing the European Marine Strategy Framework Directive-BMW and Project number: RTC-2014-2837-2, Minimizacion de la problematica del mercurio del atun y valorizacion del atun como alimento saludable-SELATUN.