Can biomass time series be reliably assessed from CPUE time series data only ?

In a letter to Nature, Myers and Worm present, for a wide range of oceanic ecosystems, an analysis of catches per unit of effort (CPUE) data, leading them to the conclusion of “a rapid worldwide depletion of predatory fish communities”: After a sharp initial reduction, CPUE appears stable since more than twenty years at a level of about 10% of their initial value. Such a result makes quite evident the necessity of a worldwide common “rebuilding effort” for stocks and ecosystems. Through the estimation of initial CPUE their results also provide a very useful benchmark for goal identification and estimation of the needed level of this rebuilding effort. Those conclusions are based on the critical assumption that CPUE is proportional to abundance. This means (i) that catchability is constant and (ii) that all the biomass is catchable. If so, relative variations in CPUE indicate the same relative variations in biomass. Myers and Worm consider the first part of this hypothesis as wrong because of increasing efficiency of fishing. Therefore, a stable CPUE level, with increasing catchability means a further decrease in biomass, which makes conservatives the obtained results on biomass decreases and stronger the conclusions they present. Myers and Worm do not consider the second part of the hypothesis, and hence make the implicit assumption of entirely catchable biomass. Those results and conclusion may appear contradictory. The initial decrease indicates a reduction in a few years below the half of the initial biomass, which means that fishing mortality was above FMSY level in the very first years of fishing. As nominal effort and cathability may be assumed both increasing since the beginning of the fishery, this means that there should be no more fish... This leads to the question of the remarkable result of stable CPUE from another point of view. The equation used by Myers and Worm is Nt= N0 ( (1-δ) exp t) + δ), (1)