Assessment of energy content indices for exploited small pelagic fish

Energy content is a reliable indicator of an individual's body condition, influencing growth, reproduction, and survival with cascading effects on population dynamics and ecosystems through energy transfer. Energy density can be measured through bomb calorimetry or estimated using proxies such as morphometric indices, water content, or proximate composition. Direct methods are the most accurate but are also time-consuming and expensive, while there is a lack of proper assessment of the various existing proxies. This study, based on a large dataset of proximate composition and energy density for anchovy ( Engraulis encrasicolus ) and sardine ( Sardina pilchardus ) from the Bay of Biscay and the English Channel, thoroughly compared four methods of varying complexity to identify the most accurate and practical approach for estimating energy density and total energy. Morphometric-based methods performed poorly, as they fail to distinguish variations in water, protein, and lipid content, each contributing differently to energy. While proximate composition could be reasonably well estimated from length, the accuracy of this estimation did not significantly improve energy estimations compared to morphometric indices. In contrast, water content proved to be the most reliable indicator of energy density, that can be accurately estimated either directly through its negative relationship with water content, or indirectly via proximate composition. Furthermore, incorporating three levels of individual condition states into the water content based methods, improved the accuracy of energy density estimates. This comprehensive comparison highlights the importance of standardizing water content measurements as a cost-effective alternative to calorimetry, enabling routine energy monitoring in fish surveys.