Conceived and designed the experiments: PWR DPC DEC JPGR JLH SES. Performed the experiments: PWR DPC DEC JPGR CDC MAF CG KTG JLH LAH CEK JLM SMM BIM SHP SES NMT SVA KY. Analyzed the data: PWR DEC CG JLH SHP SES. Contributed reagents/materials/analysis tools: DPC DEC JPGR. Wrote the paper: PWR DPC DEC JPGR MAF CG KTG JLH CEK JLM SMM BIM SHP SES NMT SVA.
The authors have declared that no competing interests exist.
The mesopelagic zone of the northeast Pacific Ocean is an important foraging habitat for many predators, yet few studies have addressed the factors driving basin-scale predator distributions or inter-annual variability in foraging and breeding success. Understanding these processes is critical to reveal how conditions at sea cascade to population-level effects. To begin addressing these challenging questions, we collected diving, tracking, foraging success, and natality data for 297 adult female northern elephant seal migrations from 2004 to 2010. During the longer post-molting migration, individual energy gain rates were significant predictors of pregnancy. At sea, seals focused their foraging effort along a narrow band corresponding to the boundary between the sub-arctic and sub-tropical gyres. In contrast to shallow-diving predators, elephant seals target the gyre-gyre boundary throughout the year rather than follow the southward winter migration of surface features, such as the Transition Zone Chlorophyll Front. We also assessed the impact of added transit costs by studying seals at a colony near the southern extent of the species’ range, 1,150 km to the south. A much larger proportion of seals foraged locally, implying plasticity in foraging strategies and possibly prey type. While these findings are derived from a single species, the results may provide insight to the foraging patterns of many other meso-pelagic predators in the northeast Pacific Ocean.
Marine apex predators are an important, yet highly vulnerable, component of pelagic ecosystems
There are several alternate avenues of research that can bypass at least some of the logistical barriers while still yielding informative results. For example, habitat models utilize animal movement data combined with environmental variables to predict distributions within a study range and allow informed extrapolations for novel regions
Adult female elephant seals are ideal research platforms to identify key habitats of mesopelagic predators because they dive continuously
We expand on previous studies of this species by 1) exploring foraging behavior metrics and associated inter-annual variability in the context of empirically measured foraging success and natality, 2) conducting a spatial analysis identifying persistent mesopelagic foraging habitats across the northeast Pacific Ocean and 3) discussing how other mesopelagic predators may use and respond to changes in the northeast Pacific Ocean. Together, these analyses allow a glimpse into the physical and biological dynamics of the mesopelagic zone and provide a context for examining the foraging patterns of other pelagic predators.
The animal use protocol for this research was reviewed and approved by the University of California at Santa Cruz Institutional Animal Care and Use Committee and followed the guidelines established by the Canadian Council on Animal Care and the ethics committee of the Society of Marine Mammalogy. Research was carried out under National Marine Fisheries Service permits: #786-1463 and #87-143.
Season | Year | Total | Complete TDR | Complete Track | Paired Track/TDR | Foraging Success | Natality | Known Age |
Post-breeding | 2004 | 7 | 5 | 5 | 4 | 4 | – | 4 |
2005 | 19 | 18 | 15 | 15 | 18 | – | 18 | |
2006 | 21 | 17 | 15 | 15 | 17 | – | 19 | |
SABE2006 | 10 | 7 | 4 | 4 | 0 | – | 0 | |
2007 | 20 | 16 | 17 | 16 | 15 | – | 18 | |
2008 | 23 | 22 | 21 | 21 | 22 | – | 22 | |
2009 | 19 | 14 | 13 | 13 | 14 | – | 13 | |
2010 | 24 | 21 | 18 | 17 | 22 | – | 21 | |
ANNU PBTotal | 133 | 113 | 104 | 101 | 112 | – | 115 | |
Post-molting | 2004 | 25 | 21 | 10 | 9 | 22 | 23 | 19 |
2005 | 25 | 17 | 17 | 12 | 22 | 22 | 22 | |
SABE2005 | 10 | 9 | 10 | 9 | 6 | 6 | 0 | |
2006 | 24 | 12 | 15 | 8 | 19 | 20 | 21 | |
2007 | 21 | 14 | 19 | 14 | 17 | 19 | 18 | |
2008 | 20 | 13 | 11 | 10 | 13 | 14 | 15 | |
2009 | 8 | 7 | 6 | 6 | 7 | 7 | 5 | |
2010 | 21 | 14 | 13 | 11 | 15 | 15 | 16 | |
ANNU PMTotal | 144 | 98 | 91 | 70 | 115 | 120 | 116 | |
Total Deployments | ANNU | 277 | 211 | 195 | 171 | 227 | 120 | 231 |
SABE | 20 | 16 | 14 | 13 | 6 | 6 | 0 | |
Overall | 297 | 227 | 209 | 184 | 233 | 126 | 231 |
Sample sizes by year, season, tagging location, and dataset. The two tagging locations were Año Nuevo, California (ANNU) and Islas San Benito, Mexico (SABE). Years without a prefix are from ANNU.
The map includes 195 tracks from the Año Nuevo, CA, USA colony (red point) and 14 tracks from the Islas San Benito, B.C., Mexico colony (yellow point).
Adult female northern elephant seals (
Season | Year | # Females | # Pups | Natality | Mass Gain(kg) | SD | Rate Mass Gain (kg day) | SD | % MassGain | SD | EnergyGain | SD | Rate EnergyGain (MJ/day) | SD |
Post-breeding | 2004 | 4 | – | – | 51.9 | 21.6 | 0.6 | 0.2 | 17.9 | 7.6 | 1047.4 | 605.3 | 12.8 | 7.2 |
2005 | 18 | – | – | 72.3 | 23.7 | 0.9 | 0.3 | 22.9 | 8.7 | 1105.6 | 563.0 | 14.11,2 | 7.0 | |
2006 | 19 | – | – | 69.0 | 24.7 | 0.9 | 0.3 | 21.8 | 8.9 | 1157.8 | 566.4 | 14.53,4 | 7.5 | |
2007 | 18 | – | – | 82.4 | 19.1 | 1.1 | 0.3 | 25.8 | 6.3 | 1413.6 | 471.9 | 19.6 | 7.5 | |
2008 | 22 | – | – | 74.1 | 25.1 | 1.0 | 0.3 | 22.4 | 7.8 | 1239.0 | 530.4 | 16.7 | 7.3 | |
2009 | 13 | – | – | 87.8 | 19.9 | 1.2 | 0.2 | 26.8 | 7.6 | 1727.3 | 760.0 | 23.61,3 | 9.8 | |
2010 | 21 | – | – | 81.3 | 19.2 | 1.1 | 0.3 | 23.1 | 6.2 | 1645.2 | 569.9 | 22.32,4 | 8.1 | |
ANNU PB Mean | – | – | – | 75.4* | 21.6 | 1.0 | 0.3 | 23.1* | 7.5 | 1321.2* | 576.9 | 17.6* | 8.0 | |
Post-molting | 2004 | 23 | 22 | 95.7 | 267.0 | 40.2 | 1.2 | 0.2 | 95.5 | 14.3 | 4369.91,2 | 677.2 | 19.5 | 2.9 |
2005 | 22 | 18 | 81.8 | 266.9 | 65.4 | 1.2 | 0.2 | 98.0 | 21.7 | 4146.0 | 912.4 | 19.2 | 3.3 | |
SABE 2005 | 6 | 6 | 100.0 | 286.6 | 36.0 | 1.3 | 0.2 | 120.7 | 18.2 | 4108.7 | 592.7 | 18.5 | 3.2 | |
2006 | 20 | 17 | 85.0 | 239.6 | 84.5 | 1.1 | 0.3 | 89.0 | 30.2 | 3458.31 | 1161.3 | 15.7 | 4.5 | |
2007 | 19 | 13 | 68.4 | 249.7 | 55.1 | 1.1 | 0.2 | 91.4 | 21.8 | 3484.72 | 846.0 | 15.2 | 3.5 | |
2008 | 14 | 12 | 85.7 | 260.8 | 77.6 | 1.2 | 0.3 | 94.1 | 30.7 | 3913.2 | 1323.6 | 17.5 | 5.5 | |
2009 | 7 | 6 | 85.7 | 271.4 | 58.0 | 1.3 | 0.2 | 103.2 | 34.2 | 4552.2 | 752.2 | 21.8 | 3.5 | |
2010 | 15 | 13 | 86.7 | 274.8 | 52.2 | 1.2 | 0.2 | 94.9 | 18.0 | 3630.1 | 650.6 | 16.3 | 2.1 | |
ANNU PM Mean | – | – | 84.1 | 261.5* | 61.9 | 1.2 | 0.2 | 95.2* | 24.4 | 3864.1* | 903.3 | 17.9 | 3.6 |
All values are determined from empirical measurement of body composition and mass estimates calculated as the difference between deployment and recovery, after correction for time on land. Identical numeric superscripts denote annual differences within seasons. (*) denotes significant differences across seasons. SABE animals were not included in the statistical comparisons.
Most females that skipped breeding returned outside of the typical breeding season (January – February).
In general, healthy adult female seals were selected at random from the subset of the population carrying flipper tags, allowing us to reference each seal’s age and haulout history
The stippled region indicates the annual range of the Transition Zone Chlorophyll Front (TZCF). The location of the gyre-gyre boundary remains stable in contrast to the annual migration of the TZCF.
Tracking data were regularized to hourly positions prior to analysis and only complete trips were included (n = 195). The black line shows the monthly position of the gyre-gyre boundary, estimated from the 170 cm absolute dynamic topography climatology contour. White points indicate the position of the Transition Zone Chlorophyll Front, estimated from the 0.2 mg/m3 contour. Oceanographic climatologies include data from 2004 through 2008.
Body composition was measured at both deployment and recovery using the truncated cones technique
Season | Year | Duration - d | Max Dist – km | Total Dist - km |
Post- breeding | 2004 | 83.7 (9.5) | 2512.9 (1033.3) | 5711.6 (1910.8) |
2005 | 77.3 (7.9) | 2289.4 (511.8) | 5059.6 (1013.3) | |
2006 | 76.7 (11.1) | 2220.4 (557.6) | 5043.9 (1006.9) | |
SABE 2006 | 73.9 (14.6) | 1238.0 (1100.6) | 2935.3 (1890.6) | |
2007 | 71.2 (8.9) | 2086.4 (631.3) | 4644.8 (1220.7) | |
2008 | 74.0 (8.9) | 2012.9 (358.6) | 4813.4 (843.3) | |
2009 | 70.6 (8.2) | 2067.5 (544.0) | 4778.0 (1029.2) | |
2010 | 73.9 (5.9) | 2189.1 (488.2) | 5255.5 (789.4) | |
ANNU PBMean | 74.7 (9.3)* | 2140.7 (552.2)* | 4913.4 (1068.4) | |
Post-molting | 2004 | 223.6 (13.5) | 3344.9 (840.0) | 9355.8 (1251.2) |
2005 | 214.4 (30.4) | 3017.3 (1068.3) | 9024.6 (1721.2) | |
SABE 2005 | 210.3 (26.9) | 2909.3 (1495.1) | 7594.5 (3186.8) | |
2006 | 213.7 (26.5) | 3437.5 (964.5) | 9775.7 (1261.8) | |
2007 | 223.1 (35.2) | 3405.9 (856.3) | 10808.0 (2719.8) | |
2008 | 214.2 (30.3) | 3267.4 (706.5) | 9688.0 (1493.7) | |
2009 | 210.0 (30.9) | 2834.3 (1091.6) | 10447.9 (2670.8) | |
2010 | 221.9 (27.4) | 3079.7 (1128.4) | 10099.1 (2144.8) | |
ANNU PM Mean | 218.5 (25.9)* | 3256.9 (944.8)* | 9850.0 (1993.1)* |
(*) denotes significant differences across seasons. Inter-annual variability was not significant for any parameter. SABE animals were not included in the statistical comparisons.
Raw ARGOS/GPS tracks were truncated according to departure/arrival times identified using the diving record, then processed using a speed/turn-angle filter to remove unlikely position estimates (thresholds: 12 km hr−1 and 160°). The filter also examined the secondary position calculations reported by ARGOS and replaced the erroneous primary positions if the speed/angle filter criteria were met. Due to a high prevalence of poor quality ARGOS location classes (predominantly A and B), we used a state-space model to smooth the tracking data and obtain hourly position estimates using the CRAWL package in R
Season | Year | # Dives | % Diving | Depth - m | Duration - min | PDI - min | % Transit | % Foraging | % Drift | % Benthic |
Post-breeding | 2004 | 5121.4 (412.6) | 89.1 (1.4) | 433.1 (79.9) | 20.9 (1.5) | 2.5 (0.2) | 26.4 (4.5) | 53.4 (20.9) | 6.0 (2.7) | 14.1 (16.7) |
2005 | 4609.1 (675.8) | 90.4 (1.3) | 499.7 (85.9) | 22.0 (2.1) | 2.3 (0.3) | 26.3 (8.5) | 55.5 (14.6) | 7.5 (3.4) | 10.6 (13.7) | |
2006 | 4650.9 (686.3) | 90.9 (1.7) | 503.4 (50.2) | 21.6 (1.5) | 2.2 (0.4) | 28.1 (8.5) | 58.0 (12.4) | 8.1 (3.5) | 5.9 (5.5) | |
SABE 2006 | 4370.1 (1387.2) | 89.5 (2.9) | 477.7 (86.4) | 23.0 (5.2) | 2.6 (0.4) | 15.1 (6.9) | 56.6 (28.4) | 8.1 (4.1) | 20.2 (23.2) | |
2007 | 4473.9 (694.9) | 91.3 (1.1) | 556.0 (39.0) | 21.7 (2.8) | 2.0 (0.2) | 34.1 (11.3) | 53.9 (16.3) | 6.6 (2.0) | 5.4 (14.4) | |
2008 | 4580.1 (786.3) | 90.9 (1.3) | 541.6 (40.2) | 21.4 (1.8) | 2.1 (0.2) | 35.0 (11.6) | 54.6 (12.4) | 7.0 (2.6) | 3.4 (1.9) | |
2009 | 4222.9 (804.2) | 91.7 (0.9) | 540.6 (39.2) | 22.4 (2.2) | 2.0 (0.2) | 34.5 (10.4) | 54.6 (10.9) | 6.9 (1.8) | 4.0 (1.7) | |
2010 | 4356.3 (474.3) | 91.2 (0.8) | 547.7 (38.5) | 22.4 (1.5) | 2.2 (0.5) | 30.6 (9.2) | 58.1 (9.7) | 7.2 (2.6) | 4.1 (1.8) | |
ANNU PB Mean | 4513.7 (708.3) | 91.0 (1.3) | 527.6 (61.6) | 21.9 (2.0) | 2.2 (0.3) | 30.6 (10.4) | 56.0 (13.4) | 7.2 (2.8) | 6.1 (9.8) | |
Post-molting | 2004 | 12121.2 (1249.6) | 91.0 (0.8) | 487.4 (44.8) | 24.3 (1.7) | 2.4 (0.3) | 30.5 (9.3) | 51.7 (13.7) | 11.5 (2.5) | 6.3 (5.0) |
2005 | 11677.4 (1140.8) | 90.2 (1.3) | 497.4 (54.2) | 23.6 (3.1) | 2.5 (0.3) | 31.5 (11.7) | 49.8 (14.1) | 11.3 (3.7) | 7.4 (9.1) | |
SABE 2005 | 12591.3 (1070.1) | 89.0 (2.0) | 501.4 (64.9) | 21.9 (2.1) | 2.6 (0.3) | 29.3 (12.2) | 53.3 (12.3) | 10.1 (3.0) | 7.4 (10.2) | |
2006 | 11702.1 (942.9) | 90.3 (1.4) | 503.6 (27.9) | 22.9 (2.8) | 2.4 (0.3) | 30.7 (10.1) | 50.2 (15.2) | 14.2 (6.5) | 4.9 (3.2) | |
2007 | 11643.1 (2008.4) | 90.8 (0.9) | 504.7 (35.7) | 24.2 (2.9) | 2.4 (0.3) | 32.4 (5.6) | 51.1 (7.5) | 11.0 (2.8) | 5.5 (4.0) | |
2008 | 11558.2 (810.4) | 90.7 (0.9) | 512.5 (26.8) | 25.2 (1.7) | 2.7 (0.4) | 27.6 (8.1) | 56.6 (9.0) | 11.5 (1.6) | 4.2 (2.2) | |
2009 | 10662.4 (1278.2) | 90.9 (1.0) | 525.1 (21.5) | 25.7 (2.2) | 2.6 (0.5) | 26.7 (5.8) | 57.6 (7.9) | 11.4 (1.2) | 4.3 (2.4) | |
2010 | 11246.1 (756.6) | 91.5 (0.8) | 523.8 (26.8) | 26.1 (2.5) | 2.7 (0.5) | 27.1 (6.9) | 56.9 (7.3) | 12.2 (3.3) | 3.9 (2.6) | |
ANNU PM Mean | 11622.2 (1281.4) | 90.8 (1.0) | 503.0 (40.9) | 24.5 (2.5) | 2.5 (0.4) | 30.4 (8.8) | 51.8 (11.8) | 12.0 (3.6) | 5.8 (5.4) | |
Overall ANNU Mean | – | 90.9 (1.2) | 516.8 (53.2) | 23.1 (2.6) | 2.3 (0.4) | 30.6 (9.7) | 54.0 (12.9) | 9.5 (4.0) | 5.9 (7.8) | |
Overall SABE Mean | – | 89.3 (2.2) | 496.2 (70.7) | 22.1 (3.6) | 2.6 (0.4) | 25.6 (13.7) | 53.7 (19.3) | 9.1 (3.6) | 11.7 (17.1) |
‘PDI’ refers to the duration of the post-dive interval. The last 4 columns indicate the proportion of each functional dive type. (*) denotes significant differences across seasons. Inter-annual variation was not significant for any parameter. SABE animals were not included in the statistical comparisons.
Diving data were collected at sampling intervals between 1 s and 8 s and were sub-sampled to 8 s to facilitate comparison. Three instruments sampled with a 20-second frequency, but were otherwise similar. The raw time-series of depth measurements were analyzed in MatLab using the IKNOS toolbox (Y. Tremblay, unpublished). Dives were retained only if exceeding 32 s in duration and 15 m in depth. All dives were then classified into one of four dive types (each with a putative function) using a forced-choice classification program: active-bottom (pelagic foraging), flat-bottom (benthic foraging), drift (food-processing/rest), or v-shape (transit)
Dives are shallower in the northern half of the sub-arctic gyre and coastal regions compared to the transition zone waters.
To investigate the distribution of individuals throughout the year, we extracted hourly position estimates across all complete tracks by month and generated kernel density plots using a 200 km bandwidth. A weighting (1/# trips) was applied to eliminate the bias associated with repeat deployments on the same individual, as they tend to recapitulate their previous tracks
Areas in red indicate statistically significant clustering of foraging activity, independent of the number of seals present. Grid cells informed by only one seal were removed to avoid high leverage.
Subsurface thermal structure was explored using temperature data from two seals (one post-molting and one post-breeding) that opportunistically swam directed transects from 40°N to 50°N through the regions of peak inter-annual seal density. Temperature profiles from the ascent (up-cast) of dives were aggregated, smoothed, geo-referenced, and visualized using Ocean Data View (Schlitzer, R., Ocean Data View,
The temperature profile was created from TDR data between 28-July-2005 and 24-August-2005 (seal ID: 2005037; post-molting season). The 8°C isotherm, indicated with a black line, highlights the temperature inversion. The seal density was extracted from the inter-annual August kernel density (see fig. 5). The grey bar shows the position of the gyre-gyre boundary.
To investigate spatial patterns of foraging success across all years of study, we conducted two hotspot analyses for independent verification of trends. The tracking data were first sub-sampled to one position per day, evenly spaced in time. A daily time-scale was selected because many aspects of foraging behavior occur on a diel cycle
To identify clustering of elevated foraging activity independent of the number of observations in a particular area, we used the Hotspot Analysis tool (Getis-Ord Gi* statistic) in the Spatial Statistics toolbox of ArcGIS 10. The foraging metric (daily transit rate or number of drift dives per day) was used as the weighting variable. The ‘Zone of Indifference’ setting was used to reduce edge-effects and a radius of 100 km was selected to match the approximate maximum daily displacement of a transiting seal. The points were then converted to a raster using a mean neighborhood analysis on the Z-statistic, again with a radius of 100 km. A mask was applied to remove cells informed by only one seal. Because the two foraging metrics are based on different datasets (surface movements vs. diving behavior), results can be viewed as independent.
Annual and seasonal effects on behavior and foraging success were analyzed using linear mixed models (SAS 9.2) with individual seal as a random effect subject, and year, season, and their interaction as fixed effects. The subject effect covariance structure was chosen to minimize the model BIC (Bayesian Information Criterion). Fixed effects were evaluated using type III F-tests. When the year by season interaction was significant, post-hoc comparisons were made within seasons by comparing least square means with a Sidak adjustment for multiple comparisons. Model residuals were assessed for approximate normality.
Of the 297 foraging migrations, 184 provided a complete dataset (track and dive record) comprising: 25,079 seal-days, 1,267,563 km of horizontal movement, 1,442,695 km of vertical movement, and 1,403,866 dives. Seventy-eight percent of these migrations also had complete foraging success data (pre- and post-deployment morphometric and mass measurements). An additional 70 migrations provided a partial dataset, either a complete TDR record or a complete track, and were included in relevant analyses. Forty-three migrations had only incomplete or missing records and were removed from the analyses. A detailed summary of sample sizes across seasons, years, and locations is provided in
At the Año Nuevo colony, mass gain during foraging migrations varied with season (F1,51 = 866.5, p<0.0001) but not between years (p = 0.52). Overall mean mass gain during the winter post-breeding migration was 75.4 ± 21.6 kg and showed no significant annual variation but wide inter-individual variation. In contrast, annual mass gain during the post-molting migration was 264.6 ± 58.6 kg and varied annually (
Seals instrumented at Año Nuevo, CA, USA (ANNU) foraged throughout the northeast Pacific (
Seals spent an average of 74.7 ± 9.3 days at sea during the post-breeding migration and 218.5 ± 25.9 days during the post-molting migration (
Overall, seals dived for 91% of their time at sea, with a mean dive duration of 23.1 ± 2.6 min and a maximum of 109 minutes. Mean dive duration and mean post-dive interval were significantly longer during the post-molting migration than the post-breeding migration (F1,2 = 26.4, 36.3 respectively, p<0.05). Active-bottom dives made up the greatest percentage of dives (54.0%), followed by V-shape (30.6%), drift dives (9.5%), and flat-bottom dives (5.9%) (
The overall mean dive depth was 516 ± 53.2 m (maximum 1735 m), but dive depths showed a strong diel pattern resulting in a bimodal distribution. The deep daytime mode was centered at 619 m while the shallow nighttime mode was centered at 456 m. In addition to the diel depth patterns, a diurnal bimodality was also observed for daytime active-bottom dives in 55% of the seals (modes at 385 m and 641 m). Shallow daytime dives were present throughout the range, but occurred most frequently in the northern region of the sub-arctic gyre (
The highest density of seals occurred in a migratory corridor off the California coast extending northwest to ∼45°N (
Hotspot analysis revealed clusters of intense foraging activity by either slow transit or an elevated rate of drift dives, independent of how many animals visited a particular region (
To explore possible subsurface thermal features that may influence the distribution of prey species and other mesopelagic predators, we generated a temperature profile of the water column by using the data collected by a seal (ID: 2005037) that swam a direct and continuous transect along the ∼163°W meridian from 50°N to 40°N during the middle of the post-molting foraging migration (from late July to late August;
To address the behavioral impacts of added transit time and, by extension, reduced time in prime foraging habitat, we compared seals instrumented at Año Nuevo, California (ANNU) to concurrent deployments at the Islas San Benito, Mexico (SABE) colony 1,150 km to the southeast during the post-molting 2005 migration. While none of the diving or tracking metrics were significantly different (
We collected a dataset from female northern elephant seals that combines a large sample size, broad geographic extent, and at-sea foraging success metrics with a direct link to reproductive success. This is a unique combination that allows us to (1) describe the at-sea diving and movement behavior of foraging seals in the context of empirically measured foraging success and natality, (2) identify persistent physical features in the environment that correspond to foraging effort, and (3) discuss how other mesopelagic predators may use and respond to changes in the northeast Pacific Ocean.
In elephant seals, and capital breeding systems in general, the energy acquired during a foraging migration helps to determine whether a female will give birth to a pup and provide enough energy during the short lactation period
We collected foraging behavior data at a finer temporal resolution than possible a decade ago, but movement and diving statistics were largely consistent with previous reports
The dive depths of most seals showed a clear diel pattern, consistent with targeting vertically migrating prey species. In the northeast Pacific, prey distributions in both the vertical and horizontal dimensions are poorly understood, but acoustic studies identifying deep scattering layers generally show peak density at shallow depths relative to elephant seal foraging dives
The hotspot and kernel density analyses show the importance of the Transition Zone for elephant seals, which is consistent with prior work
For a subset of the elephant seals, the hotspot analyses also show the importance of regions farther north in the sub-arctic gyre. Relatively few animals visit this region, but the large sample size of this study facilitated sufficient coverage. When seals did visit this region, their behavior was indicative of feeding (slower transit rates and elevated frequency of drift dives). Because the hotspot analyses indicate foraging intensity independent of seal density, they are likely an indicator of prey availability. Therefore, the foraging behavior hotspot maps (
To address the effects that increased transit costs may have on the behavior and foraging success of a mesopelagic predator, we compared the Año Nuevo (ANNU) colony to the Islas San Benito (SABE) colony 1,150 km to the south. We found a mix of strategies in which most SABE seals traveled north to feed in the same areas as those from ANNU while a subset of the population remained local. The same individuals were tracked during the subsequent post-breeding migration and all seals maintained their strategies, but did not travel as far north during this shorter foraging trip. This may partially explain the findings of a previous study that uses isotopic data to suggest SABE seals feed pelagically ∼8° south of ANNU seals
The northern elephant seal is one of many predators foraging in the mesopelagic zone of the north Pacific. Sperm whales (
In this study, we used one of the largest mesopelagic predator diving and movement datasets to explore at-sea foraging behavior and inter-annual variability in the context of empirically measured foraging success and natality. We identified high-use areas along the latitudinally stable boundary between the sub-arctic and sub-tropical gyres, which explains the bulk of foraging migration trajectories during both annual migrations. We also showed that elephant seals exhibit a variety of foraging strategies at the population level, which may buffer against the impacts of environmental perturbation.
By studying a relatively accessible species over many years, we can better understand the connections between physical dynamics, predator behavior, foraging success, and demographic consequences in the north Pacific mesopelagic ecosystem. A wide variety of predators occupy this region
We thank the many field assistants, undergraduates, and Costa-lab members who helped make this work possible, especially Pat Morris, Guy Oliver, John Harley, Molly McCormley, Ann Allen, Concha Garcia, and Octavio Maravilla. We also thank the rangers and docents at Año Nuevo State Reserve, especially Gary Strachan and Terry Kiser, Sarah Allen and Brian Hatfield for assistance recovering wayward seals, Richard Condit for the creation and maintenance of the flipper tag database, the fishing cooperative Pescadores Nacionales de Abulón for providing logistic support and transportation to Islas San Benito, and Dr. Jennifer Ayers for providing the sea surface height climatologies.