Pacific bluefin tuna, Thunnus orientalis, exhibits a flexible feeding ecology in the Southern California Bight

Abstract

Pacific bluefin tuna, Thunnus orientalis, migrates from spawning grounds in the western Pacific Ocean to foraging grounds in the California Current System (CCS), where they are thought to specialize on high energy, surface schooling prey. However, there has been substantial variability in estimates of forage availability in the CCS over the past two decades. To examine the foraging ecology of juvenile T. orientalis in the face this variability, we quantified the diet and prey energetics of 963 individuals collected in the Southern California Bight (SCB) from 2008 to 2016. Using classification and regression tree analysis, we observed three sampling periods characterized by distinct prey. In 2008, T. orientalis diet was dominated by midwater lanternfishes and enoploteuthid squids. During 2009-2014, T. orientalis consumed diverse fishes, cephalopods, and crustaceans. Only in 2015-2016 did T. orientalis specialize on relatively high energy, surface schooling prey (e.g. anchovy, pelagic red crab). Despite containing the smallest prey, stomachs collected in 2009-2014 had the highest number of prey and similar total energetic contents to stomachs collected in 2015-2016. We demonstrate that T. orientalis is an opportunistic predator that can exhibit distinct foraging behaviors to exploit diverse forage. Expanding our understanding of T. orientalis foraging ecology will improve our ability to predict its responses to changes in resource availability as well as potential impacts on the fisheries it supports.

Fig 1. Summary of Thunnus orientalis sampling.

(a) Map of study area showing sampling region in the Southern California Bight within ~150 nautical miles of San Diego, CA in blue. (b) Number of T. orientalis stomachs processed per year. (c) Month of collection (proportional bars, dark grey) and tuna fork length (density plots, light grey) for each year of sampling.

Fig 2. Classification and regression tree (CART) analysis describing groups of Thunnus orientalis with similar diets.

(a) The selected tree, with splits based on responses to the condition listed at each branching point; left for affirmative, and right for negative. The mean cross validated proportion of the most abundant prey group is given below each terminal node. (b) Diet composition for prey contributing >1% mean proportional abundance in each CART group (purples = fishes, browns = cephalopods, and greens = crustaceans).

Fig 3. Differences in diet and Thunnus orientalis size structure among sampling periods.

(a) Diversity accumulation curves for T. orientalis diets by CART group. Solid lines represent interpolated diversity and dashed lines represent extrapolated diversity to double the sample size of each group. The legend in panel (a) also applies to panel (b). (b) Principal coordinate analysis of T. orientalis diet by CART group. Ellipses represent 95% confidence intervals for each group. (c) Box plots of T. orientalis mass across sampling periods with means represented by grey diamonds on each boxplot (n = 644).

Fig 4. Lengths of crustacean, cephalopod, and fish prey consumed by Thunnus orientalis in the Southern California Bight, 2008–2016.

Boxplots of prey length are given for each prey type measured per CART group. The x-axis was cropped at 15 cm to improve visualization, but the boxplots dimensions include all measured individuals. The number of individuals greater than 15 cm body length (n_>15cm) as well as the total number of measured prey (n_total) is provided. Within each prey type, asterisks indicate groups that are significantly different from all other groups. The color key and body measurements for each prey type are given at right.

Fig 5. Characteristics of the stomach contents of Thunnus orientalis.

In all panels, the mean values per CART group are given as grey diamonds. For each stomach with estimated prey mass less than 3% BM (n = 592, see Methods), the number of prey per stomach (a), mean energetic value of individual prey (b), and total energetic value of stomach contents (c) are given per CART group.

Fig 6. Feeding behaviors on prey found in at least 20% of Thunnus orientalis stomachs per CART group.

For each prey, vertical polygon dimensions describe the mean prey-specific proportion (up from the x-axis) and mean prey-specific number (down from the x-axis) for which standard deviations are represented by whiskers. The frequency of occurrence of each prey per CART group corresponds to the width of the polygons and is given as text on the x-axis.

Fig 7. Relationship between stomach contents of Thunnus orientalis and its availability to fishers in the SCB.

Silhouettes describe the identity and relative daytime depth of dominant prey families. The relative diversity, size, quantity, and energetic value of prey are also given for each sampling period.