Spatial distribution of narwhal (Monodon monoceros) diving for Canadian populations helps identify important seasonal foraging areas
Abstract
In Canada, narwhals (Monodon monoceros L., 1758) are divided into the Baffin Bay (BB) and northern Hudson Bay (NHB) populations. Satellite tracking of 21 narwhals from BB and NHB provided information on their diving behaviour and was used to identify foraging regions. Previous research from hunted narwhals indicated that narwhals in both populations depend on benthic prey to meet their dietary needs. To evaluate home ranges and define areas important for benthic foraging, we conducted kernel density analysis on narwhal locations and focused on areas where deep diving occurs, as a proxy for foraging, in the winter, spring, and migratory periods. These analyses revealed important areas for foraging for BB narwhals on the summer grounds in Eclipse Sound, and the winter grounds in Davis Strait, as well as on the migratory pathway between regions. Similarly, important areas were identified for the NHB narwhal population in northwestern Hudson Bay in summer, in NHB and Hudson Strait on the migration, and to the east of the entrance to Hudson Strait in the winter. This, along with an analysis of the absolute dive depths, provides information on seasons and regions important for foraging, which is particularly relevant with increasing industrial activities in the Arctic.
Résumé
Au Canada, les narvals (Monodon monoceros L., 1758) sont divisés en deux populations, celle de la baie de Baffin (BB) et celle du nord de la baie d’Hudson (NBH). Le suivi par satellite de 21 narvals de la BB et du NBH a fourni des renseignements sur leur comportement de plongée et a été utilisé pour délimiter des régions d’approvisionnement. Des travaux antérieurs sur des narvals chassés indiquent que les narvals des deux populations dépendent de proies benthiques pour combler leurs besoins alimentaires. Afin d’évaluer les domaines vitaux et de définir les aires importantes pour l’approvisionnement benthique, nous avons mené une analyse de la densité de noyaux des emplacements de narvals en mettant l’accent sur les zones où des plongées profondes ont lieu (comme variable substitutive de l’approvisionnement) à l’hiver, au printemps et durant les périodes de migration. Ces analyses ont fait ressortir des zones importantes pour l’approvisionnement des narvals de la BB dans les aires estivales du détroit d’Éclipse et les aires hivernales du détroit de Davis, ainsi que dans la voie de migration entre les régions. De même, des zones importantes ont été cernées pour la population de narvals du NBH dans le nord-ouest de la baie d’Hudson en été, dans le NBH et le détroit d’Hudson durant la migration et à l’est de l’entrée du détroit d’Hudson en hiver. Ces résultats, combinés à une analyse des profondeurs de plongée absolues, fournissent de l’information sur les saisons et les régions importantes pour l’approvisionnement qui s’avère particulièrement pertinente au vu de l’augmentation des activités industrielles dans l’Arctique. [Traduit par la Rédaction]
Introduction
The Arctic is currently undergoing a reduction in total sea ice, changes in sea-ice extent, and a shift in the timing of ice freeze-up and break-up (Tivy et al. 2011). These changes have facilitated increases in shipping traffic in the Arctic (Pizzolato et al. 2014) and allowed access to enormous oil and gas reserves beneath the Arctic Ocean (Reeves et al. 2014). Narwhals (Monodon monoceros L., 1758) are medium-sized (approximately 3–4 m length for adults; Garde et al. 2015) cetaceans endemic to the Arctic and may be impacted by these increases in industrial activities. There are two populations of narwhals, which are spatially and genetically distinct (Petersen et al. 2011), that frequent Canadian waters. The Baffin Bay (BB) population spends summer (approximately June–September) in the northeastern fiords and inlets in Canada and the northwestern inlets of Greenland (Dietz et al. 2001, 2008; Heide-Jørgensen et al. 2003, 2013a; Watt et al. 2012). They then begin their ∼1700 km migration to Davis Strait where they overwinter, before beginning their migration in April back to the summering grounds (Heide-Jørgensen et al. 2003; Watt et al. 2012; Fig. 1). Narwhals in this population are divided into different management units, referred to as stocks, based on their summering aggregations, and there are at least four defined stocks in northern Canada (DFO 2012). The second Canadian population is the northern Hudson Bay (NHB) population. Whales in this population spend summer in northern Hudson Bay, and then migrate ∼1250 km east to their wintering grounds in the eastern Hudson Strait (Richard 1991; Fig. 1).
Fig. 1.
The BB narwhal population is estimated to be approximately 140 000 individuals (Doniol-Valcroze et al. 2015) and the NHB population has approximately 12 500 individuals (Asselin et al. 2012). Narwhals are listed as special concern by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC 2004). This was a change in status from their “not at risk” listing in 1987, primarily because of the uncertainty with the population size and levels of sustainable hunting, as well as potential impacts of climate change (COSEWIC 2004). At the time of the listing, industrial activities were of little concern, other than potential competition for Greenland halibut (Reinhardtius hippoglossoides (Walbaum, 1792)) with the active fishery that occurs in the summer in the region where narwhals overwinter. However, since then, an iron-ore mine has become operational on northern Baffin Island and the port of Churchill is expanding (Reeves et al. 2014). There is no information on how these industrial activities may impact narwhals, but having an understanding of areas important for narwhal foraging may allow us to evaluate how much overlap there will be in areas shared by narwhals and industry.
Currently, our understanding of narwhal diet comes from satellite tag studies, stomach contents, and stable isotope analysis (Laidre and Heide-Jørgensen 2005; Watt et al. 2013, 2015). For the BB population, previous stomach content studies have identified deep-dwelling prey, such as Greenland halibut that live at depths of 400–1400 m (Peklova et al. 2012), as being important to diet, particularly in the winter months (Laidre and Heide-Jørgensen 2005). Based on stomach contents and dive behaviour, it has been suggested that narwhals forage intensively on Greenland halibut in the winter months (Laidre et al. 2003; Laidre and Heide-Jørgensen 2005). Stable isotopes identified benthic prey, such as Greenland halibut and northern shrimp (Pandalus borealis Krøyer, 1838), as contributing substantially to BB narwhal summer diet (>50% of their diet; Watt et al. 2013). In addition, dive behaviour indicated that these whales spend a significant amount of time in and make dives to 75%–100% of total bathymetric depth at all times of the year (Watt et al. 2015). Stomach content studies have never been conducted on narwhals from NHB, but stable isotopes confirmed that benthic prey makes up the majority of their summer diet (up to 70% for males and 60% for females; Watt et al. 2013) and these results were supported by dive behaviour that confirmed narwhals from this population make a significant number of dives to 75%–100% of total bathymetric depth (∼26% of all dives were to this portion of the water column; Watt et al. 2015). Although benthic foraging contributes substantially to the diet of narwhals in both populations (Watt et al. 2013), no studies of where this foraging is occurring have been conducted for either population. An understanding of areas critical for narwhal foraging is important for determining how much regional overlap exists between industry and narwhals, and for predicting anthropogenic impacts on narwhals.
Deep-diving marine mammals are limited in their foraging time because of their oxygen requirements at the surface. The time they need to spend at the surface increases with dive length, and this must be traded off with the fact that longer dives increase the chances of the animals finding and capturing prey (Kooyman and Ponganis 1998). Thus, diving marine mammals need to offset the high costs of diving by foraging on lipid-rich and (or) abundant prey to optimize their energy budget (Bluhm and Gradinger 2008; Davis 2014). Because of this selectivity, animals may focus on specific areas of the water column and this can indicate where foraging is focused (Laidre et al. 2003; Hauser et al. 2015). Narwhal are specially adapted for deep diving (Laidre et al. 2003) and are known to forage heavily on Greenland halibut (Laidre and Heide-Jørgensen 2005; Watt et al. 2013), which are lipid-rich benthic prey (Lawson et al. 1998). Because deep diving is so energetically expensive, it is often assumed that targeted deep dives indicate foraging (Laidre et al. 2003; Robinson et al. 2012), and we assume narwhals would only make such dives for foraging or some other unknown reason, but which also must be of great importance. Therefore, we used dives close to the bottom (75%–100% of total bottom depth) as a proxy for regions important for narwhal foraging or other life-history traits. Home-range analysis of satellite tag data has been conducted for both narwhal populations, which has provided information on areas used by the animals (Heide-Jørgensen et al. 2002; Westdal et al. 2010). However, a study on where dives close to the bottom are occurring has never been conducted and may inform more specifically on foraging regions, and identify areas to target for conservation efforts.
In this study, we assessed the number of dives to different depth categories for narwhals from BB and NHB and identified seasonal regions where targeted dives close to the bottom are occurring as a proxy for foraging areas. This was an extension of a study done in 2015 which had identified that dives close to the benthos occurred in all seasons for both populations (Watt et al. 2015). In this study, we evaluated the number of dives narwhals made to 75%–100% of total bottom depth and found dives to this portion of the water column varied across seasons for the BB narwhals but not for the NHB whales. From this work, we wanted to determine where these dives close to the bottom were occurring in all seasons to assess important foraging areas, assuming that targeted dive depths can be an indication of foraging (Laidre et al. 2003; Robinson et al. 2012; Hauser et al. 2015). Given that we already had an understanding of where narwhals spend summer and winter and how they migrate between the regions (Richard 1991; Dietz et al. 2001, 2008; Heide-Jørgensen et al. 2003, 2013a; Watt et al. 2012), we had a good idea of where the whales would be located and assumed that they would forage on both the summer grounds (in northwestern Hudson Bay for NHB narwhals and Eclipse Sound for BB narwhals from the Eclipse Sound stock) and the winter grounds (just to the east of the entrance to Hudson Strait for NHB narwhals and in the Davis Strait for BB whales). However, this was somewhat exploratory in nature because it has been thought that narwhal forage mostly in the winter with little foraging on the summering grounds (Laidre and Heide-Jørgensen 2005), particularly by late summer (Finley and Gibb 1982). We believed narwhal would forage, at least to some extent, on the summering grounds because our previous study had found whales made a significant number of dives and spent significant time close to the bottom (75%–100% of total depth) in summer, suggesting some foraging on benthic prey (Watt et al. 2015). However, we also know that the summer area is used for calving (Koski and Davis 1994), whereas mating occurs on the winter range (Best and Fisher 1974) or during the migration (Heide-Jørgensen and Garde 2011). As a result, we hypothesized that (i) narwhals would use only a subset of their winter and summer ranges for foraging, rather than the entire home range, because a portion of these areas may be used exclusively for other activities.
Previous research has shown that winter is considered an important time for narwhals to gain energy reserves and it has been suggested that this is the most important season for foraging on benthic prey, such as Greenland halibut (Laidre et al. 2003; Laidre and Heide-Jørgensen 2005). As a result, we also hypothesized (ii) that the deepest dives would occur on the wintering grounds when narwhals are known to inhabit deep regions and feed on Greenland halibut (Laidre and Heide-Jørgensen 2005) and (iii) that the areas used for foraging in the winter would be larger than those used in the summer because feeding is thought to be more intensive in winter (Laidre et al. 2003; Laidre and Heide-Jørgensen 2005). During the migratory phase, narwhals typically migrate between summer and winter areas quite quickly (depending on ice break-up) and it was thought that migration was primarily characterized by horizontal movements with little vertical movements (Heide-Jørgensen et al. 2002); however, we did identify some dives close to the benthos at this time for both populations suggesting that a small amount of foraging may be occurring (Watt et al. 2015). As a result, our final hypothesis was that (iv) the migration corridor between summer and winter ranges would not be used as much for foraging, and we would expect only a small area of the entire range to be used as a foraging area. Determining how much of the seasonal home ranges are used for foraging and how much overlap may exist with ongoing or proposed industrial activities in the Arctic will help to better manage any potential interactions between narwhal and industry.
Materials and methods
Study areas
Narwhals were tagged near the communities of Pond Inlet and Repulse Bay, Nunavut, Canada. We only tagged one stock of narwhals from the BB population; whales from the Eclipse Sound stock were tagged near Pond Inlet in Tremblay Sound (72°21′N, 81°06′W) in 2010 and 2011 (Fig. 1; Table 1). From the NHB population, narwhals were tagged near the community of Repulse Bay; five narwhals were tagged in Lyon Inlet (66°30′N, 84°00′W) in August 2006 and four narwhals were tagged in Repulse Bay (66°31′N, 86°14′W) in August 2007 (Fig. 1; Table 1).
Table 1.
Note: Date format is “day/month/year”. M, male; F, female.
*
Indicates the final date for high-quality locations within each of the seasons.
†
Indicates tags that went into another summer season and date represents the first date for the following summer.
Capture and tagging
The whale capture and satellite tagging methods used in this study were previously developed and used successfully by Orr et al. (2001), Dietz et al. (2001, 2008), and Heide-Jørgensen et al. (2003). Narwhals were caught in nets set perpendicular to the shore. When a narwhal was detected in the net, zodiac boats would drive out and pull the narwhal(s) to the surface; a shore crew would then pull the net into shore and position the whale with its tail in the shallowest water. A looped rope with rubber coating was placed around the tail. For females, a hoop net was placed over the head; for males, the tusk was held by one or two people. Once stabilized, the satellite tag was attached with two or three 10 mm nylon pins through the fat and blubber under the dorsal ridge. All work was conducted under a DFO License to Fish and prior approval was obtained from the Freshwater Institute Animal Care Committee (NHB: FWI-ACC-2006-2007-009 and FWI-ACC-2007-2008-037; BB: FWI-ACC-2010-001 and FWI-ACC-2011-016). The entire tagging process followed approved protocols and was monitored by a veterinarian.
Narwhals were equipped with Wildlife Computers SPLASH tags and were programmed to transmit daily from 1 July to 31 September and subsequently on a 3-day duty cycle. Although daily transmissions provide more detailed movements of narwhals, the duty cycle allows for a longer deployment period due to battery-life considerations. Data on location of all deployed tags were obtained from the ARGOS system (CLS America). ARGOS data files were extracted using WC-DAP version 3.0 build 69 software (Wildlife Computers). Dive information was grouped into depth bins and represented the 6 h time span directly prior to the transmission. The depth reading to determine the start and end of a dive was set at 4 m. Due to the differences in the ocean bathymetry for the different populations, tags were programmed to collect data within different dive depth bins. For the BB population, the number of dives to different depths were binned into 6, 8, 10, 12, 15, 20, 100, 200, 400, 800, 1000, 1400, 1800, and >1800 m bins, whereas dives from the NHB population were binned in 6, 8, 10, 12, 15, 20, 25, 50, 100, 200, 300, 400, 500, and >500 m depth bins.
Data analysis
The number of dives to a particular depth bin was associated with a location that occurred within the 6 h binned time frame on that day. Location information from the tags was categorized based on the accuracy of the transmission, varying from poor to good: classes A and B and classes 0 to 3. Classes A and B provide no estimation of error; class 0 includes an error range of >1500 m; class 1 includes an error range of 500–1500 m; class 2 includes an error range of 250–500 m; class 3 includes an error range of <250 m (Lopez et al. 2014). Shape files of ocean bathymetry were obtained from IBCAO (international bathymetric chart of the Arctic Ocean), which provide 500 m2 grid files (Jakobsson et al. 2012) and were used to estimate a depth for each narwhal location through extraction using ArcGIS. Only class-2 and class-3 locations were used in analysis (61% of all locations), as they are both within the error range associated with the IBCAO depths. We only considered high-quality locations because we needed as accurate bottom bathymetry measures as possible and did not want to add any extra error into the estimates. Generally, home-range estimates do not improve after ∼50 observations (Seaman et al. 1999) and our study had >50 observations for each season for both populations; however, because of the use of only high-quality locations, our home-range estimates should be considered minimum estimates. After bathymetry data for each location was obtained, dives to a particular depth bin were converted into a percentage of total depth. For the deep-diving analysis, we only included dives that were 75%–100% of the total depth (essentially dives close to the bottom; see Watt et al. 2015). Because bottom bathymetries are not mapped well in this area, we used a conservative estimate for what constituted bottom diving to ensure that we included all benthic dives. When narwhals were making 25% or more of all their dives in the 6 h binned period to 75%–100% of total bottom depth, we considered them to be targeting this region (Hauser et al. 2015) and likely a time when they would be foraging on benthic prey. Bathymetric depths sometimes exceeded the programmed depth readings of the tags for the NHB population, where tags were only programmed to up to 500 m and then all dives >500 m fell into one bin together. Dives occurred in the >500 m bin, but this bin was always included in the 75%–100% depth category because narwhals from this population were never in waters deeper than 659 m according to the IBCAO depths.
A repeated-measures ANOVA with dive depth and season as fixed effects and whale nested within depth as a random variable was used to evaluate absolute dive depths for each population (Underwood 1997). When a significant interaction between season and depth was found, a model with dive depth as a fixed effect and whale nested within depth as a random variable was conducted for each season; Tukey’s honest significant difference (HSD) tests were used to determine where significant differences occurred (Underwood 1997). Normality was assessed using normal probability plots. The Mauchly criterion (Mauchly 1940) was used to assess sphericity; if sphericity was violated, then the adjusted Greenhouse–Geisser degrees of freedom were used (Greenhouse and Geisser 1959).
All data were imported into ArcGIS version 10.2 and all good-quality locations (classes 2 and 3) where dives were recorded, as well as only a subset of the locations where the deepest dives occurred, were then analyzed using the Spatial Analyst toolbox add-in for estimating the home range of an animal using a kernel density estimate in an environment with barriers to movement (MacLeod 2013). This allowed us to calculate a home-range estimate for narwhals in the different seasons based on all locations and then solely on locations where deep diving occurred; thus, representing an estimate of foraging range. Percent volume contours (PVC), which represent the boundary of the area that contains a certain percentage (both 50% and 95% were calculated) of the volume of a probability density distribution, were then calculated based on the home-range estimates. The 50% PVC, which contains 50% of the location records for the population, is often taken to represent the core range of the critical foraging of the population, whereas the 95% PVC is representative of the entire range, excluding only extreme outliers (MacLeod 2013). Both 50% and 95% PVC were calculated for the wintering and summering regions, as well as for the migration pathway between the two regions.
Summering and wintering grounds were defined spatially based on previous studies. Summer range for NHB narwhals encompassed Repulse Bay, Frozen Strait, and north to Lyon Inlet, whereas the wintering region was just outside of Hudson Strait (Richard 1991; Westdal et al. 2010). When whales moved out of these defined areas, they were considered to have left the summering or wintering regions and were on the migratory route. The definition of these areas resulted in dates of the seasons varying between whales (Table 1). Migration encompassed both the fall and spring movements since narwhals have to follow the same route through Hudson Strait for both periods. However, as a result of the tag programming and the duration of the tags, the migration season for NHB narwhals was dominated by the fall migration (n = 83), with few locations from the spring migration (n = 9).
The summering range for narwhals from Eclipse Sound had been defined previously as the Eclipse Sound region (DFO 2012). However, whether Admiralty Inlet and Eclipse Sound are separate stocks or one stock has not been resolved (Watt et al. 2012). Because satellite-tagged narwhals from Eclipse Sound travelled into Admiralty Inlet in August, we included both regions in the summer home range. The winter range was defined based on Heide-Jørgensen et al. (2002). This resulted in seasonal dates varying for individual whales (Table 1). Spring and fall migrations were also combined for BB narwhals because the migratory routes are generally the same for both seasons following ice formation and recession (Dietz et al. 2008; Heide-Jørgensen et al. 2003); however, when all dives were considered, there was greater representation from the fall season (n = 279) compared with the spring season (n = 167).
Results
NHB population
Sample sizes varied across the seasons for each population due to the programmed duty cycle for the tags, the dates that were chosen to represent the seasons (based on narwhal movements), and the quality of the locations (because only locations of class 2 or class 3 were used in analysis). For the NHB population, we had 240, 245, and 92 high-quality locations with dive behaviour for the summer, winter, and migration seasons, respectively. Of these dives, 128 (53%), 163 (67%), and 54 (59%) during the summer, winter, and migration, respectively, represented locations where narwhals were making at least 25% of their dives to 75%–100% of total bottom depth.
For the NHB population, we found a significant interaction between depth bin and season (F[9,2284] = 52.24, P < 0.0001). Simple-effects models found a significant effect of depth bin in the summer (F[4,1008] = 32.88, P < 0.0001), winter (F[4,922] = 211.03, P < 0.0001), and migration (F[3,286] = 34.82, P < 0.0001) seasons. Narwhals in NHB made most dives to the 0–6 and 300–400 m depth bins and the least number of dives to the >500 m depth bin during the summer and the migration seasons (P < 0.05; Fig. 2). In winter, NHB narwhals made most dives to depths >500 m and the 0–6 m depths, and the fewest dives to the 300–500 m depths (P < 0.05; Fig. 2).
Fig. 2.
Kernel home-range analysis for all locations where narwhals were diving in NHB revealed a 50% PVC of 615, 946, and 143 km2 for the winter, summer, and migration seasons, respectively (Figs. 3a, 3c, and 3e). The 95% PVC represented a total area of 1852, 3954, and 1394 km2 for the winter, summer, and migration seasons, respectively (Figs. 3a, 3c, and 3e). Narwhals from NHB made deep dives in their summering and wintering regions and on the migration routes between the two areas (Figs. 3b, 3d, and 3f). In total, the area encompassed by the 50% PVC for deep diving was 630, 469, and 228 km2 in the summer, winter, and during the migratory phase, respectively, for narwhals from NHB (Figs. 3b, 3d, and 3f). The 95% PVC for deep diving in the winter, summer, and the migratory phase were 2499, 1896, and 718 km2, respectively (Figs. 3b, 3d, and 3f).
Fig. 3.
BB population
For the BB population, we used 2101, 993, and 446 high-quality locations for the summer, winter, and migration seasons, respectively. Of these dives, 837 (40%), 273 (27%), and 145 (33%) represented locations where narwhals were making at least 25% of their dives to 75%–100% of total bottom depth. For the BB population, absolute dive data was transformed to improve normality using a log(x + 1) transformation, and adjusted Greenhouse–Geisser degrees of freedom were used to meet the assumption of sphericity. An analysis of the absolute dive depths for narwhals from the BB population found a significant interaction between dive depth and season (F[3,5901] = 66.24, P < 0.0001). Simple-effects models found a significant effect of depth bin in the summer (F[1,17] = 123.86, P < 0.0001), winter (F[2,19] = 46.80, P < 0.0001), and migration (F[2,18] = 39.23, P < 0.0001) seasons. Narwhals in BB made most dives to the 0–6 m depth bin followed by the mid-depths in all seasons and the least number of dives to the deepest depths (≥1000 m) (P < 0.05; Fig. 4).
Fig. 4.
Kernel home-range analysis for all locations where narwhals were diving in BB revealed a 50% PVC of 1955, 1608, and 898 km2 for the winter, summer, and migration seasons, respectively (Figs. 5a, 5c, and 5e). The 95% PVC represented a total area of 14 244, 6 995, and 6 123 km2 for the winter, summer, and migration seasons, respectively (Figs. 5a, 5c, and 5e). Narwhals from the Eclipse Sound stock also made deep dives on the summering and wintering grounds and during the migration. The 50% PVC for deep dives were 628, 1051, and 227 km2 for the winter, summer, and the migration seasons, respectively (Figs. 5b, 5d, and 5f), whereas the 95% PVC were 3475, 5203, and 1691 km2 for winter, summer, and migration seasons, respectively (Figs. 5b, 5d, and 5f).
Fig. 5.
Discussion
We were able to identify important foraging regions in all seasons (summer, winter, and the migration period) for both populations using deep diving as a proxy for foraging. Previous research has shown that narwhals gain a significant portion of their prey from benthic sources (Laidre and Heide-Jørgensen 2005; Watt et al. 2013, 2015) and the present study demonstrates that narwhal carry out deep dives during all seasons, suggesting some year-round foraging. This is not to say that shallower dives within the pelagic zone could not indicate foraging; however, we are unable to conduct such an analysis because we cannot determine where in the water column a narwhal is foraging with the available data. Advances in tag technology have permitted the use of stomach temperature pills in narwhals (Heide-Jørgensen et al. 2014), which can pinpoint when a foraging event occurs; however, these pills are short lived (maximum 2 weeks and most <48 h) and are logistically difficult to deploy in narwhals. As a result, we used dives close to the bottom to define a minimum area used for foraging that may be important for narwhals. This is the first study to evaluate foraging areas for each population and this is discussed below with respect to shipping traffic (Pizzolato et al. 2014), seismic exploration (Reeves et al. 2014), and expanding fisheries (Jørgensen and Arboe 2013) for each population.
NHB population
Narwhals from NHB used in this study have been analyzed previously for home-range analysis in the month of August. This resulted in a summer home range of 7900 km2 in 2006 and 4600 km2 in 2007 (Westdal et al. 2010). Our results, which represent a smaller subset of total movements but a much larger date span, estimated a home range of ∼4000 km2 for both years combined (sample sizes were too small to investigate individual years). The discrepancy between the two studies is in part due to the fact that Westdal et al. (2010) used all locations, even those with lower quality, and only defined summer up until 31 August. Our estimate of foraging range for this population represents a new analysis where deep diving is occurring and indicates narwhals are using ∼48% of their total summer home range for foraging and making most dives to the 200–400 m range (400 m is the deepest bottom depth in their summer range). Generally, shipping and oil and gas operations are very minimal in the summer range of NHB narwhals (AMSA 2009; Reeves et al. 2014), and although there is potential for seismic exploitation, the large hydrocarbon reserves are primarily south of the critical foraging habitat (Reeves et al. 2014); thus, we would expect little interaction between industry and narwhals at this time.
No study has previously evaluated narwhal foraging during migration; most focus on summer or winter diet (Finley and Gibb 1982; Laidre and Heide-Jørgensen 2005). However, whales in this population made many deep dives (>300 m) on the migration route and we identified a small area in Hudson Strait used for foraging at this time (∼700 km2). When compared with the PVC for all dives, narwhals use ∼52% of the total migration range for foraging, and make more deep dives (>500 m) on the migration route compared with the summer season, which suggests that migration may be an important foraging time for NHB narwhals. At this time of year, narwhals may be interacting with shipping vessels because all shipping in and out of Hudson Bay must travel through Hudson Strait (AMSA 2009) and there is an operational well in the Strait (Reeves et al. 2014). This interaction may present a threat of ship strikes or noise pollution that could impact narwhal communication; however, currently there is no information on how this increase in shipping may impact narwhals.
Our study found ∼1850 km2 made up the winter home range for the tagged narwhals from NHB; however, a greater area was actually used for deep diving (∼2500 km2). Despite the fact that fewer locations were used in the analysis, we ended up with a smaller overall range when all dives were included. This suggests that deep diving may have been excluded as the 5% outliers when all dives were considered but were included when only a smaller subset was used. It could also have been impacted by the fact that for proper comparison, the same bandwidth was used for both analyses. Regardless, this suggests that deep diving is occurring throughout the entire winter range and that this season is likely very important for foraging. Narwhals also made most of their deep dives (>500 m) in the winter season, confirming the importance of foraging at this time. The location of this foraging range overlaps a demersal fish stock assemblage and current Greenland halibut fishing region (DFO 2006); thus, increases in fishing pressure (Jørgensen and Arboe 2013) could result in competition with narwhal for prey, which is part of the reason that they are listed as special concern by COSEWIC (COSEWIC 2004).
BB population
Previous home-range analysis has been conducted on six narwhals tagged in Eclipse Sound in 1999. They found the 95% summer home range was 3417 km2 and included location information for August (Heide-Jørgensen et al. 2002). The summer home-range size for narwhals from BB in our study of whales tagged in 2010–2011 was almost double that found by Heide-Jørgensen et al. (2002). This may be due in part to the greater date range and the larger sample of narwhals investigated in our study (12 compared with 6). Dates were based on narwhal movements and we found that most narwhals had not left the summering grounds until after 31 August, which may reflect a change in the timing of migration for Eclipse Sound narwhals. However, the biggest difference between the narwhals tagged in our study compared with those in 1999 is that narwhals travelled out of the Eclipse Sound region and into Admiralty Inlet. This movement between what have been defined as independent summering regions (DFO 2008) had not been documented previously and may indicate a greater overall summer home range for the Eclipse Sound population. Why they travelled into Admiralty Inlet is unknown, but prey availability or presence of killer whales (Orcinus orca (L., 1758)) in the area in 2010 (G. Freund, personal communication) may have contributed to their movements. It has been noted previously that killer whales elicit behavioural changes in narwhals. Narwhals used twice as much area when killer whales were present and shifted their distribution away from the attack site during an encounter in Admiralty Inlet in 2005 (Laidre et al. 2006). It is possible that narwhals may have moved out of Tremblay Sound and into Admiralty Inlet because of the presence of killer whales. However, deep diving also occurred in Admiralty Inlet, suggesting that foraging may have also been occurring because narwhals are not known to dive to evade predators, rather they tend to make quiet movements close to the shoreline at the surface (Laidre et al. 2006).
Overall, narwhals from Eclipse Sound used almost 75% of their total summer home range for deep diving, suggesting there is some foraging occurring at this time, which would be counter to previous studies that have found many empty stomachs in summer (Finley and Gibb 1982). Dietz et al. (2007) tagged narwhals with Crittercams and DTAGS in Admiralty Inlet in 2003 and 2004 and found narwhals spent only 12% of their time at the bottom, where they spent 80% of their time upside down. This upside-down behaviour is not unusual for narwhals, as many observers have seen narwhals roll upside down in the water, and most whales in this study rolled upside down after tag attachment and upon release. However, why narwhals roll is unknown. Dietz et al. (2007) suggested that the behaviour may assist with foraging by orientating the tusk toward the benthos and stirring up prey that may be there, but no foraging was seen on the Crittercam footage (Dietz et al. 2007). These tags were relatively short lasting (maximum ∼15 h) and the maximum depths obtained were between 100 and 200 m (Dietz et al. 2007). Shortly after narwhals were tagged, they may not be inclined to forage. Whales tagged in our study made the majority of their dives at 400–800 m depths, and thus may have been foraging.
Narwhal may also dive for reasons unrelated to foraging. Evading predators has been discussed and is unlikely based on behavioural observations of narwhals in the presence of killer whales (Laidre et al. 2006). Molting is another possibility. Narwhals may be diving to the bottom to rub off skin; much like what has been seen in beluga whales (Delphinapterus leucas (Pallas, 1776)) (St. Aubin et al. 1990). Another possible explanation is that narwhals are diving to avoid hunters at the surface. It has been noted that narwhals are very sensitive to sound and have dove right before rifle shots (Gonzalez 2001), possibly because they can hear the rifle being loaded or sense movement on the water or footsteps on the ice. Although this is possible, in 2010 and 2011 in Tremblay Sound, hunting had ceased prior to tagging, so it is unlikely that this was the cause of such deep dives. Regardless of whether narwhals are spending time diving to the depths because of foraging or some other reason, these regions should still be considered important areas for narwhal because deep dives are energetically expensive, and no matter what is eliciting them, it must be of importance.
In BB, ∼28% of the migration range was used for deep diving. Most deep dives occurred along the shelf break and occasionally in fiords between Pond Inlet and Clyde River (Fig. 5d). In 2014, the National Energy Board approved a proposal to conduct a 5-year seismic testing survey off the coast of Baffin Island (Speers-Roesch 2014). Community members from Clyde River have since launched a legal fight to have the decision reversed, as they fear seismic testing may deter marine mammals from the area. Previous research suggest seismic testing may be responsible for entrapment events of narwhals (Heide-Jørgensen et al. 2013b) and subsequent drilling can be clearly heard underwater at distances up to 38 km (Kyhn et al. 2014). Both the summer and migratory foraging grounds of narwhals experience intermediate levels of shipping traffic (AMSA 2009; Reeves et al. 2014) and oil and gas activity (Reeves et al. 2014), but it is unknown how these activities may impact narwhal residence time or migration into and out of the summering regions.
Previous 95% winter kernel home-range estimates for one narwhal tagged in Eclipse Sound in 1999, which included dates from 10 November to 17 March, was 12 360 km2, whereas another whale’s winter home range was 13 457 km2 (Heide-Jørgensen et al. 2002). These values were comparable with what was found for the winter home range for BB narwhals when all dives were investigated (∼14 000 km2). Narwhals in BB used a much smaller proportion of their home range specifically for foraging activities. For instance, in the winter, of the ∼14 000 km2 home range, only ∼3 500 km2 was used for deep diving (∼24%) and a greater total area was actually used for deep diving in the summer (∼5 200 km2). This may suggest narwhals are foraging more in the summer, or are foraging in a more condensed region in winter, which makes sense given the limited open water available during the winter and the need for narwhals to recover at the surface after deep dives (Kooyman and Ponganis 1998). This small area may be very important for foraging and is also known as an important region for Greenland halibut (DFO 2006). There has been an increase in the landed catches of Greenland halibut in recent years (Jørgensen and Arboe 2013). Competition between fisheries and narwhals in this region has been a concern in the past and resulted in closing an area that overlaps the winter foraging range of narwhals (DFO 2007, 2014). However, we found that much of the narwhals diving is spread out across the winter range, suggesting that there may still be some competition between the Greenland halibut fishery and narwhals outside of the closed fishing zone (DFO 2014). How or if this overlap in fishing and narwhal foraging will impact narwhals is unknown, but it may have little effect because the biomass and abundance of Greenland halibut in the region is stable or increasing (Treble 2016).
Although the diet of narwhals has been considered quite specialized in the past (Laidre et al. 2008), recent research results suggest that the diet has shifted both inter- and intra-annually, indicative of some plasticity to altering food webs (Watt et al. 2013; Watt and Ferguson 2015). Adaptability may be particularly important because narwhals face increased shipping traffic, seismic testing, drilling operations, and enhanced fisheries within their foraging range. Of particular importance for narwhal populations are industrial activities on the migration route. We have shown that in the migration season, narwhals are not just making horizontal movements at the surface but are diving 75%–100% of the total water depth available to them. We suggest that this deep-diving behaviour may indicate foraging, and thus interactions between narwhal foraging and industry may occur at this time. Further research is needed to determine what affect this interaction could have on narwhal foraging and movements. By determining important areas for narwhal foraging, we are better able to predict when and where industry may interact with narwhals and build partnerships with industry to better mitigate any potential impacts of increasing anthropogenic disturbances in the Arctic.
Acknowledgements
We thank the many dedicated people in the research field camps for their assistance with capturing and handling the narwhals, particularly S. Black, DVM, who offered veterinarian assistance. We also thank P. Richard and K. Westdal for assistance with tagging/data retrieval for NHB narwhals. Thanks go to the Hunters and Trappers Organizations in Repulse Bay and Pond Inlet, Nunavut, Canada, for all of their support and the Polar Continental Shelf Program for logistic support. We thank two external reviewers for comments on the paper, which improved the final version. Fisheries and Oceans Canada, the Nunavut Wildlife Management Board, Nunavut Implementation Fund, World Wildlife Fund Canada, ArcticNet, and the US National Scientific Fund all provided research funding. The Natural Sciences and Engineering Research Council of Canada (NSERC), the Northern Scientific Training Program, the E. Scherer Memorial scholarship, and the Garfield Weston Foundation provided personal funding to C.A.W.
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Canadian Journal of Zoology
Volume 95 • Number 1 • January 2017
Pages: 41 - 50
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Received: 19 July 2016
Accepted: 27 September 2016
Accepted manuscript online: 3 November 2016
Version of record online: 3 November 2016
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C.A. Watt, J.R. Orr, and S.H. Ferguson. 2017. Spatial distribution of narwhal (Monodon monoceros) diving for Canadian populations helps identify important seasonal foraging areas. Canadian Journal of Zoology.
95(1): 41-50. https://doi.org/10.1139/cjz-2016-0178
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