Physical and Ecological Factors Explain the Distribution of Ross Sea Weddell Seals During the Breeding Season
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Vol. 612: 193–208, 2019 MARINE ECOLOGY PROGRESS SERIES Published March 7 https://doi.org/10.3354/meps12877 Mar Ecol Prog Ser Physical and ecological factors explain the distribution of Ross Sea Weddell seals during the breeding season Michelle A. LaRue1,*, Leo Salas2, Nadav Nur2, David G. Ainley3, Sharon Stammerjohn4, Luke Barrington5, Kostas Stamatiou6, Jean Pennycook3, Melissa Dozier6, Jon Saints6, Hitomi Nakamura7 1Department of Earth Sciences, University of Minnesota, Minneapolis, MN 55455, USA 2Point Blue Conservation Science, Petaluma, CA 94954, USA 3H. T. Harvey and Associates Ecological Consultants, Los Gatos, California 95032, USA 4Institute of Arctic and Alpine Research, University of Colorado-Boulder, Boulder, CO 80303, USA 5Google, Mountain View, CA 94043, USA 6DigitalGlobe, Inc., Westminster, CO 80234, USA 7Polar Geospatial Center, University of Minnesota, St. Paul, MN 55108, USA ABSTRACT: Weddell seal Leptonychotes weddellii populations can potentially serve as indicators of change in Southern Ocean food web structure, but tracking populations at regional to continen- tal scales has so far been impossible. Here, we combined citizen science with remote sensing to learn about environmental and biological factors that explain fine-scale distribution of Weddell seal haul-outs in the Ross Sea, Antarctica. We employed the crowd-sourcing platform Tomnod (DigitalGlobe) to host high-resolution (~0.5−0.6 m) satellite imagery of the Antarctic fast ice dur- ing November in 2010 and 2011 and asked volunteers to identify seals on images. We created a 5 km × 5 km grid of seal presence per year, and modeled habitat suitability for seals using a gen- eralized linear model. The top Ross Sea-wide model that best explained seal presence included proximity to fast-ice cracks, deep water, and emperor penguin Aptenodytes forsteri colonies. This model also revealed that seal presence decreased with proximity to Adélie penguin Pygoscelis adeliae colonies and size of the nearest emperor penguin colony, suggesting the potential for trophic competitive exclusion by large penguin colonies. With respect to 3 sub-regions within the Ross Sea (North and South Victoria Land in the western Ross Sea, and Marie Byrd Land in the east), we found that 3 habitat variables differed in their effects among sub-regions: proximity to emperor penguin colonies, proximity to deep water, and relative ice width. Our results represent a step toward effectively monitoring Weddell seal population trends, and disentangling biological and environmental factors influencing locations of Weddell seal haul-outs around Antarctica. KEY WORDS: Leptonychotes weddellii · Antarctica · Citizen science · Generalized linear model · High-resolution satellite imagery · Trophic interaction · Tomnod Resale or republication not permitted without written consent of the publisher 1. INTRODUCTION adaptations to cope with such variability (Laws 1964; see also Siniff et al. 2008). Fast ice (ice ‘fas- The Antarctic sea ice zone is especially dynamic tened’ to the Antarctic coastline) occurs mostly both seasonally and inter-annually, and Weddell where the coastline is sheltered from strong winds seals Leptonychotes weddellii have developed or currents, and is an extremely limited habitat geo- *Corresponding author: [email protected] © Inter-Research 2019 · www.int-res.com 194 Mar Ecol Prog Ser 612: 193–208, 2019 graphically (in some cases, also temporally), but 2011, Ainley et al. 2015a). Given that the diet of Weddell seals show unique adaptations for using it Weddell seals depends on the commercially impor- (summarized by Siniff et al. 2008, Southwell et al. tant Antarctic toothfish (Ponganis & Stockard 2007, 2012), such as breath-holding capacity that allows Ainley & Siniff 2009, Salas et al. 2017) and silverfish the use of tide cracks many kilometers away from (one of the principal forage fish in coastal Antarctic open water. Hence, Weddell seals may be among waters; La Mesa & Eastman 2012), the ability to the most vulnerable species to climate change (Siniff assess population size and distribution of Weddell et al. 2008) because fast-ice prevalence is sensitive seals at regional scales may render the species a to changes in temperature and strength and persist- reliable indicator of ocean health, and thus a target ence of wind (Massom et al. 2009, Ainley et al. 2010, of the Convention for the Conservation of Antarctic Fraser et al. 2012, Nihashi & Ohshima 2015, Kim et Marine Living Resources (CCAMLR) Ecosystem Moni- al. 2018). As climate models predict continued win- toring Program (CEMP; www.ccamlr.org) and/or the ter warming and increasing winds, Weddell seal incipient Research and Monitoring Program for the habitat will likely continue to shrink and could alter Ross Sea Region Marine Protected Area (Dunn et al. the distribution and abundance of Weddell seals 2017). (Siniff et al. 2008). Given recent dramatic swings in Herein we investigated the distribution of Weddell Antarctic climate, describing the baseline distribu- seals in the Ross Sea using a combination of VHR, tion of Weddell seals is particularly important and citizen science, and crowd-sourcing. Based on seal time sensitive (Ainley et al. 2010, Turner & Comiso presence/absence data derived from searches con- 2017). ducted by the crowd of citizen scientists and an The natural history, physiology, and demography expert, we sought to understand the environmental of Weddell seals of the Ross Sea’s Erebus Bay have and biological variables that explain locations of been studied for nearly 50 yr (Stirling 1969b, Weddell seal haul-outs (5 km spatial scale). We chose Cameron & Siniff 2004, Chambert et al. 2015); how- to study Weddell seals during the November pup- ever, very limited information exists for other popula- ping season, when the species is most concentrated tions of this species (e.g. Lindsey 1937, Stirling 1969c, on fast ice (Stirling 1971). We compared the predic- Ainley et al. 2015a). Enumerating seal populations tive relationships among 3 sub-regions: the north- beyond the local level (Southwell et al. 2004, 2012) is west, southwest, and eastern portions of the Ross difficult due to logistical challenges associated with Sea. These sub-regions differ in several parameters surveying fast ice zones. During the 1960−1980s, var- that are potentially of great importance to the Wed- ious regional attempts were made to estimate seal dell seals, as the southwest sub-region is entirely on populations in pack-ice habitat, relate those numbers the continental shelf, while the northwestern and to physical factors such as ice floe characteristics, and eastern sub-regions have both shelf and continental then produce a density estimate to extrapolate to slope waters. larger areas (e.g. Erickson et al. 1971, Gilbert & We evaluated the hypothesis that physical factors Erickson 1977, Erickson & Hanson 1990). That work such as cracks constrain the presence of reproductive made use of strip censuses from ice breakers, helicop- females. Other factors (such as proximity and size of ters, or fixed-wing aircraft, which are logistically com- penguin colonies) could be an additional constraint plex (e.g. Bengtson et al. 2011, Forcada et al. 2012, via competition or niche partitioning, or may reflect a Southwell et al. 2012). As a re sult, the regional distri- preference if similar habitat features attract compet- butions of colonies and their habitat features are not ing species. In that regard, Adélie penguins Pygoscelis well understood, especially in areas far from major adeliae prefer immediate proximity to open water research facilities (e.g. Lake et al. 2008, Ainley et al. (Ainley 2002), in contrast to emperor penguins Apten- 2015a). odytes forsteri that prefer fast ice (Burns & Kooyman More recently, monitoring of Antarctic ice-obligate 2001). Under a ‘constraint’ hypothesis, physical vari- species has been attempted through remote sensing ables would be more important than the ecological (Weddell seals: LaRue et al. 2011; penguins: Fret - variables. We hypothesized that physical and oceano - well et al. 2012, LaRue et al. 2014, Lynch & LaRue graphic variables, such as cracks in the ice and prox- 2014). Because Weddell seals are dark and large imity to shore, would be the primary factors in ex - (400− 600 kg, 1.5 × 3−4 m), very high-resolution plaining seal presence, and be consistently important (VHR) satellite imagery has been used to quantify across sub-regions, while ecological effects would be their distribution and numbers even in the most less important and more variable in their influence remote sections of the Antarctic coast (LaRue et al. among sub-regions. LaRue et al.: Weddell seal breeding distribution 195 2. MATERIALS AND METHODS square and others are long swaths of images ~15 km wide; from the Polar Geospatial Center) for Novem- 2.1. Study area ber 2010 and 2011 over the Landsat Image Mosaic of Antarctica basemap (Bindschadler et al. 2008). Dur- We focused our research along the coast of the Ross ing November, Weddell seals are the only seals pres- Sea (Fig. 1), the most productive region in the South- ent in coastal fast-ice areas (Smith 1965, Stirling ern Ocean (Arrigo et al. 1998, 1999), characterized by 1969b, Rotella et al. 2009). Note that there are small high-salinity Ross Sea shelf water that overlies the concentrations of non-breeding individuals, which continental shelf and upper slope (Smith et al. 2014). we cannot distinguish from the breeders, and pups The Ross Sea is covered by sea ice (mostly pack ice) are too small to be detectable on VHR in November for most of the year, except for the large or persistent (LaRue et al. 2011). We extracted image extents for polynyas located in front of the Ross Ice Shelf and in consideration using several criteria: existence of fast Terra Nova Bay and McMurdo Sound, with other ice, <20% cloud cover, panchromatic (i.e. black and smaller more ephemeral polynyas located over Pen- white), high quality (not over-exposed or striped), nell Bank and in the Ross Passage (see Jacobs & and of lowest off-nadir angle (the angle at which an Comiso 1989, Arrigo et al.