Eurytemora affinis egg mass size-a commonly used demographic parameter-was surveyed in the Gironde estuary. Its decrease between 1978 and 2003 contrasted with the stability of population density during this long-term period. Different hypotheses were tested to explain this paradox. (i) Upstream shift of the population: Even though the population of E. affinis shifted upstream due to a greater penetration of marine water, no relationship between the population shift and demographic parameters was observed: the egg mass size exhibited the same evolution upstream. (ii) Water warming: No early egg production and no change of female size that could metabolically limit the egg mass size was observed. (iii) Food limitation: The change in food quality observed did not result in any change in total copepod abundance. (iv) Faster renewal of egg masses: The paradox cannot be explained by this factor and the number of adults observed nowadays is higher than the number of eggs produced by females, whereas fecundity rates strongly increased. The paradox is probably explained by the expulsion of viable subitaneous eggs. This may represent a recent behaviour adaptation of the copepod to environmental changes.