Because of natural complexity, field studies are often inconclusive regarding the final cause of changes observed in populations of wild animals. We favour the use of experimental ecology studies to evaluate the effects of environmental changes on marine fish biology. We identify the advantages and constraints of experimental ecology. We present some examples of the contribution of experimental ecology to the study of the effects of environmental factors in Atlantic cod (Gadus morhua). M. Suquet, MJ Rochet, JL. Gaignon. Aquat. Liv. Resour, 18 : 251-259.