SC/60/BRG28

Mitochondrial DNA diversity in the Baffin Bay stock of bowhead whales (Balaena mysticetus)

L. Bachmann, C. Lindqvist, Ø. Wiig, Natural History Museum, University of Oslo, P.O. Box 1172 Blindern, N-0318 Oslo, Norway

L. Postma, L. Dueck, M. Lindsay, Fisheries and Oceans Canada, Central and Arctic Region, 501 University Crescent, Winnipeg, Manitoba, R3T 2N6, Canada

M.P. Heide-Jørgensen, K. L. Laidre, Greenland Institute of Natural Resources, Box 570, DK-3900 Nuuk, Greenland

Abstract

We determined the genetic structure of bowhead whales from Disko Bay, West Greenland and the Hudson Bay-Foxe Basin, East Canada through sequencing of a 453 bp stretch of the control region of the mitochondrial DNA. There was a significant genetic differentiation of the Baffin Bay stock from the Holocene Spitsbergen and the extant Bering-Chukchi-Beaufort (BCB) stocks. The Baffin Bay stock is characterized by a lower number of haplotypes that occur on average at higher frequencies. We found no clear evidence for the assumption that the recent increase in population size of the Baffin bay stock can be attributed to immigrants from the BCB stock.

Introduction

Five geographical stocks of bowhead whales (Balaena mysticetus) are currently recognized in (i) the Bering-Chukchi-Beaufort (BCB) Seas, (ii) the Okhotsk Sea, (iii) the Davis Strait-Baffin Bay, (iv) the Hudson Bay-Foxe Basin, and (v) in the areas around Svalbard (Spitsbergen). Recent genetic studies provided evidence that challenge the validity of these stocks (e.g. Rooney et al. 2001, Borge et al. 2007). In addition, recent information on movements and genetics suggest that the Davis Strait-Baffin Bay and the Hudson Bay-Foxe Basin stocks are just one population (Heide-Jørgensen et al. 2006, SC/60/BRG20), although locally structured with respect to age classes and sex. Accordingly, it has been suggested that this stock should be named the Baffin Bay stock that is also for simplicity used in this study,

For many years the abundance of bowhead whales in the Davis Strait-Baffin Bay and the Hudson Bay-Foxe Basin was considered very low, and no surveys were dedicated to assess the abundance of this widely scattered population. Extensive aerial surveys for assessing the distribution of marine mammals in the eastern Canadian Arctic in the 1980s provided evidence that the numbers of bowhead whales were low. It was estimated at 1500 individuals in 1981 based on a survey of the wintering ground in Hudson Strait (Davis and Koski 1980, Koski et al. 2006). A large scale survey of the Canadian eastern Arctic in 2002-2003 indicated that the population size had increased significantly (COSEWIC 2005, Heide-Jørgensen et al. 2007). For West Greenland bowhead whales an index series extending back to 1981 shows a sudden increase in relative abundance after 2000 (Figure 1, Heide-Jørgensen et al. 2007). The two Canadian surveys (1981 and 2002-03) also indicated a dramatic population growth (Heide-Jørgensen et al. 2007, Cosens et al. 2006). Even though past surveys are not entirely compatible with the more recent efforts, they covered sufficient large areas to conclude that the density of bowhead whales in West Greenland and the Canadian eastern Arctic was low throughout the 1980s and 1990s. If previous and recent population size estimates are considered correct, the detected population growth exceeds what can be explained by natural reproduction alone. The pertinent question is therefore if part of the increase can be explained by immigration of whales from neighbouring stocks. Given that the presence of the Spitsbergen stock is hardly detectable (Wiig et al. 2007) the only candidate population that could have provided substantial numbers of whales is the BCB stock.

Since 2000 skin biopsies have been obtained from a considerable number of adult bowhead whales at the feeding ground in Disko Bay, West Greenland (SC/60/19). The sampling indicates that it is primarily adult females that migrate to this particular feeding ground. The lack of recaptures between sampling years documents limited individual fidelity to this aggregation site although the predictable occurrence in Disko Bay demonstrates a clear site fidelity for the stock. The limited number of recaptures within sampling years indicates that the individulas monitored in Disko Bay provide a good representation of mitochondrial DNA diversity of the stock. The overrepresentation of females at this aggregation site is no problem for assessing the diversity of this maternally inherited marker.

The mitochondrial DNA diversity has been previously determined for the contemporary BCB and the historic Spitsbergen using parts of the control region as marker (Rooney et al. 2001, Borge et al. 2007). Here we report the mitochondrial DNA diversity of West Greenland and eastern Canadian Arctic bowhead whales and discuss the possible relationships to the BCB and the Holocene Spitsbergen stocks.

Material and Methods

Skin biopsies from some 250 adult bowhead whales have been taken at the aggregation site of Disko Bay, West Greenland, during the spring seasons 2000 – 2007 (SC/60/BRG19). Extraction of total genomic DNA was following the protocol of the EZNA Tissue DNA kit (Omega Bio-tek).

A 453 bp stretch of the mitochondrial control region, corresponding to position 15 473–15 925 in the complete mitochondrial genome of the bowhead whale (Arnason et al. 1993, GenBank Accession no. AP006472) was amplified as described by Borge et al. (2007). The obtained PCR products were purified using 10x diluted exoSAP-IT (USB Corporation) and subsequently sequenced according to the instructions of the BigDye 1.1 sequencing kit (Applied Bioscience).

The obtained nucleotide sequences were aligned and edited with the software sequencher 4.1 (GeneCodes) and subsequently analyzed. Estimates of genetic diversity, molecular variance, and population differentiation were obtained using the computer programs DNASP 4.10.9 (Rozas et al. 2003) and Arlequin Ver 2.000 (Schneider et al. 2000).

Results

After cleaning the dataset for recaptures within and between sampling years 209 bowhead whale sequences of a 453 bp stretch of the mitochondrial control region were aligned and analyzed for the samples obtained at the aggregation site of Disko Bay, West Greenland. Further details on the age structure and sex distribution of the sampled individuals can be obtained from SC/60/BRG19.

A total of 31 different haplotypes were detected, out of which 7 (22.6%) were only detected in one individual. Further details on haplotype and nucleotide diversity within the West Greenland sample of bowhead whales are summarized in Table 1. In comparison to the previously published datasets on the Holocene Spitsbergen stock (Borge et al. 2007) and the extant Bering-Chukchi-Beaufort stock (Rooney et al. 2001) 11 haplotypes (35.5%) detected in the West Greenland sample are specific.

We further included a sample of 279 control region samples of the Hudson Bay-Foxe Basin stock that have been collected during 2004. The sample comprises of 38 haplotypes, out of which 7 (18.4%) were only detected in one individual. When comparing the West Greenland and the Hudson Bay-Foxe Basin samples 8 haplotypes are specific for West Greenland sample and 13 are specific for Hudson Bay-Foxe Basin. When combining both samples according to the concept of the suggested Baffin Bay stock, the resulting dataset includes 488 individuals and 45 haplotypes, out of which 10 (22.2%) were only detected in one individual. Twenty-two haplotypes are Baffin Bay stock specific and have not been detected in the Holocene Spitsbergen stock (Borge et al. 2007) and the extant BCB stock (Rooney et al. 2001). The Baffin Bay stock specific haplotypes account for 28.1% of the dataset (137 individuals).

As has been described earlier for the Holocene Spitsbergen stock (Borge et al. 2007) and the extant BCB stock (Rooney et al. 2001), haplotype BWS1/F is the most common one with frequencies of 0.201 (West Greenland), 0.244 (East Canada), and 0.225 (combined Baffin Bay) as compared to 0.242 (Holocene Spitsbergen) and 0.081 (BCB). With respect to the high number of mitochondrial haplotypes in bowhead whales, there are some further halotypes that deserve particular attention, since they occur with relatively high frequencies (> 0.03) in the datasets included here. These are listed in Table 2 along with the respective frequencies.

The genetic differentiation between the Davis Strait-Baffin Bay, Hudson Bay-Foxe Basin, the combined Baffin Bay stock, the Holocene Spitsbergen (Borge et al. 2007), and the extant Bering-Chukchi-Beaufort (Rooney et al. 2001) stocks were estimated in terms of FST values (Table 2). The Holocene Spitsbergen and the BCB stocks are significantly differentiated from both the Davis Strait-Baffin Bay and the Hudson Bay-Foxe Basin samples. However, when the the Davis Strait-Baffin Bay and Hudson Bay-Foxe Basin datasets are pooled there is no longer a significant differentiation from the Spitsbergen stock. In disagreement with the recently suggested concept of a Baffin Bay stock (Heide-Jørgensen et al. 2006), the calculated FST values indicate also a significant genetic differentiation between the the Davis Strait-Baffin Bay and Hudson Bay-Foxe Basin samples.

It can be noticed that the West Greenland and the East Canada datasets as well as the combined Baffin Bay sample have a substantially lower proportion of stock specific haplotypes than the Holocene Spitsbergen and the BCB stocks, respectively. In addition, the average haplotype frequencies are also higher, which also causes lower gene diversity estimates (Figure 2, Table 1). It is noteworthy that the nucleotide diversity estimates are very similar for all datasets included.

Discussion

The Spitsbergen stock of bowhead whales is separated from the West Greenland/East Canada populations by the landmass of Greenland, although the occurrence of ancient skulls from bowhead whales on the shore of North Greenland and recent sightings in the Robeson Channel (Davis and Koski 1980) suggest that movements north of Greenland was possible at times.

The situation is different with respect to the geographic separation of the West Greenland/East Canada populations from the BCB stock. The complex archipelago of the Canadian high Arctic does not constitute a natural barrier to movements between the Beaufort Sea and the Prince Regent Inlet/Gulf of Boothia that is the nucleus of the summer distribution of bowhead whales from the Baffin Bay stock. Migration along Northern Canada is expected to be first of all restricted in times when the M’Clintock Channel is blocked by the M’Clintock Channel sea-ice plug, but may be possible otherwise. Furthermore, satellite tracking has shown that bowhead whales are capable of moving long distances through apparently impenetrable dense pack ice cover by using leads and cracks that are invisible to low resolution remote sensing of sea ice. The prevailing sea ice conditions during the past five years in this area leaves little doubt that bowhead whales can easily travel between the Beaufort Sea and the areas used by the Baffin Bay stock (Reference). However, there is currently no direct evidence from tracking or re-identification of individual whales that such movements actually do occur.

At least in theory the BCB stock of bowhead whale may have been a source population for immigrants to the Baffin Bay stock in order to explain the rapid increase in population size during recent years. Such a scenario is certainly not applicable for the Spitsbergen stock that is currently too small in number to contribute significantly to any neighbouring stock. In the absence of any direct evidence genetic data may allow to evaluate potential migration between bowhead whale stocks.

In the current study we detected significant mitochondrial DNA differentiation between the Baffin Bay, the Holocene Spitsbergen, and the extant BCB stocks. In the Baffin Bay stock the number of observed haplotypes was substantially lower and the average haplotype frequency was higher than in the Holocene Spitsbergen and the extant BCB stocks. There is a number of haplotypes in the Baffin Bay stock with relatively high frequencies that have not been observed in the Spitsbergen or the BCB stocks or have only been recorded with very low frequencies. The data may indicate that the Baffin Bay stock went through a genetic bottleneck, which is in line with previous estimates of population sizes. However, the data can also be taken as evidence against the hypothesis of significant immigration of individuals from the BCB stock into the distribution area of the Baffin Bay stock. Accordingly, the recent increase in population size of the Baffin Bay stock is likely due to other reasons. Of course, we cannot entirely rule out that the samples used in this study are not representative for the stock. However, sample sizes are relatively large and the female bias of the samples from Disko Bay, West Greenland, should not have an effect on estimates based on the maternally inherited mitochondrial DNA. However, there may be arguments against the assumption of immigrants being a good representation of the source population, for example related to social biases in movement patterns and samples. Future studies need to address such questions in more detail.

References

Arnason, U., Gullberg, A., Widegren, B. 1993. Cetacean mitochondrial DNA control region: sequences of all extant baleen whales and two sperm whale species. Molecular Biology and Evolution 10960–10970.

Borge, T., Bachmann, L., Bjørnstad, G., Wiig, Ø. 2007. Genetic variation in Holocene bowhead whales from Svalbard. Molecular Ecology 16, 2223-2235.

Cosens, S. E., Cleator, H., Richard, P. 2006. Numbers of bowhead whales (Balaena mysticetus) in the eastern Canadian Arctic, based on aerial surveys in August 2002, 2003 and 2004. DFO Can. Sci. Advis. Sec. Rec. Doc. – 2006/52.

COSEWIC. 2005. COSEWIC assessment and update status report on the bowhead whales Balaena mysticetus in Canada. Committee on the Status of Endangered Wildlife in Canada, Ottawa. Viii+51 pp.

Davis, R. A. and Koski, W. R. 1980. Recent observations of the bowhead whale in the Eastern Canadian High Arctic. Rep. int. Whal. Commn. 30, 439-444.

Heide-Jørgensen, M. P., Laidre, K. L.,Jensen, M. V., Dueck, L. and Postma, L. D. 2006. Dissolving stock discreteness with satellite tracking: Bowhead whales in Baffin Bay. Marine Mammal Science 22, 34-45.

Heide-Jørgensen, M.P., Laidre, K., Wiig, Ø., Bachmann, L., Lindqvist, C., Postma, L., Dueck, L., Lindsay, M., Tenkula, D. 2008. Segregation of sexes and plasticity in site selection of bowhead whales. SC/60/BRG19

Heide-Jørgensen, M.P., Cosens, S.E., Dueck, L.P., Laidre, K., Postma, L. 2008. Baffin Bay-Davis Strait and Hudson Bay-Foxe Basin Bowhead whales: A reassessment of the two-stock hypothesis. SC/60/BRG20

Heide-Jørgensen, M. P., Laidre, K. L., Borchers, D., Samara, F. I. P. 2007. Increasing abundance of bowhead whales in West Greenland. Biology Letters 3, 577-580. doi:10.1098/rsbl.2007.0310