The twilight zone contains the largest biomass of the world’s ocean. Identifying its role in the trophic supply and contaminant exposure of marine megafauna constitutes a critical challenge in the context of global change. The white shark (Carcharodon carcharias) is a threatened species with some of the highest concentrations of neurotoxin methylmercury (MeHg) among marine top predators. Large white sharks migrate seasonally from coastal habitats, where they primarily forage on pinnipeds, to oceanic offshore habitats. Tagging studies suggest that while offshore, white sharks may forage at depth on mesopelagic species, yet no biochemical evidence exists. Here, we used mercury isotopic composition to assess the dietary origin of MeHg contamination in white sharks from the Northeast Pacific Ocean. We estimated that a minimum of 72% of the MeHg accumulated by white sharks originates from the consumption of mesopelagic prey, while a maximum of 25% derives from pinnipeds. In addition to highlighting the potential of mercury isotopes to decipher the complex ecological cycle of marine predators, our study provides evidence that the twilight zone constitutes a crucial foraging habitat for these large predators, which had been suspected for over a decade. Climate change is predicted to expand the production of mesopelagic MeHg and modify the mesopelagic biomass globally. Considering the pivotal role of the twilight zone is therefore essential to better predict both MeHg exposure and trophic supply to white sharks, and effectively protect these key vulnerable predators.

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Madigan et al. (2021) recently reported a supposedly illegal fishery targeting white sharks in the Gulf of California (GC), Mexico. While we are strongly against illegal fishing and recognize the need to enhance enforcement to pro-This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

How animals use a set of physical characteristics and resources in a habitat, together with the time over which they use them, is known as habitat use (Hall et al., 1997). Energy is the most precious asset in an ecosystem (Lawson et al., 2019), such that an animal must always be concerned with conserving it by balancing its acquisition and loss (MacArthur & Pianka, 1966) while minimizing predation risk (Bartumeus & Catalan, 2009). Consequently, animals must make different decisions throughout the day, optimizing their energy use (e.g., whether to hunt, take refuge, socialize for reproductive purposes or not, and rest) (Heithaus et al., 2001;Papastamatiou et al., 2011). This will lead to specific patterns of activity depending on the physical conditions (tides, time of day, temperature, wind, vegetation, currents, bathymetry, etc.) and external biological conditions (distribution and density of their predators and prey), as well as physiological requirements, at a given time (Houston & McNamara, 2014). In general terms, the more linear movements (and fewer