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Large marine predators such as tunas and sharks play an important role in structuringmarine food webs. Their future populations depend on the environmental conditions they en -counter across life history stages and the level of human exploitation. Standard predator−prey rela-tionships suggest favorable conditions (high prey abundance) should result in successful foragingand reproductive output. Here, we demonstrate that these assumptions are not invariably validacross species, and that somatic condition of Atlantic bluefin tuna Thunnus thynnus in the Gulf ofMaine declined in the presence of high prey abundance. We show that the paradox of decliningbluefin tuna condition during a period of high prey abundance is explained by a change in the sizestructure of their prey. Specifically, we identified strong correlations between bluefin tuna bodycondition, the relative abundance of large Atlantic herring Clupea harengus , and the energeticpayoff resulting from consuming different sizes of herring. This correlation is consistent withoptimal foraging theory, explaining why bluefin tuna condition suffers even when prey is abundant.Furthermore, optimal foraging principles explain a shift in traditional bluefin tuna foraging areas,toward regions with a higher proportion of large herring. Bluefin tuna appear sensitive to changesin the size spectrum of prey rather than prey abundance, impacting their distribution, reproductionand economic value. Fisheries managers will now face the challenge of how to manage for highabundance of small pelagic fish, which benefits benthic fishes and mammalian predators, andmaintain a robust size structure beneficial for top predators with alternative foraging strategies.KEY WORDS: Bluefin tuna · Herring · Optimal foraging · Condition ·

The paradox of the pelagics: why bluefin tuna can go hungry in a sea of plenty