Trends and cohort resonant effects in age-structured populations

1. Trends and fluctuations in populations are determined by complex interactions between extrinsic forcing and intrinsic dynamics. As an example, the dynamics of many marine fish are characterized by age-structured dynamics forced by stochastic recruitment. 2. In this study we develop stochastic age-structured models for two case studies, the Atlantic bluefin tuna and the Atlantic cod. The former exemplifies intracohort interactions and density-dependent reproduction, the latter exemplifies density-dependent survival and intercohort interactions. 3. We use transfer functions and delay-coordinate models to study how the combination of age-structured interactions and stochastic recruitment can induce low-frequency variability. 'Cohort resonance', as we dub this effect, can induce apparent trends in abundance and may be common in age-structured populations. 4. Our study complements the theory of structured populations that focuses on cycles and chaos (high-frequency dynamics). 5. The innate low-frequency fluctuations we describe can potentially mimic or cloak critical variation in abundance linked to environmental change, over-exploitation or other types of anthropogenic forcing. 6. From a management and conservation viewpoint, it will be important to find ways to separate anthropogenic forcing from cohort resonant effects and/or to understand the way they interact.