1. Introduction
The Bering Strait region has been occupied for at least 4000 years [
1]. The cultural traditions of the Inupiat, Yupik, and St. Lawrence Island Yupik people who live here are still practiced in contemporary times, such as hunting various marine mammals for meat, oil, and other subsistence foods and materials. An estimated 2700 pounds of marine mammals are consumed annually as food by the Inupiat, Yupik and St. Lawrence Island Yupik households in the Bering Strait region, and sea mammal foods have tremendous cultural importance [
2,
3]. Long-term climatic change may result in a shift or loss of habitat, which could influence marine mammal population distributions and affect the communities that depend upon them [
4,
5], even though there is no documentation of climate-change related shifts in migration patterns of marine mammals in the existing scientific record [
6]. Marine mammal populations in the Bering Strait may experience a variety of stressors related to health impacts of climate change [
7], increasing resource exploration, underwater seismic testing, increasing surface vessel traffic [
8,
9], and commercial fishing activities [
10].
Global climate model projections call for a warming Arctic and declining sea ice throughout the 21st century [
11]. Arctic sea ice is a dynamic polar phenomenon, covering approximately 15 million km
2 at its March maximum each year, and shrinking to approximately 7 million km
2 by September [
12]. The September minimum sea ice extent decreased by an average linear rate of 79,000 km
2/yr between 1979 and 2009, and the March maximum extent is projected to be 25% less by 2050 and 60% less by 2100, with the ice-free period, which currently consists of 5.5 months on average, projected to increase to a median of 8.5 months [
13]. Arctic temperatures up to 2.9 °C warmer when comparing the 2001–2009 mean to the 1951–2000 mean [
14] accompany an overall loss of 1.59 m of September sea ice thickness when comparing 2003–2007 to 1958–1976 [
15]. The majority of sea ice loss occurred on the Pacific side of the Arctic [
16], thus increasing the strategic importance of the Bering Strait.
Commercial interests identify the Bering Strait as an important route for trade and for exploration and development of natural resources [
17,
18,
19,
20]; the oil reserves of the Outer Continental Shelf in the Beaufort and Chukchi Seas are estimated to be worth $193–312 billion, and the Bering Strait is the passageway to markets in Asia, North America, and Europe [
21]. The United States Coast Guard has estimated that the number of transits through the Bering Strait increased from 245 in 2008 to more than 400 in 2011 [
21]. This substantial increase in ship traffic and general utilization of Bering Strait has considerable ramifications, and the economic interests of outsiders may conflict with the subsistence practices of the indigenous residents who live and hunt there. This subsistence economy revolves around the abundant and diverse marine mammals that inhabit the Bering Strait [
3,
22,
23,
24].
To help guide complex, multinational policy discussions that will shape the future of the Bering Strait, a need exists for enhanced knowledge and characterization of marine mammal habitats in order to help plan for the conservation of these animals as climate- and human-induced changes occur across the area. Marine spatial planning efforts incorporate physical information about habitats within the marine environment, characteristics of marine species found within those habitats, and humans who interact with the marine environment and its resources [
25,
26]. Geospatial analysis and mapping techniques have been used for various threatened and endangered marine species such as mussels [
27], turtles [
28], and monk seals [
29] to aid in understanding environmental conditions within habitats and to help guide policy decisions for managing environmental and human pressures upon habitats. In the Arctic, however, these analyses have been fairly limited.
The increasing availability of remotely sensed and other environmental data has allowed ecologists to develop spatially explicit maps of habitat suitability and species distributions using only a limited number of field observations. This is accomplished by modeling relationships between species observations and predictor variables measured from remote sensing [
30,
31,
32,
33,
34]. Habitat classification at a variety of spatial scales using multiple sources of spatial data is a common application of remote sensing analysis in both terrestrial [
35,
36,
37] and marine [
38,
39,
40,
41,
42] environments; however, the majority of habitat classification research has focused on tropical and temperate latitudes, with relatively few studies at high latitudes. One challenge to developing these models for non-stationary, migratory species in remote, high latitude regions (like the Bering Strait) is the lack of training and validation data derived from Western science field observations of animal numbers, locations, and movement patterns [
43]. Here, we use the term ‘Western science’ to refer to research conducted by individuals trained in the Western scientific method, with roots in European philosophy and a focus on reductionist and standardized experimental design and academy-based knowledge.
The bearded seal (
Erignathus barbatus) is a marine mammal inhabiting the Bering Strait that is highly dependent on sea ice and is an important subsistence resource for Alaska Native hunters with a wide range of traditional uses [
44,
45]. The bearded seal is a protected species under the Marine Mammal Protection Act and has recently been listed as threatened under the Endangered Species Act [
46]. Bearded seals are found throughout the Arctic region and prefer to remain in close proximity to broken sea ice, preferentially hauling out onto ice rather than the shore, and they tend to avoid massive shore-fast ice packs [
47]. Adult bearded seals are primarily benthic feeders; they consume fish, invertebrates, and other bottom-dwelling prey items found at depths of 500 m or less, but more typically at depths of 200 m or less [
48,
49]. Adult bearded seals associated with the Bering Strait tend to migrate north as the pack ice shrinks northward in warmer months and south as the pack ice expands southward in colder months, moving with the active ice edge that produces fractures, areas of thin ice, and other features that provide haul-out surfaces and protection from predators [
49]. Published maps of bearded seal range or habitat typically encompass the majority of the Bering and Chukchi Seas, and the entirety of the Bering Strait [
47,
49,
50]. Bering Strait region indigenous hunters and elders have explained that while bearded seals are found throughout the region, they are more concentrated in desirable feeding areas [
45]. Identifying seal concentration areas not located near human communities can be difficult, as much of the region is remote with both environmental conditions that make obtaining observations difficult, as well as international boundaries between nations with a history of poor cooperation in scientific ventures [
49].
As of 2015, the most systematic experimental data on bearded seals has been gathered through two very recent campaigns: (1) aerial surveys conducted when the seals are hauled out on the sea ice in springtime [
51]; and (2) Global Positioning System (GPS)-enabled tags affixed to a small number (
n = 4) of bearded seals to track their movements in the Bering Sea [
43]. These data are recently acquired (and, thus, have not been subject to quality control or further analysis) and limited in scope for a migratory species. Bearded seals are more difficult to observe in summer and fall, when those remaining in the region are in the water or hauled out on shore, and much of the population has traveled north with the sea ice. Summer and fall bearded seal habitat is typically driven by fish and coastal features, rather than ice [
45]. As sea ice is projected to retreat rapidly in the coming decades, understanding and delineating critical bearded seal habitat in the absence of sea ice will become increasingly important. At this time, however, little Western science data has been collected on bearded seals during ice-free periods.
An alternative source of information on bearded seals during summer and fall seasons is indigenous hunters and community elders, who have detailed multi-generational knowledge and observations of seals and their hunting areas; this is more frequently termed Traditional Ecological Knowledge (TEK). TEK has been defined as “
a cumulative body of knowledge, practice, and belief, evolving by adaptive processes and handed down through generations by cultural transmission, about the relationship of living beings (including humans) with one another and with their environment” [
52]. The TEK of Alaska Native subsistence hunters includes information from those who have observed, tracked, and harvested seals in the Bering Strait region, and who have passed down that knowledge over successive generations [
1,
53,
54]. While there is increasing interest in integrating TEK and Western scientific approaches, it has proved difficult for a variety of reasons, including different contexts, values, goals, and approaches [
24].
From 2010 to 2013, Kawerak, Inc., the Alaska Native non-profit entity for the Bering Strait region, worked closely with 82 local experts, defined as hunters and elders with extensive marine mammal hunting experience, to map the TEK of seal harvest and habitat areas near nine communities in the region [
45]. Local experts repeatedly noted that there were many important habitat areas that were beyond their harvest and travel areas, and several encouraged collaboration with scientists in order to incorporate advanced technology. Additionally, local experts were frustrated when Western scientific studies conducted in the region neglected TEK and produced conclusions that were easily invalidated by local observations (Gadamus, personal observation). As such, Kawerak shared the traditional knowledge data for this study in the hopes of (a) supporting a novel method for better integration of TEK and Western science; and (b) producing region-wide maps of summer/fall bearded seal habitat.
This study focuses on developing and proofing a method to utilize remotely sensed environmental data and geospatial training data derived from the Kawerak TEK data to create habitat suitability maps for marine mammals in Arctic environments. Specifically, the objectives include: (1) identifying an approach for converting TEK data to training and validation points for classification processes; (2) determining which predictor variables contribute the most information to the classification process; (3) ascertaining whether time series analysis outputs improve the accuracy of the classification process; and (4) developing habitat suitability maps that can inform policy discussion in the Bering Strait.