Reconstructing individual food and growth histories from biogenic carbonates

Environmental conditions experienced by aquatic organisms are archived in biogenic carbonates such as fish otoliths, bivalve shells and coral skeletons. These calcified structures present an accretionary growth and variations in optical properties (color or opacity) that are used to reconstruct time. However, full and reliable exploitation of the information extracted from these structures is often limited as the metabolic processes that control their growth and optical properties are poorly understood. Here, we propose a new modeling framework that couples both the growth of a biogenic carbonate and its optical properties with the metabolism of the organism. The model relies on well-tested properties of the Dynamic Energy Budget (DEB) theory. It is applied to otoliths of the Bay of Biscay anchovy Engraulis encrasicolus, for which a DEB model has been previously developed. The model reproduces well-known otolith patterns and thus provides us with mechanisms for the metabolic control of otolith size and opacity at the scale of an individual life span. Two original contributions using this framework are demonstrated. (1) The model can be used to reconstruct the temporal variations in the food assimilated by an individual fish. Reconstructing food conditions of past and present aquatic species in their natural environment provides key ecological information that can be used to better understand population dynamics. (2) We show that non-seasonal checks can be discriminated from seasonal checks, which is a well-recognized problem when interpreting fish otoliths. We suggest further developments of the model and outline the experimental settings required to test this new promising framework.