Ecological drivers of movement for two sympatric marine predators in the California current large marine ecosystem

Abstract

Background: An animal's movement reflects behavioral decisions made to address ecological needs; specifically, that movement will become less directional in regions with high prey availability, indicating foraging behavior. In the marine realm, animal behavior occurs below the sea surface and is difficult to observe. We used an extensive satellite tagging dataset to explore how physical and biological habitat characteristics influence blue (Balaenoptera musculus) and fin (B. physalus) whale movement and foraging behavior in the California Current Ecosystem across four known bioregions.

Methods: We fitted movement models to 14 years of blue whale satellite tracking data and 13 years of fin whale data to characterize their movement persistence, with higher move persistence values representing more directional movement and lower move persistence values representing less directional movement. Models were evaluated against a range of physical and biological environmental predictors to identify significant correlates of low move persistence (i.e., presumed intensified foraging behavior). We then used data from a subset of sensor-equipped tags that monitored vertical behavior (e.g., dive and feeding), in addition to movement, to test the relationship between vertical behavior and movement persistence.

Results: Low move persistence was strongly correlated with shallower water depth and sea surface height for both species, with additional effects of chlorophyll-a concentration, vorticity and marine nekton biomass for blue whales. Data from sensor-equipped tags additionally showed that low move persistence occurred when whales made more numerous feeding dives. Temporal patterns of bioregion occupancy coincided with seasonal peaks in productivity. Most blue whale low-move-persistence movements occurred in the northern, nearshore bioregion with a late-season peak in productivity and were evenly distributed across all bioregions for fin whales.

Conclusions: We demonstrated that low move persistence is indicative of increased feeding behavior for both blue and fin whales. The environmental drivers of low move persistence were similar to those previously identified for survey-based species distribution models, linking environmental metrics to subsurface behavior. Occupancy and movement behavior patterns across bioregions indicate both species moved to exploit seasonal and spatial variability in productivity, with blue whales especially focusing on the bioregion of highest productivity during late summer and fall.

Keywords: Biologging; Blue Whale; California current; Feeding behavior; Fin Whale; Move persistence; Movement behavior; Satellite tags.

Fig. 1

(A) Boundaries of four CCE bioregions based on spatial and temporal patterns in chlorophyll-a concentration from [74]. (B) The deployment locations for satellite tags attached to blue and fin whales off the U.S. West Coast and Mexico from 1998–2018. (C) The number of tags deployed during each month of the year by species

Fig. 2

Spatial distribution of mean move persistence values for satellite-tagged blue (left) and fin (right) whales tracked in the eastern north Pacific Ocean from 1998–2018. Values are presented on a 0.25-degree grid. Low movement persistence indicates less-directional movement while higher values indicate directional movement

Fig. 3

Distribution of blue (top) and fin (bottom) whale move persistence values and timing of occurrence in four biogeographic regions in the California Current Ecosystem described by [74]. The four regions were derived based on spatial and temporal patterns in chlorophyll-a concentration: SummerMin represents regions with elevated chlorophyll-a levels in winter, spring and late fall; LatePeak represents a spring peak in chlorophyll-a during April and May, then a longer, late summer elevated period from July through October; Flat is characterized by weak seasonality, and EarlyPeak describes where chlorophyll-a increases from November through May and decreases from its peak in early June to a minimum in late October

Fig. 4

Ecological drivers of blue whale movement behavior. Results from the top move‑persistence mixed model for fixed (thick blue line) and random (individual whales; thin yellow lines) effects. Low move-persistence is indicative of less directed movement while high values indicate directed movement. Predictors were scaled and centered prior to model fitting

Fig. 5

Ecological drivers of fin whale movement behavior. Results from the top move‑persistence mixed model for fixed (thick blue line) and random (individual whales; thin yellow lines) effects. Low move-persistence is indicative of less directed movement while high values indicate directed movement. Predictors were scaled and centered prior to model fitting

Fig. 6

Diel variability of maximum dive depth recorded from RDW-tagged blue and fin whales tracked in the CCE during summer - fall 2016 and 2017

Fig. 7

(Top) Relationship between move persistence and feeding dive rate, (bottom) relationship between move persistence and dive depth. Lower move-persistence values indicate less directional movement. Data are derived from satellite tags attached to blue and fin whales off California during summer and fall 2016 and 2017. Shaded regions indicate the 95% confidence intervals. Feeding dives rate is a metric representing the hourly rate of feeding dives that occurred during the data available for each time step