Résumé
In the Gulf of Lion in the Mediterranean Sea, there has been a steady decrease in total biomass of European sardines, Sardina pilchardus since 2007. This is due to a decline in body condition and the disappearance of older age classes. Overfishing, predation pressure and pathogens have been discounted as explanatory factors and the current hypothesis is a 'bottom up' effect for this planktivorous species, linked to a change in the plankton community, towards smaller species. To test this, sardines were reared for 7 months on four diets, comprising combinations of small or large pellets in small or large quantities. The effects were studied at different levels of integration, from whole-organism to cellular and sub-cellular levels. Sardines fed with large quantities of large pellets showed positive growth and above average body condition, linked to high mitochondrial respiration rates. By contrast, sardines fed with small pellets regardless of the quantity, and with large pellets in small quantities, exhibited a lower growth rate (up to -87%) and a poor body condition (up to -25%). This was associated with reduced maximal uncoupled respiration in red muscle mitochondria (-28%), but a better coupling efficiency in terms of ATP produced per molecule of oxygen consumed (+32%) for sardines that received small pellets. By contrast, the maximum rates of oxygen consumption (+36%) and ATP synthesis (+37%) were higher in fish fed on large pellets, regardless of the quantity. Our results show that, although both size and quantity of food particles had strong impacts on growth and body condition, only food size affected mitochondrial bioenergetics. Despite cellular plasticity leading to more efficient and economic metabolism in sardines fed small food particles, a decline in plankton size likely plays a role in the reduction of sardine size and body condition in the wild.