Low-enthalpy geothermal fluids are being used in Porto Lagos, Greece, to prevent freezing in an open (uncovered) pond connected to the sea where various fish species spend the winter. Despite the low temperature of the fluids and the moderate flow, the system proved to be satisfactory, increasing the pond temperature by more than 5 degrees C. This prevented the formation of an ice layer on the surface of the pond that would otherwise have caused a massive reduction in the fish population, as has happened in the past. A numerical model was developed and validated to simulate the thermal behavior of an open pond, based on hourly temperature measurements collected from various depths and at various sites in the pond during the last heating period. The simulator was subsequently applied to the evaluation of various alternative scenarios of exploitation. The hourly pond and ambient temperatures were also correlated in such a way as to obtain a semi-empirical expression for the heat losses from the pond and to estimate the time response of the system, so as to regulate the flow of geothermal fluids into the pond. (c) 2005 CNR. Published by Elsevier Ltd. All rights reserved.