Trade-offs in prey quantity and quality in gray whale foraging

Abstract

To forage optimally, predators face complex decisions regarding target prey distribution, quantity, and quality. We paired theodolite tracking of gray whales Eschrichtius robustus in Port Orford, Oregon, USA, with concurrent sampling of their zooplankton prey to examine foraging decisions relative to prey quantity (abundance) and quality (caloric content). We tested the hypotheses that whales (1) feed more than search or transit in areas with high quantity and quality prey and (2) select foraging habitat dominated by the calorically rich mysid Neomysis rayii. Relative prey abundance was assessed through standardized image analysis of camera drops, and zooplankton prey community was determined from net tows. These data were spatially interpolated and modeled to generate daily layers of species-specific prey abundance and calories (20 m grid) for comparison to whale behavior derived from tracking data. Whales fed significantly more in areas with higher prey abundance and calories than where they searched and transited. Whales increased foraging effort as overall prey availability increased, yet foraging probability was significantly correlated with the quantity and quality of the mysid Holmesimysis sculpta, which has significantly lower calories than N. rayii. However, during the study period, the maximum abundance of N. rayii was 4 times lower than that of H. sculpta and never reached the quantity threshold determined by a logistic regression needed to support whale foraging behavior. Hence, gray whale prey selection involves trade-offs between prey quantity and quality to maximize energetic gain, and prey quality should be considered alongside abundance in ecological studies investigating predator decision-making.

Exploring indirect effects of a classic trophic cascade between urchins and kelp on zooplankton and whales

“…Data for this study were collected as part of a larger study on fine-scale gray whale foraging ecology 30 near the coast of Port Orford (Fig. 1 ), Oregon, U.S.A, between July and August from 2016 to 2023.…”

“…Our video processing methods described below were specifically designed to accommodate our study design and enable use of eight years of standardized data collection. While our methods are unconventional and do not measure absolute levels of urchin density or kelp condition, our approach derives accurate relative measurements that enable us to assess correlations between two co-occurring taxa in video data initially collected for other purposes 30 . We validated our unconventional methods against more traditional methods for assessing urchin and kelp density (see ‘ Validation of non-traditional methods ’ Section below).…”

“…As described by 30 , still images were extracted every 5 s from the upcast of the GoPro videos and divided into a 3 × 3 cell grid (each cell measured 533 × 953 pixels). Each grid cell was assigned a score from 0 to 5 according to the relative amount of zooplankton in the grid cell using standardized methods based on reference images (with obscured and low visibility cells assigned NA values; see Fig.…”

“…Theodolite locations of whale surfacings were analyzed and behaviorally classified using RST 30 , 43 into transit, search, or feed states. The number of feed and search points within each site on each day were summed to estimate foraging time in minutes.…”

“…Mysid shrimp ( Neomysis rayii and Holmesimysis sculpta ) are energetically profitable and consistently available prey items of PCFG whales 27 . These mysid species are found strongly associated with kelp forests and canopies within the PCFG range 28 – 30 . Mysids associate with kelp-dominated reef habitat for a number of reasons.…”