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

Diet data are critical for describing resource use and partitioning among
competitors. However, timeseries of diet data 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 timeseries spanning
two decades (1998-2018), consisting of 2,749 stomachs from ten 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 examined feeding
habits with respect to prey taxonomy, length, vertical habitat, and
horizontal habitat. From 1998-2015, 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 increased abundance of
northern anchovy (Engraulis mordax) starting in 2016, the ecological
niches of bluefin tuna and swordfish converged. Both predators fed more
heavily on northern anchovy and other prey that occupy shallow nearshore
habitats. Changes in resource availability likely varied with depth across
our time series. We show that pelagic predators can maintain resource
partitioning under a wide range of conditions. However, we also observed
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.