The French part of the continental shelf of the Bay of Biscay (northeast Atlantic) is the habitat of an anchovy (Engraulis encrasicolus L.) population of scientific interest because of its economic importance and the trend towards greater interannual fluctuations in its abundance, particularly as a consequence of variations in recruitment. Each year a 1-month survey of this population and its pelagic environment is undertaken. Among the descriptors of ecosystem function characterized, mesozooplankton aspartate transcarbamylase (ATC) activity is measured with the goal of defining overall productivity of the mesozooplankton communities. Diverse physical forcing factors are responsible for the enhancement of productivity in the Bay of Biscay, and their respective influences vary at interannual scales. We present the results of ATC activity measurements carried out within the anchovy habitat during six consecutive breeding seasons (from 2000 to 2005). A strong correlation was found between mean ATC activity and variations in interannual biomass for the anchovy population (R = 0.928; p < 0.01). During the study period the anchovy population collapsed, and a particularly low level of ATC activity in the mesozooplankton preceded this event 1 year earlier. Conversely, an increase in anchovy abundance in the year following the collapse was preceded by a return to substantially higher levels of ATC activity. We hypothesize that this relationship may be robust and generally applicable. We speculate on the environmental descriptors necessary to confirm this result, in view of its potential application to the monitoring of this valuable fish population in the Bay of Biscay.