Understanding chinstrap penguin and elephant seal migrations in the Southern Ocean

doi:10.21203/rs.3.rs-4947354/v1

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Understanding chinstrap penguin and elephant seal migrations in the Southern Ocean

https://doi.org/10.21203/rs.3.rs-4947354/v1

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Characterizing the high-latitude winter foraging habitats of migratory marine predators is necessary for conservation and management in Antarctica. Tracking data from chinstrap penguins (Pygoscelis antarcticus) and southern elephant seals (Mirounga leonina), key Antarctic predators with different diets and foraging habits, indicate that approximately 12% and 5%, respectively, of tagged penguins and seals undertake long-distance winter migrations to remote regions south of 60°S between 120°W and 170°W. Tracking data revealed reduced daily swimming speeds and two hotspots of increased use, consistent with increased foraging efforts, for both species in this region. Light limitation during winter, however, precludes the use of optical satellite data to characterize marine productivity here, but biogeochemical-Argo floats can provide year-round chlorophyll data. These data inform the Biogeochemical Southern Ocean State Estimate (B-SOSE), which provides year-round estimates of marine productivity. Overlapping the predator hotspots, B-SOSE predicts two areas with year-round elevated surface chlorophyll levels, consistent with previous studies indicating enhanced mixing in those areas. We hypothesize that persistent areas of elevated chlorophyll centered near 160°W and 120°W near the boundaries of the Ross Gyre and the southern boundary of the Antarctic circumpolar current support a productive food web capable of supporting the diverse foraging niches of pelagic species.

Earth and environmental sciences/Biogeochemistry

Earth and environmental sciences/Ocean sciences

Chinstrap penguins

Ross Gyre

Amundsen Sea

SOCCOM data

Southern elephant seals

Identifying and characterizing the foraging areas used by seabirds and marine mammals during their winter migrations is an important task for understanding population trends and managing living marine resources. In particular, temporal variability in the availability and productivity of winter foraging habitats in the Southern Ocean may help explain trends in breeding populations of Antarctic predators ¹. For example, prior work with Pygoscelid penguins suggested the importance of winter foraging conditions for explaining divergent population trends (Hinke et al. 2007). However, measuring marine productivity and other biological characteristics of presumed foraging areas in the Southern Ocean during the austral winter is difficult because low light levels preclude traditional satellite-based observations that require visible light. Here, we use model output informed from biogeochemical-Argo float data ² to examine the winter distribution of marine productivity to assess and characterize a suspected remote foraging habitat used by migratory chinstrap penguins (Pygoscelis antarcticus) and southern elephant seals (Mirounga lionina).

Chinstrap penguins (CHPE) have a global population of roughly 8 million individuals ³ and are primarily consumers of Antarctic krill (Euphausia superba) and pelagic fishes ^4,5. Southern elephant seals (SES) have a global population of roughly 325,000 individuals ⁶ and are primarily consumers of squids and pelagic fishes ⁷. These abundant Antarctic predators account for large proportions of total krill and squid consumption in the Southern Ocean among seabirds and pinnipeds, respectively ⁸. Both species routinely move long distances throughout the Southern Ocean during their winter migrations, with maximum recorded movements from breeding locations to winter destinations exceeding 4000 km ^9,10. Available tracking data from CHPE and SES record movements of several individuals, in multiple years, into a remote,high-latitude region south of 60°S between 120°W and 170°W that spans the western boundary of the Amundsen Sea and the Ross Gyre to the northern Ross Sea ^1,10. This remote sector of the Southern Ocean is relatively understudied ^11,12 and, consequently, the ecological importance of this region for pelagic food webs is largely unknown ¹³.

The interaction between the Antarctic Circumpolar Current (ACC) and the Pacific Antarctic Ridge in this region leads to enhanced mixing and nutrient upwelling ^14,15. In particular, the area around 140°W is the location of the narrowest restriction of the ACC, due to topographical steering from the underlying Udintsev Fracture Zone (UFZ) at 144°W ¹⁶. Downstream of the UFZ, between 140°-135°W and 56°-58°S, is an area of enhanced eddy kinetic energy ¹⁶ and strengthening gradients in the circumpolar fronts ¹⁷. Previous analyses of Southern Ocean satellite data report elevated summer chlorophyll in this region which has been attributed to the enhanced mixing caused by the interaction of the ACC and bottom topography ^15,18,19.

Characterizing marine productivity and the prey field in high-latitude foraging areas used by migratory animals during winter months is more difficult. While satellite ocean color data is often used to map ocean productivity, it requires the presence of both visible light and cloud-free conditions. Hence passive optical satellite sensors cannot provide chlorophyll data during Antarctic winters due to insufficient light. Thus, the high-latitude foraging habitats of Antarctic predators during winter are usually modeled on the basis of physical habitat descriptors including sea ice coverage, sea surface temperature, depth, or proximity to frontal features of the ACC ^10,20,21. While these physical variables provide important proxies of foraging habitat, a fuller exploration of biological indices in remote foraging areas is needed to strengthen understanding of variation in animal behavior and potential population-level consequences of using particular habitats.

Biogeochemical-Argo (BGC-Argo) floats, deployed by the Southern Ocean Carbon and Climate Observations and Modeling program (SOCCOM; http://soccom.princeton.edu), provide year-round, in situ observations of chlorophyll, as well as data from under-ice ^22,23. Despite the large number of floats deployed by SOCCOM (over 250 since 2014) the spatial coverage of float data remains relatively sparse. Thus, direct mapping of presumed animal foraging locations to raw float data remains untenable. However, the BGC-Argo float data has been incorporated into the Biogeochemical Southern Ocean State Estimate (B-SOSE), a general circulation model of the Southern Ocean. The B-SOSE model generates year-round estimates of biogeochemical properties for the Southern Ocean ², including chlorophyll concentrations, which can be used to assess the general productivity of the system. The spatially resolved estimates of marine productivity during winter may help understand animal occupancy in remote high-latitude pelagic regions.

Here we analyze overwinter tracking data of CHPE and SES along with B-SOSE model output to examine whether variation in migratory movements in the western Pacific sector of the Southern Ocean correspond to unique features of B-SOSE predictions of ocean productivity. We also provide comparisons of B-SOSE output and satellite-derived surface chlorophyll measurements during summer to help validate B-SOSE predictions. Our results suggest that consistently elevated marine production centered near 160°W and 120°W in association with the Ross Gyre and southern boundary of the ACC supports the long-distance winter migrations of these two iconic Southern Ocean predators.

Telemetry Data

We used publicly-available location estimates of the overwinter migrations of CHPE and SES collected from ARGOS satellite tracking data. Tracking data collected by the U.S. Antarctic Marine Living Resources Program for CHPE and reported previously ²⁴ were obtained from the Seabird Tracking Database, available at https://www.seabirdtracking.org. Additional CHPE tracking data were obtained from a published supplemental data file {Updating}available at https://doi.org/10.1371/journal.pone.0226207.s008. Telemetry data for SES were obtained from the Marine Mammals Exploring the Oceans Pole to Pole (MEOP) program ²⁵ available at https://www.meop.net/ and the Tagging of Pelagic Predators (TOPP) program ²⁶ available at https://coastwatch.pfeg.noaa.gov/erddap/.

Data from 97 CHPE tagged in the northern Antarctic Peninsula region in February or March of six different years between 2000 to 2017 were analyzed. The initial SES dataset included 1029 animals tagged all around Antarctica between 2004 and 2019. Since we are interested in movements into a specific region of the Southern Ocean, the SES dataset was subset to include only animals that were tagged within 90° of 140°W, i.e. between 50°W and 130°E. Because we were interested in analyzing winter migration behavior, CHPE and SES tracks that did not contain data extending into April were excluded. We also removed any tracks with less than 10 days of data. These criteria reduced the dataset to 74 CHPE and 315 SES tracks (Table 1).

Table 1

Statistics for the CHPE and SES tags analyzed, including the total number of tags, the total locations, the average number of locations per tag, the average number of days recorded in a tag, and the average maximum distance traveled. The statistics are given for the entire dataset, and for the subset which migrated to the area centered around 140°W.
	CHPE	CHPE (in study area)	SES (50°W-130°W)	SES (in study area)
Total tags	74	9 (12%)	315	16 (5%)
Years	2000, 2004, 2006, 2010, 2011, 2017	2006, 2010, 2011, 2017	2004–2019	2005, 2006, 2008, 2009, 2010, 2012, 2013
Total locations	28270	3638 (13%)	165027	2953 (2%)
N per tag	382 ± 317	722 ± 467	524 ± 326	599 ± 214
Days per tag	94 ± 49	151 ± 53	191 ± 81	209 ± 56
Maximum distance	1085 ± 123	2426 ± 734	960 ± 1035	3615 ± 959

We assessed all 389 winter tracks to compare the general winter migratory behaviors of CHPE and SES. We then subset the data to isolate animals with directed movements into the region north of 65°S between 110°W and 165°W. This large region, which we will refer to as the study region, was selected to encompass ice-free areas of the northern and eastern Ross Sea and western Amundsen Sea. Nine CHPE and 16 SES entered the study region.

For the subset of CHPE and SES that entered the study region, we identified areas of high use during winter months (May-November) by using a 2-dimensional kernel density estimation (KDE) implemented in the MASS package ²⁷ in R ²⁸. The kernel was specified based on 100 grid points in x and y directions with the bandwidth set as the mean of the two estimators to achieve a balance between two default methods that provide, given the tracking data, a smoother ²⁷ and a more-granular ²⁹ density surface.

To differentiate transit periods from potential foraging periods we calculated an average daily speed along each track. Periods with higher speeds can be interpreted as directed, migratory transit periods, while slower average speeds indicate increased local foraging efforts ³⁰, similar to metrics of move persistence ³¹ or residence time ³². Because the satellite tags were programmed to report positions at different temporal frequencies depending on the species and year of deployment, we standardized the data by computing a daily average position. The estimated mean daily locations and associated mean times were used to estimate a daily swim speed based on the shortest geodesic distance between positions ³³. We grouped daily swim speed estimates into 5° longitudinal bins to examine patterns in swimming speed along the recorded tracks. For reference, note that a mean daily swim speed of 0.5 m/s equates to a daily displacement of 43.2 km.

Model data

The B-SOSE model (Verdy and Mazloff, 2017) was used to examine the distribution of phytoplankton over the study area. B-SOSE is a general circulation model that assimilates observations from biogeochemical-Argo floats, shipboard data, and satellites to produce a realistic estimate of the ocean’s physical and biogeochemical states. The model output used is “Iteration 135” and available at http://sose.ucsd.edu/. This iteration runs from 2013 through 2019 with 1/6° longitudinal grid spacing ³⁴. The meridional spacing varies with latitude such that Δx = Δy in meters. There are 52 vertical thickness-varying levels with 33 levels in the upper 750 m. Bathymetry is derived from ETOPO1 ³⁵. The biogeochemical model is the Nitrogen version of the Biogeochemistry With Light, Iron, Nutrients (N-BLING), which is evolved from the model by ³⁶. The B-SOSE model is of intermediate complexity, with nine prognostic tracers: dissolved inorganic carbon, alkalinity, dissolved oxygen, nitrate, phosphate, dissolved inorganic iron, dissolved organic nitrogen, dissolved organic phosphorus, and phytoplankton biomass.

Satellite Data

South of 60°S satellites can only measure chlorophyll from ocean color sensors between October and March when there is sufficient light. To help validate the B-SOSE predictions, we compared the climatological values for February, averaged over 2013–2019, from the model-generated chlorophyll output with the chlorophyll data from the VIIRS sensor on the SNPP satellite. This period of comparison matches the time period of the B-SOSE model run used to estimate chlorophyll levels during winter months.

Migration Patterns

The overwinter migrations of CHPE and SES (Fig. 1) are similar in several respects. Both species exhibited a mix of movement patterns, with a predominance of relatively short movements (< 500 km) near tagging locations, but with some individuals undertaking much longer migrations (Fig. 2). In general, the mean maximum distance from the initial tagging locations that were achieved by the CHPE (854 ± 1158 km) and SES (887 ± 1006 km) were similar (Fig. 2). However, the subset of animals that moved into the study region had the longest annual migration distances (Fig. 1), with several tracks exceeding 4000 km regardless of tagging location (Table 1). The longest penguin migration, originating in the South Shetland Islands region, reached 171.7°W, 4680 km from its tagging location, while the longest SES migration, which also originated from the South Shetland Island region, was 5480 km (Table 1).

Tracks for the subset of CHPE and SES that move into the study region are shown in Fig. 3. The CHPE generally stayed between 60°-70°S in the southern portion of the ACC (Fig. 3a). The SES tracks are distributed over a wider portion of the Southern Ocean than the penguins, going both south and north of the ACC boundaries, with extensive use of the Ross Sea region (Fig. 3b). The SES that moved into the study region originated from both the Antarctic Peninsula and the Macquarie/Campbell Islands.

KDE analysis

The kernel density analysis reveals distinct CHPE and SES hotspots in the western sector of the Southern Ocean. There are two hotspots used by CHPE, one near 155°W and one between 120–140°W; both are between 60–65°S (Fig. 3c). Both hotspots occur along the southern boundary of the ACC. There are also two SES hotspots. The strongest occurs around 170°W, between 65–67°S, occurring slightly southwest of the CHPE hotspot at 155°W (Fig. 3d). This hotspot is centered slightly south of the southern boundary of the ACC. Several regions with elevated SES density also occur between 120°-140°W, which overlaps with the larger CHPE hotspot (Fig. 3d). The hotspots east of 140°W are located downstream of the narrow constriction in the ACC (Fig. 3c,d).

Speed analysis

Both species exhibited slower daily mean swim speeds between 120°-170°W relative to other longitudes (Fig. 4). For CHPE and SES, respectively, mean speeds slowed from 0.59 ± 0.29 m/s and 0.72 ± 0.37 m/s outside to 0.45 ± 0.29 m/s and 0.36 ± 0.26 m/s inside this region. This slowing is consistent with increased foraging effort relative to directed migratory movement. The reduced speeds equate with a reduction in daily displacement by 12 km for CHPE and 30 km, for SES. The relatively low swim speeds in the far east and west of the longitudinal range likely reflect local movements near breeding or molt locations.

B-SOSE Model output

Output from the B-SOSE model shows the presence of enhanced chlorophyll year round between 110°W and 140°W (Fig. 5a, b). The largest winter concentration, averaging over 0.17 mg/m³, occurs between 115°-135°W from the sea-ice edge into the ACC. There is a smaller bloom with a magnitude of 0.13 mg/m³ at the Polar Front and sea-ice edge in the Bellingshausen Sea (100°-80°W). Blooms of magnitude ~ 0.10 mg/m³ are seen at the sea-ice edge of the central Ross Sea sea-ice edge (~ 165°-145°W). The hotspots for CHPE and SES overlap these areas of elevated chlorophyll.

We have identified two areas in the Pacific sector of the Southern Ocean that a small proportion of CHPE and SES populations appear to target annually during their long (> 4000 km) winter migrations. Animal behavior within the hotspots was characterized by slower average swim speeds, consistent with increased foraging efforts relative to directed migratory movements. The two hotspots occurred mainly between 60°and 65°S. One was centered north of the Ross Sea between 150°W and 170°W, where the CHPE and SES hotspots exhibited some spatial separation. The second was located between 120–140°W at the boundary between the Amundsen Sea and the Ross Gyre, and is primarily occupied by CHPE, though elevated use by SES relative to the surrounding regions is also apparent here. These hotspots overlap with regions predicted to have persistent and elevated chlorophyll during winter, indicating the importance to migratory marine predators of bottom-up processes in remote ocean regions.

These two areas of consistent use by marine predators have not previously been identified as biological hotspots, despite a number of studies that have used tracking data to identify important habitats in the Southern Ocean. An analysis of tracks from 17 bird and mammal species in the Southern Ocean identified large Areas of Ecological Significance (AESs) in the Atlantic and Indian sectors of the Southern Ocean, but none in the Pacific sector ^13,37. Species examined in this analysis included SES, but not CHPE, although five other types of penguins were part of the dataset ³⁷. A study of seal and penguin tracking data discovered areas of productivity created by seamounts in the Atlantic and Indian sectors of the Southern Ocean but did not examine any data from the Pacific sector ³⁸. Hinke et al. ¹⁰ analyzed a single year of the data presented here and suggested that individual variability in migratory movements was a dominant driver for the broad-scale patterns of habitat use observed across the Southern Ocean. By extending that data set to include multiple years of tracking data from two species, however, we show that evidence for specific, repeatable patterns of habitat use during winter exists. Such predictable locations provide an opportunity for future work to study the effects of winter foraging conditions on migratory animals and to link variation in migratory habitats with population trends for these important Antarctic predator species.

The observation that two different species repeatedly migrated to the same general area of the western Pacific sector of the Southern Ocean suggests that this region hosts a productive and predictable habitat that long-distance migrants can reliably occupy. Output from the B-SOSE model supports that idea, as it shows year-round elevated chlorophyll in the hot spot regions. In particular, the CHPE hotspot between 120°–140°W overlaps with the area with the highest chlorophyll values in this sector. The model output also shows a region with elevated winter chlorophyll values that occurs near 160°W, just south of the southern ACC boundary. There, the CHPE hotspot occurs slightly north of the bloom, while the SES hotspot occurs west of the bloom. Since CHPE and SES target different prey (Bradshaw et al. 2003, Miller and Trivelpiece 2008; Rombolá, Marschoff, and Coria 2010), and forage within different portions of the water column, it is possible that the spatial separation of their hotspots arise from spatial variability of prey resources associated with the area of elevated chlorophyll there. For example, CHPE typically forage within the upper 50 m of the water column ^39,40, while elephant seals can dive much deeper and exploit resources at depths in excess of 1000 m ⁴¹. The persistence of elevated chlorophyll may enhance local production of prey resources (krill, fish, squids), whose distributions are likely to be heterogeneous. Alternatively, the spatial separation of the CHPE and SES winter hotspots north of the Ross Sea may be due to the presence of sea ice in the region. CHPE do not typically enter into the marginal ice zone (MIZ), favoring ice-free habitats ⁴². The location of the bloom is likely to be partially covered by sea ice during mid-winter periods, forcing CHPE to remain further north relative to SES that can exploit the MIZ ⁴³. Regardless, the presence of consistently elevated local chlorophyll levels provides a bottom-up mechanism to attract and retain migratory predators in this remote habitat.

The winter chlorophyll blooms observed in the B-SOSE model output can not be compared to satellite chlorophyll data since there is no satellite data south of 45°S from May-Oct due to insufficient light in winter. Climatological chlorophyll values for February from B-SOSE and the VIIRS satellite, however, exhibit similar qualitative patterns of elevated chlorophyll, although the magnitudes from B-SOSE are lower and the blooms smoother than the satellite data (Fig. 5b, c). Both show areas of elevated chlorophyll extending along the southern boundary of the ACC where the CHPE and SES hotspots occur, and also a notable bloom from about 140°W to 120°W where the CHPE hotspot occurs. A summer chlorophyll bloom in this area has also been seen in previous analyses of satellite data ^15,18,19 and, in part, is attributable to the turbulence resulting from the interaction of the ACC with the Pacific Antarctic Ridge. In particular, the UFZ around 144°W is a location of extreme topographical steering leading to heightened eddy activity downstream between 140°-135°W and 56°-58°S ¹⁶. This physical mechanism for enhanced mixing supports the hypothesis that the elevated primary production near the boundary of the ACC, Amundsen Sea, and Ross Gyre promotes a diverse foraging niche capable of supporting multiple pelagic predator species.

The SOCCOM project has generated a large amount of year-round, subsurface biogeochemical data for the remote Southern Ocean from the deployment of BGC-Argo floats. Our study was motivated by an interest in utilizing this novel data set in a project related to higher-trophic level predators and fishery management issues in the Southern Ocean, thus addressing a key priority of SOCCOM ⁴⁴. Despite the extensive number of SOCCOM floats deployed, however, the raw data remain too sparsely distributed for effectively overlaying them on the estimated locations from animal telemetry, as is standard practice when using gridded data products from remotely-sensed satellite observations. We therefore used the gridded fields of biogeochemical parameters from the output of the B-SOSE model which assimilates the BGC Argo float data. The year-round availability of BGC Argo float data in this remote part of the ocean is not only a valuable source of data for ocean ecosystem modeling, but we show that it is also useful for understanding the ecology of remote habitats used by migratory marine predators. Such ecological understanding can, in turn, help advance ocean management initiatives ranging from spatial protections like marine protected areas, to informing ecosystem-based fisheries management actions that are informed by the status and trends of non-target species. To maximize the use of these valuable BGC Argo float data for future ecological study and ecosystem management applications, we argue that it is important to continue efforts to make the float data available in gridded format, as either statistically mapped products, or as the output of models which assimilate the float data, as was used here. Similarly, efforts are needed to make these gridded products easily discoverable and accessible, specifically by providing means to both visualize the data and subset the data when downloading it. Issues of data accessibility are especially important to consider as the array of floats expands with the GO-BGC program ⁴⁵.

Roughly 10% of the CHPE and SES populations tracked over the last 3 decades have undertaken long-distance winter migrations, often traveling > 4000 km, to a remote region in the Pacific Sector of the Southern Ocean. We identified two hotspots of increased use by CHPE and SES that corresponded to broader regions of elevated chlorophyll predicted by the B-SOSE model. By leveraging in situ observations of biological variables during winter periods that derive from BGC Argo floats, we have shed light on a large-scale phenomenon invisible to traditional satellite sensors and linked long-distance migration patterns of two Antarctic marine predators with areas of consistently elevated chlorophyll levels. While the hotspots in the region are known areas of enhanced mixing ^14–16, this is the first study to show that it is likely a productive habitat capable of supporting diverse foraging niches exemplified by southern elephant seals and chinstrap penguins.

Competing interests

The authors declare no competing interests.

Author Contribution

C.W. managed the project. J.H. provided the penguin data. M.M. developed the B-SOSE model. All authors contributed to data analysis and writing of the manuscript.

Acknowledgement

This project grew out of a case study that was done for the BGC-Argo & Fisheries Workshop that was held at Scripps Institution of Oceanography in June 2022. M.R.M. acknowledges funding from NSF awards OPP-1936222 and OPP-2149501, and NASA award 80NSSC24K0243.

Data Availability

The chinstrap penguin telemetry data for 2000, 2004, 2006, 2010, and 2011 is available from the Seabird Tracking Database at http://seabirdtracking.org. Chinstrap penguin telemetry data from 2017 is available at https://doi.org/10.1371/journal.pone.0226207.s008. The elephant seal telemetry data was obtained from two sources: the "Marine Mammals Exploring the Oceans Pole to Pole" (MEOP) consortium website at https://www.meop.net/ and the Tagging of Pelagic Predators dataset at https://coastwatch.pfeg.noaa.gov/erddap/tabledap/gtoppAT.html. Output from the B-SOSE model is available at http://sose.ucsd.edu/SO6/ITER135/. The VIIRS/SNPP data was obtained from the NASA Goddard Space Flight Center, Ocean Ecology Laboratory, Ocean Biology Processing Group. Visible and Infrared Imager/Radiometer Suite (VIIRS) Chlorophyll Data; NASA OB.DAAC, Greenbelt, MD, USA. doi:10.5067/SUOMI-NPP/VIIRS/L3M/CHL/2022. Accessed on 4 March 2020.

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No competing interests reported.

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Understanding chinstrap penguin and elephant seal migrations in the Southern Ocean

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Version 1

Abstract

Figures

Introduction

Material and Methods

Telemetry Data

Model data

Satellite Data

Results

Migration Patterns

KDE analysis

Speed analysis

B-SOSE Model output

Discussion

Conclusions

Declarations

Competing interests

Author Contribution

Acknowledgement

Data Availability

References

Additional Declarations

Status:

Version 1