On the importance of fine scale analyses in cephalopod population studies

Because cephalopods in the Northeast Atlantic have long been considered as minor commercial resources data acquisition is often scanty and fisheries scientists are tempted to apply DLS (Data Limited Stocks) methods to these stocks. However, progress made in the understanding of cephalopod population dynamics or of their role in the ecosystem reveals that cephalopod population assessment is data demanding. Each progress, from the adaptation of Leslie-DeLury methods to its generalized version required data at a finer temporal resolution than in most fin fish stock assessments. The difficulty and huge effort in obtaining finer scale information on a routine basis has led to some approaches being abandoned (like age-based methods). The ecosystem approach also requires a better integration of cephalopods in trophic models and accurate diet data. The challenges that we are facing with possible spatial distribution shifts or the need to understand the consequences of local dynamic fishing pressure are not likely detected by DLS tools and will also require data at a fine spatial and temporal scale. Examples such as "the September 2017 cuttlefish rush in the Southwest of England" or the interaction between small-scale and large-scale fisheries, underline the difficulties in estimating the level of fishing mortality in short-lived migrating populations. Recent diagnostics about large management units assessed on an annual basis do not confirm fishers’ observations