Fronts-i.e., the boundaries between water masses-are ubiquitous in the world oceans and have been shown to significantly influence pelagic ecosystems with enhanced local productivity and increased abundances of forage fish and top predators. Here we use data from archival tags to document how four juvenile albacore tunas foraged at and exploited a thermal front. Of the 3098 observed trips, the albacore mainly swam across the front between the warm side above the thermocline and the cold side below the thermocline with an average of 78 6 20.4 cross-frontal trips per fish per day. The warm frontal surface waters provided a thermal resource, allowing the tuna to maintain higher body temperatures and thus forage more efficiently in the food-rich waters of the cold side of the front. Foraging success of the tunas decreased as the cross-front thermal gradient weakened. This first look into small-scale use of fronts by a top predator demonstrates that ephemeral, submesoscale oceanic features can play a significant role in pelagic ecology.Oceanic fronts, strong horizontal gradients in temperature, salinity and/or density, are biogeochemical hotspots caused by the physics associated with the abutment of water masses with different properties (L evy et al. 2012;Woodson and Litvin 2014). The increased remote and in situ sampling of our oceans in recent decades has demonstrated that fronts are ubiquitous features, occurring across a spectrum of temporal and spatial scales (L evy et al. 2012). Although the term "front" covers a wide array of physical features, most higher trophic level, ecological work has focused on mesoscale fronts (spanning 10-100 km) that are both persistent (lasting from weeks to months) and recurring (seasonally prominent 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. Scientific Significance StatementThe boundaries between water masses, known as "fronts," are oceanic hotspots-known to harbor increased abundances of bacteria, plankton, forage fish, and pelagic predators. Fronts are complex structures that vary in both time and space. The influence of these dynamics on open ocean ecosystems is still an active area of research, and it is not clear how large predators, such as tunas, exploit these features. We used a novel approach to show that albacore tuna use fronts as both a temperature and a food resource, but that their foraging success deteriorates as the horizontal scale of the front increases in size.

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