Year-round Energy Dynamics of Sardine and Anchovy in the North-western Mediterranean Sea

Variability in body condition and energy storage has important implications for fish recruitment and ecosystem structure. Understanding strategies for energy allocation to maintenance, reproduction and growth is essential to evaluate the state of the fish stocks. In this study, we address the energetics dynamics of the annual cycle of anchovies (Engraulis encrasicolus) and sardines (Sardina pilchardus) in the north-western Mediterranean Sea using indirect and direct condition indices. We assessed and validated the use of morphometric, biochemical and energetic indices for both species. Annual patterns of the relative condition index (Kn), gonadosomatic index (GSI), lipid content (% lipids) and energy density (ED) were linked to the energy allocation strategy. Our results highlight that anchovy mainly rely on income energy to reproduce, while sardine accumulate the energy during the resting period to be used in the reproduction period. Consequently, variability in the lipid content and ED between seasons was lower in anchovy than in sardine. In both species, we observed an early decline in energy reserves in late summer-early fall, which may be related to unfavourable environmental conditions during spring and summer. Regarding the use of different condition indices, both direct indices, lipid content and ED, were highly correlated with Kn for sardine. ED was better correlated with Kn than lipid content for anchovy. For the first time, a relationship between ED of gonads and GSI for sardine and anchovy was provided, highlighting the importance of the energy invested in reproduction. This work provides new insights into the energy dynamics of sardine and anchovy. We also demonstrate which are the most suitable indices to measure changes in the physiological condition of both species, providing tools for the future monitoring of the populations of these two commercially and ecologically important fish species.