Resource partitioning among pelagic predators remains stable despite annual variability in diet composition

Diet data are critical for describing predator resource use and partitioning among competitors. However, time series needed to properly assess variability in resource use and partitioning are limited, especially in pelagic (open ocean) ecosystems where predators and prey make broad use of horizontal and vertical habitats. We examined a diet time series spanning two decades (1998-2018) consisting of 2749 stomachs from 10 pelagic predators in the southern California Current Ecosystem (SCCE): albacore tuna (Thunnus alalunga), Pacific bluefin tuna (Thunnus orientalis), swordfish (Xiphias gladius), blue shark (Prionace glauca), shortfin mako (Isurus oxyrinchus), common thresher shark (Alopias vulpinus), bigeye thresher shark (Alopias superciliosus), short-beaked common dolphin (Delphinus delphis), long-beaked common dolphin (Delphinus capensis) and northern right whale dolphin (Lissodelphis borealis). We quantified feeding habits with respect to prey taxonomy, length, vertical habitat and horizontal habitat. From 1998 to 2015, each predator exhibited diet variability but maintained consistent resource partitioning with the other predators. Across years, the diets of predators feeding mostly on shallow-living prey (<200 m) were more variable than those feeding on deeper-dwelling prey (>200 m). Following an increase in the abundance of northern anchovy (Engraulis mordax) in the SCCE starting in 2015, the ecological niches of Pacific bluefin tuna and swordfish converged. During 2016-2018, both predators fed more heavily on northern anchovy and other prey that occupy shallow nearshore habitats. We show that pelagic predators can maintain resource partitioning under a wide range of conditions. However, we also observe that drastic changes in resource availability can alter the degree of niche partitioning among competitors, providing new perspectives on the flexibility of predator niches. As climate change continues to alter food webs, understanding how predators forage will be essential for anticipating changes to pelagic ecosystem structure and services.