Quantifying the causes and consequences of variation in satellite-derived population indices: a case study of emperor penguins

© The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Labrousse, S., Iles, D., Viollat, L., Fretwell, P., Trathan, P. N., Zitterbart, D. P., Jenouvrier, S., & LaRue, M. Quantifying the causes and consequences of variation in satellite-derived population indices: a case study of emperor penguins. Remote Sensing in Ecology and Conservation, (2021), https://doi.org/10.1002/rse2.233.Very high-resolution satellite (VHR) imagery is a promising tool for estimating the abundance of wildlife populations, especially in remote regions where traditional surveys are limited by logistical challenges. Emperor penguins Aptenodytes forsteri were the first species to have a circumpolar population estimate derived via VHR imagery. Here we address an untested assumption from Fretwell et al. (2012) that a single image of an emperor penguin colony is a reasonable representation of the colony for the year the image was taken. We evaluated satellite-related and environmental variables that might influence the calculated area of penguin pixels to reduce uncertainties in satellite-based estimates of emperor penguin populations in the future. We focused our analysis on multiple VHR images from three representative colonies: Atka Bay, Stancomb-Wills (Weddell Sea sector) and Coulman Island (Ross Sea sector) between September and December during 2011. We replicated methods in Fretwell et al. (2012), which included using supervised classification tools in ArcGIS 10.7 software to calculate area occupied by penguins (hereafter referred to as ‘population indices’) in each image. We found that population indices varied from 2 to nearly 6-fold, suggesting that penguin pixel areas calculated from a single image may not provide a complete understanding of colony size for that year. Thus, we further highlight the important roles of: (i) sun azimuth and elevation through image resolution and (ii) penguin patchiness (aggregated vs. distributed) on the calculated areas. We found an effect of wind and temperature on penguin patchiness. Despite intra-seasonal variability in population indices, simulations indicate that reliable, robust population trends are possible by including satellite-related and environmental covariates and aggregating indices across time and space. Our work provides additional parameters that should be included in future models of population size for emperor penguins.Geospatial support for this work was provided by the Polar Geospatial Center under NSF-OPP awards 1043681 and 1559691. NCAR- PPC visitor funds and Ian Nisbet that supported the internship of LV. WWF-UK supported PNT and PTF under grant GB095701. DZ was supported by The Penzance Endowed Fund and The Grayce B. Kerr Fund in Support of Assistant Scientists. To SJ, ML, SL, LV, NSF OPP 1744794

Quantifying the causes and consequences of variation in satellite‐derived population indices: a case study of emperor penguins

Very high-resolution satellite (VHR) imagery is a promising tool for estimating the abundance of wildlife populations, especially in remote regions where traditional surveys are limited by logistical challenges. Emperor penguins Aptenodytes forsteri were the first species to have a circumpolar population estimate derived via VHR imagery. Here we address an untested assumption from Fretwell et al. (2012) that a single image of an emperor penguin colony is a reasonable representation of the colony for the year the image was taken. We evaluated satellite-related and environmental variables that might influence the calculated area of penguin pixels to reduce uncertainties in satellite-based estimates of emperor penguin populations in the future. We focused our analysis on multiple VHR images from three representative colonies: Atka Bay, Stancomb-Wills (Weddell Sea sector) and Coulman Island (Ross Sea sector) between September and December during 2011. We replicated methods in Fretwell et al. (2012), which included using supervised classification tools in ArcGIS 10.7 software to calculate area occupied by penguins (hereafter referred to as ‘population indices’) in each image. We found that population indices varied from 2 to nearly 6-fold, suggesting that penguin pixel areas calculated from a single image may not provide a complete understanding of colony size for that year. Thus, we further highlight the important roles of: (i) sun azimuth and elevation through image resolution and (ii) penguin patchiness (aggregated vs. distributed) on the calculated areas. We found an effect of wind and temperature on penguin patchiness. Despite intra-seasonal variability in population indices, simulations indicate that reliable, robust population trends are possible by including satellite-related and environmental covariates and aggregating indices across time and space. Our work provides additional parameters that should be included in future models of population size for emperor penguins

The call of the emperor penguin: Legal responses to species threatened by climate change

Species extinction risk is accelerating due to anthropogenic climate change, making it urgent to protect vulnerable species through legal frameworks in order to facilitate conservation actions that help mitigate risk. Here, we discuss fundamental concepts for assessing climate change risks to species using the example of the emperor penguin (Aptenodytes forsteri), currently being considered for protection under the US Endangered Species Act (ESA). This species forms colonies on Antarctic sea ice, which is projected to significantly decline due to ongoing greenhouse gas (GHG) emissions. We project the dynamics of all known emperor penguin colonies under different GHG emission scenarios using a climate-dependent meta-population model including the effects of extreme climate events based on the observational satellite record of colonies. Assessments for listing species under the ESA require information about how species resiliency, redundancy and representation (3Rs) will be affected by threats within the foreseeable future. Our results show that if sea ice declines at the rate projected by climate models under current energy system trends and policies, the 3Rs would be dramatically reduced and almost all colonies would become quasi-extinct by 2100. We conclude that the species should be listed as threatened under the ESA

Marine ecosystem assessment for the Southern Ocean: birds and marine mammals in a changing climate

The massive number of seabirds (penguins and procellariiformes) and marine mammals (cetaceans and pinnipeds) – referred to here as top predators – is one of the most iconic components of the Antarctic and Southern Ocean. They play an important role as highly mobile consumers, structuring and connecting pelagic marine food webs and are widely studied relative to other taxa. Many birds and mammals establish dense breeding colonies or use haul-out sites, making them relatively easy to study. Cetaceans, however, spend their lives at sea and thus aspects of their life cycle are more complicated to monitor and study. Nevertheless, they all feed at sea and their reproductive success depends on the food availability in the marine environment, hence they are considered useful indicators of the state of the marine resources. In general, top predators have large body sizes that allow for instrumentation with miniature data-recording or transmitting devices to monitor their activities at sea. Development of scientific techniques to study reproduction and foraging of top predators has led to substantial scientific literature on their population trends, key biological parameters, migratory patterns, foraging and feeding ecology, and linkages with atmospheric or oceanographic dynamics, for a number of species and regions. We briefly summarize the vast literature on Southern Ocean top predators, focusing on the most recent syntheses. We also provide an overview on the key current and emerging pressures faced by these animals as a result of both natural and human causes. We recognize the overarching impact that environmental changes driven by climate change have on the ecology of these species. We also evaluate direct and indirect interactions between marine predators and other factors such as disease, pollution, land disturbance and the increasing pressure from global fisheries in the Southern Ocean. Where possible we consider the data availability for assessing the status and trends for each of these components, their capacity for resilience or recovery, effectiveness of management responses, risk likelihood of key impacts and future outlook

First description of in situ chlorophyll fluorescence signal within East Antarctic coastal polynyas during fall and winter

Antarctic coastal polynyas are persistent and recurrent regions of open water located between the coast and the drifting pack-ice. In spring, they are the first polar areas to be exposed to light, leading to the development of phytoplankton blooms, making polynyas potential ecological hotspots in sea-ice regions. Knowledge on polynya oceanography and ecology during winter is limited due to their inaccessibility. This study describes i) the first in situ chlorophyll fluorescence signal (a proxy for chlorophyll-a concentration and thus presence of phytoplankton) in polynyas between the end of summer and winter, ii) assesses whether the signal persists through time and iii) identifies its main oceanographic drivers. The dataset comprises 698 profiles of fluorescence, temperature and salinity recorded by southern elephant seals in 2011, 2019-2021 in the Cape-Darnley (CDP;67˚S-69˚E) and Shackleton (SP;66˚S-95˚E) polynyas between February and September. A significant fluorescence signal was observed until April in both polynyas. An additional signal occurring at 130m depth in August within CDP may result from in situ growth of phytoplankton due to potential adaptation to low irradiance or remnant chlorophyll-a that was advected into the polynya. The decrease and deepening of the fluorescence signal from February to August was accompanied by the deepening of the mixed layer depth and a cooling and salinification of the water column in both polynyas. Using Principal Component Analysis as an exploratory tool, we highlighted previously unsuspected drivers of the fluorescence signal within polynyas. CDP shows clear differences in biological and environmental conditions depending on topographic features with higher fluorescence in warmer and saltier waters on the shelf compared with the continental slope. In SP, near the ice-shelf, a significant fluorescence signal in April below the mixed layer (around 130m depth), was associated with fresher and warmer waters. We hypothesize that this signal could result from potential ice-shelf melting from warm water intrusions onto the shelf leading to iron supply necessary to fuel phytoplankton growth. This study supports that Antarctic coastal polynyas may have a key role for polar ecosystems as biologically active areas throughout the season within the sea-ice region despite inter and intra-polynya differences in environmental conditions

Sous la banquise Antarctique : écologie alimentaire des éléphants de mer des îles Kerguelen, influence des paramètres océanographiques et de glace de mer

Understanding how physical properties of the environment underpin habitat selection of large marine vertebrates is crucial in identifying how and where animals acquire resources necessary for locomotion, growth and reproduction and ultimately their fitness. The Southern Ocean harbors one of the largest and most dynamic marine ecosystems on our planet which arises from the presence of two majors physical features, (i) the Antarctic Circumpolar Current and (ii) the seasonal sea ice cover region. In the Antarctic, marine predators are exposed to climate-induced shifts in atmospheric circulation and sea ice. However, because these shifts vary regionally, and because much remains to be understood about how individual animals use their environment, it has been difficult to make predictions on how animals may respond to climate variability. Southern elephant seals (Mirounga leonina) are a major consumer of Southern Ocean resources and use two main large scale foraging strategies, (i) feeding in the frontal zone of the Southern Ocean, or (ii) feeding in the seasonal sea ice region. In the present thesis I examined the winter post-moulting foraging strategies of 46 male and female Kerguelen southern elephant seals which utilized the second strategy. Using an eleven year time-series of tracking, diving, and seal-collected hydrographic data (from 2004-2014) I assessed their movements and foraging performance in relation to in situ hydrographic and sea ice conditions. The influence of both the spatio-temporal and inter-annual variability of sea ice around seal locations was investigated, and an investigation on the role of polynya for male elephant seal during winter conducted.Les mammifères marins de l'Océan Austral sont des éléments essentiels des écosystèmes marins antarctiques et des sentinelles de l’état des océans polaires. Comprendre comment les conditions océanographiques déterminent leurs habitats préférentiels est essentiel pour identifier de quelle façon et dans quelle région ces mammifères acquièrent les ressources nécessaires à leur déplacement, leur croissance, leur reproduction et donc leur survie. Les éléphants de mer du Sud (Mirounga leonina) se déplacent dans l’océan austral à l’échelle des bassins océaniques pour s’alimenter en plongeant en moyenne à 500 m et jusqu’à 2000 m de profondeur. En fonction de leur colonie d’origine, de leur sexe, et de leur âge, ils exploitent des régions radicalement différentes de l’océan austral, mettant ainsi en œuvre des stratégies alimentaires diversifiées. Les éléphants de mer de Kerguelen utilisent deux zones préférentiellement: la zone du Front Polaire ou la zone Antarctique couverte par la banquise. Dans cette thèse, les stratégies alimentaires des voyages post-mue Antarctiques de 46 mâles et femelles éléphants de mer de Kerguelen ont été étudiées. Une série temporelle de 11 années (2004-2014) de données de déplacement, de plongées et de données hydrologiques a été analysée pour déterminer le rôle des paramètres océanographiques et de glaces de mer impliqués dans l’acquisition des ressources alimentaires des éléphants de mer en Antarctique. L’influence de la variabilité spatio-temporelle et interannuelle de la glace de mer associée à la position des mâles et des femelles ainsi que le rôle des polynies côtières sur les stratégies alimentaires des mâles en hiver ont été examinés

Massive and infrequent informed emigration events in a species threatened by climate change: the emperor penguins

Dispersal is a ubiquitous phenomenon which affects the population dynamics and evolution of natural populations, hence it is a fundamental process in driving species persistence under global changes. However, dispersal rates and dispersal range are difficult to measure in most species, and remain unknown for many. In addition, informed behaviors, whereby individuals leave their natal area and select a breeding habitat non-randomly, may play an important role in species' responses to global change, and are even more difficult to comprehend. By developing new models combing demographic and genetic data, we reveal dispersal rates, range and behaviors for emperor penguin, a species threatened by climate change and living in the most remote place on earth where measurements of dispersal are nonexistent. We found that emperor penguins have a short distance of dispersal compared to their capacity to cover large distances during seasonal migration. On average, emigration rates are small, with emigration occurring when the local habitat becomes unsuitable (informed emigration). While overall dispersal rates remain low, occasional mass emigration events can occur in some regions. We also detect environmental and demographic drivers of emigration using new independent data:emigration is more likely to occur for habitats far from open water and with low food availability. Finally, we provided new global population forecasts for emperor penguins that incorporate dispersal processes into a sea-ice dependent demographic model that can inform conservation actions in Antarctica. Our method can be applied to other species and data-poor systems to estimate dispersal processes

Massive and infrequent informed emigration events in a species threatened by climate change: the emperor penguins.

Dispersal is a ubiquitous phenomenon which affects the population dynamics and evolution of natural populations, hence it is a fundamental process in driving species persistence under global changes. However, dispersal rates and dispersal range are difficult to measure in most species, and remain unknown for many. In addition, informed behaviors, whereby individuals leave their natal area and select a breeding habitat non-randomly, may play an important role in species' responses to global change, and are even more difficult to comprehend. By developing new models combing demographic and genetic data, we reveal dispersal rates, range and behaviors for emperor penguin, a species threatened by climate change and living in the most remote place on earth where measurements of dispersal are nonexistent. We found that emperor penguins have a short distance of dispersal compared to their capacity to cover large distances during seasonal migration. On average, emigration rates are small, with emigration occurring when the local habitat becomes unsuitable (informed emigration). While overall dispersal rates remain low, occasional mass emigration events can occur in some regions. We also detect environmental and demographic drivers of emigration using new independent data:emigration is more likely to occur for habitats far from open water and with low food availability. Finally, we provided new global population forecasts for emperor penguins that incorporate dispersal processes into a sea-ice dependent demographic model that can inform conservation actions in Antarctica. Our method can be applied to other species and data-poor systems to estimate dispersal processes

First odyssey beneath the sea ice of juvenile emperor penguins in East Antarctica

International audienceAdult emperor penguins Aptenodytes forsteri breed on fast ice and forage within sea ice in winter. However, it remains unknown whether juveniles exhibit similar foraging behavior during their early life at-sea movements, and how it links with the oceanographic conditions. We investigated the first at-sea odyssey of 15 juvenile emperor penguins from Terre Adélie in 2013-2014. The average tracking duration was 167 ± 110 d SD (range 86-344 d). After departing the colony in December/January, the juveniles traveled north up to 53.76°S before heading south in April/May to forage within the sea ice. The juveniles spent 49 ± 14% of their total recorded trips (n = 12) in the sea ice, over both the continental slope and deep ocean regions. The penguins dived primarily during daylight. Within sea ice, the juveniles performed both shallow and deep dives, with the proportion of each varying seasonally. The switch to primarily deep dives in the autumn and winter within sea ice may be a consequence of (1) a seasonal change in the krill distribution from surface to deep waters and/or (2) the presence of macrozooplankton at depth due to a reduced/absent diel migration. Furthermore, we showed for the first time that the diving behavior of juveniles was associated with the mixed layer depth. We suggest they feed on mesopelagic prey aggregating near the thermocline. This study provides insight into an important, but poorly understood, part of the emperor penguin life cycle, essential to predict their response to future climate change