Disentangling the influence of density dependence, size dependence and environmental effects on fish population dynamics

Improving our understanding of the ecological and demographic mechanisms that underlie changes in wild population productivity is critical to support ecosystem-based management. Yet, population productivity results from a combination of demographic rates (such as mortality, growth and recruitment) driven by intrinsic and extrinsic factors which interact through density-, size- or environment-dependent mechanisms. The interdependence between demographic rates, and intrinsic and extrinsic factors, has rarely been studied throughout the life cycle of a wild population. This is particularly needed for short life species, such as small pelagic fish, which are subject to inter-annual abrupt changes in abundances. Here, we developed an age-based life-cycle model to investigate the relative influence of intrinsic and extrinsic factors on successive transitions rates along the life cycle of a system composed of two small pelagic species of the Bay of Biscay, the European anchovy (Engraulis encrasicolus) and European sardine (Sardina pilchardus). Our model allows to disentangle the influence of density dependence, size dependence and environmental factors throughout the life cycle of short life species. Our results highlight that sardine demography is density-dependent, with significant effects of density on natural mortality at age 1 and age 3. In contrast, anchovy demography is both size- and density-dependent, with density-dependent effects on natural mortality at age 1, age 2 and on recruitment, and a size-dependent effect on age-1 mortality. Moreover, we found that natural mortalities of both anchovy and sardine were related to large-scale environmental indicators, such as the Atlantic Multidecadal Oscillation and the North Atlantic Oscillation. The originality of this approach lies in its ability to integrate the effects of size, density and environment on a multitude of demographic processes throughout the life cycle for wild species, and can be seen as a step towards supporting ecosystem-based management.