A “stock” of animals is a term often used in wildlife management. The stock is the basic unit of management, mainly because it describes a group of animals that is reproductively and often physically isolated from other groups. Animals in a stock generally share similar life history characteristics, such as age of sexual maturity, rate of breeding etc. Most importantly, animals within a stock mate and breed primarily with other members of that stock.
It is important to try to determine whether narwhal are all members of one stock, or if there are different stocks, for a number of reasons. Narwhal are hunted throughout Nunavut in Canada, and in Greenland. In order to set appropriate harvest levels, managers need to know if narwhals are being taken from one or more stocks, and which stocks those are. It is also important to know if there are different stocks in order to make good population estimates.
Stock identity is determined in various ways: through analyzing the genetic makeup of individuals, tagging studies, studies of stable isotope ratios, use of traditional ecological knowledge, and studies of morphology.
In general, narwhal appear to have low genetic diversity and there is little differentiation among stocks (NAMMCO 2013). There are two possible reasons for this: either a “bottleneck”, or population restriction, occurred in the population at some ancient date, or there is currently a high rate of gene flow between areas. Either or both explanations could be correct.
Despite this overall low genetic diversity, genetic studies have found that narwhals in East Greenland are highly distinct from other groups (NAMMCO 2013). Within west Greenland, far northern (Uumanaaq) narwhals are different than those found further south. Differences are also seen between Baffin Bay, Northern Hudson Bay and East Greenland populations (Petersen et al. 2011, NAMMCO 2013), and less distinct differences between narwhal that summer in Jones Sound and the Somerset Island area (Petersen et al. 2011).
Similar results were found using stable isotope analysis of carbon and nitrogen from skin samples. A clear difference was seen between northern Hudson Bay, East Greenland, and all other stocks (Watt et al. 2012b).
East Greenland narwhals are also distinct in their cranial morphology (Wiig et al. 2012a), however, no differences were seen between West Greenland and Canadian narwhals.
Another method which has been used to try to identify stocks is analysis of organochlorine (OC) contaminant levels in narwhal tissues. Differences in contaminant levels can be caused by differences in feeding, which may depend on prey selection, feeding patterns in summering/wintering areas or on migration routes, and on the feeding behaviour of the individual. In a 2003 study, narwhals sampled from Repulse Bay could be distinguished from all other narwhals studied. Apart from that group, it was not possible to clearly distinguish other stocks using this method (de Marche and Stern 2003).
Tagging studies have been done in a number of locations throughout the narwhal's range. These studies suggest interannual site fidelity, which means that narwhals generally return to the same summering and wintering areas year after year. There are exceptions, however: one narwhal tagged in Admiralty Inlet wintered in Northern Foxe basin rather than Davis Strait (Watt et al. 2012a, NAMMCO 2013), and one whale tagged in northern Greenland migrated to Somerset Island in the summer (Heide-Jørgensen et al. 2013).
Most narwhal stocks from northern Canada and West Greenland mix in the winter in Baffin Bay/ Davis Strait (Heide-Jørgensen et al. 2013). Mating, however, occurs during the spring migrations to the summering areas, which maintains summer stock identity (NAMMCO 2013).
Tracklines of narwhals tagged at various summering locations (stars) to their wintering area in Baffin Bay and Davis Strait. From Heide-Jørgensen et al. (2012).
The "summering aggregations" (areas where narwhals are found in the summer) currently provide the basis for the determination of management stocks, mainly because 1) narwhal stocks tend to be geographically isolated during the summer and 2) because it is during the summer and at these locations that narwhals are usually hunted. Overall, the current thinking is that there are five stocks in Canada, two in West Greenland and one in East Greenland (NAMMCO 2013). There is not enough information about the narwhals found in Jones Sound, Smith Sound and in areas further north and west in Nunavut to determine if these form one or more separate stocks. Inuit Qaujimajatuqangit, or Inuit traditional knowledge suggests that narwhal in Parry Channel, Smith Sound and Jones Sound exhibit physical and behavioural differences from other stocks (White 2012).
Summer aggregations of narwhals, and where they are harvested. From Heide-Jørgensen et al. (2013)
There is very little information on the narwhals that inhabit this area. Generally narwhals were historically sighted and sometimes caught by Norwegian sealers and whalers in the area between East Greenland, Svalbard, the New Siberian Islands and Novaya Zemlya (Gjertz 1991, Lydersen et al. 2007). Narwhals are most frequently observed in the northwest and northeast parts of the Svalbard archipelago, but this probably reflects the numbers of observers rather than the number of animals. Generally narwhals in this area appear to be widely dispersed and do not form large predictable aggregations as they do in other areas. The wintering area of Northeast Atlantic narwhals is not known, but it is likely that they overwinter within the extensive pack ice in this area. Farther east, sightings of narwhal are very rare or non-existent.
Approximate summer distribution of narwhals around Svalbard, Novaya Zemlya and Franz Josef Land
During the summer narwhal are found all along the East Greenland coast north of 66° N, and concentrations are found in Scoresby Sound (Ittoqqortormiit) and Tasiilaq/Kangerlussuaq (Heide-Jørgensen et al. 2010). Narwhal have a scattered distribution along the entire coast and periodically occupy many fjords in the area. Available information from satellite tracking of narwhal from Scoresby Sound suggest that these narwhals do not mix with those from other areas during the summer, and that they should therefore be considered a separate stock (NAMMCO 2013). Narwhal tagged in Scoresby Sound wintered off the East Greenland coast south of 69° N, at water depths of 500 to 1500 m (NAMMCO 2013). Given their geographical separation from other stocks, as well as genetic and other differences, East Greenland is therefore considered to have at least two summer stocks of narwhal, with their summer distributions centred at Scoresby Sound (Ittoqqortormiit)and Tasilaq/Kangerlussuaq.
Narwhal are found off West Greenland at all times of the year. During the summer there are concentrations at Inglefield Bredning and farther south near the glacier fronts of Melville Bay. During the fall, winter and spring they are found in the ice pack of the West Greenland coast from Smith Sound to just south of Disko Bay (Heide-Jørgensen et al. 2010). While the summering aggregations are considered to be stocks for management purposes, at other times of the year narwhals off West Greenland are likely composed of a mixture of narwhals from several summering areas (Heide-Jørgensen et al. 2012).
Summer stocks in Canada and West Greenland.
There are two main groups of narwhals in Canada that are nearly completely isolated from one another at all times of the year.
The North Hudson Bay narwhal summer in northwestern Hudson Bay, particularly near Repulse Bay and into Foxe Basin. Tagging studies indicate that this group winters in the pack ice at the eastern end of Hudson Strait (DFO 2010a, Heide-Jørgensen et al. 2012). The Northern Hudson Bay stock can be separated from other Canadian stocks through genetics, contaminant analysis and by the fact that it’s distribution does not overlap with that of any other group of narwhal.
All other Canadian stocks winter in southern Baffin Bay/northern Davis Strait, and have summer concentrations throughout the eastern Canadian Arctic Archipelago. Five main summering stocks have been identified, mainly because these are summer aggregation areas: Jones/Smith Sounds, Eclipse Sound, Admiralty Inlet, Somerset Island, and East Baffin Island. Tagging studies have indicated that there is little mixing between areas during the summer, although it does sometimes occur. The distribution of all these stocks overlaps in the winter, and not surprisingly they cannot be readily differentiated genetically or by other means. Nevertheless these divisions are useful for management purposes as it allows a stock allocation to hunting areas, which is a more conservative approach than assuming that there is only one “panmictic” stock of narwhals in the area, as the latter approach could result in local depletions if an underlying stock structure existed.