Importance of age structure in models of the response of upper trophic levels to fishing and climate change

There is a growing effort to use predictions of the physical state of the ocean under climate change to forecast the response of marine ecosystems. Many of these forecasts use ecosystem models rather than age-structured population models to describe upper trophic level (UTL) species. We illustrate the potential effects of climate on age-structured populations, then illustrate the ways in which ecosystem models might not depict adequately: (i) long-term changes in abundance, and (ii) variability attributable to cohort resonance. We simulated two generic species with different life histories, a short-lived semelparous species (e.g. salmon), and a long-lived iteroparous species (e.g. cod). For both species, juvenile survival was varied, first with white noise, then with the Pacific Decadal Oscillation as environmental signals. Variability in recruitment increased with fishing and became particularly sensitive to forcing at time-scales near the mean age of reproduction, consistent with the cohort resonance effect. Ecosystem models without age structure do not predict this behaviour, particularly when the ecosystem model incorrectly predicts the effective steepness of the stock recruitment relationship, or the age structure is approximated by a stage-structured model. We suggest that ecosystem models of UTLs include full representations of age structure, fitted to available population data.