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Keywords:

  • Maui's dolphin;
  • Hector's dolphin;
  • Cephalorhynchus ;
  • genetic monitoring;
  • dispersal;
  • migrant;
  • assignment

Abstract

  1. Top of page
  2. Abstract
  3. Materials and Methods
  4. Results
  5. Discussion
  6. Acknowledgments
  7. Literature Cited
  8. Supporting Information

For endangered populations with low genetic diversity, low levels of immigration could lead to genetic rescue, reducing the risk of inbreeding depression and enhancing chances of long-term species survival. Our genetic monitoring of Maui's dolphins revealed the first contemporary dispersal of their sister subspecies, Hector's dolphin, from New Zealand's South Island into the Maui's dolphin distribution along ~300 km of the North Island's northwest coast. From 2010 to 2012, 44 individuals were sampled within the Maui's dolphin distribution, four of which were genetically identified as Hector's dolphins (two living females, one dead female, one dead male). We also report two Hector's dolphins (one dead female neonate, one living male) sampled along the North Island's southwest coast, outside the presumed range of either subspecies. Together, these records demonstrate long-distance dispersal by Hector's dolphins (≥400 km) and the possibility of an unsampled Hector's dolphin population along the southwest coast of the North Island. Although two living Hector's dolphins were found in association with Maui's dolphins, there is currently no evidence of interbreeding between the subspecies. These results highlight the value of genetic monitoring for subspecies lacking distinctive physical appearances as such discoveries are not detected by other means, but have important conservation implications.

When a population becomes isolated and loses genetic diversity, natural dispersal by even one individual per generation can increase genetic diversity and bring about genetic rescue (Vila et al. 2003, Adams et al. 2011). Genetic rescue is thought to reduce the risk of inbreeding depression and enhance the chances of long-term species survival (Ingvarsson 2001). For endangered species with natural or anthropogenic limitations to dispersal, human-mediated translocations are sometimes used to maintain or restore genetic diversity (Griffith et al. 1989, Wolf et al. 1996, Benson et al. 2011). For endangered dolphins, however, genetic rescue via natural dispersal has never been documented and human-mediated translocation has never been attempted. Although human-mediated translocation was considered for the baiji (Lipotes vexillifer), the species went extinct before implementation of the plan (Wang et al. 2006).

The New Zealand endemic Hector's dolphin (Cephalorhynchus hectori van Beneden 1881) is thought to have declined in distribution and abundance as a result of fisheries-related mortality since the 1970s (Martien et al. 1999, Slooten and Dawson 2010, Slooten and Davies 2012). This species was classified as two subspecies—the Maui's dolphin (C. h. maui) and the Hector's dolphin (C. h. hectori)—by Baker et al. (2002) and supported by a later review (Perrin et al. 2009). The critically endangered Maui's dolphin is surviving as a remnant population along ~300 km of the west coast of New Zealand's North Island, with the core concentration occurring within only 150 km of this distribution (Fig. 1; Reeves et al. 2008; Oremus et al. 2012; Baker et al., in press). The more abundant South Island subspecies retains the common name of Hector's dolphin and is divided into three genetically differentiated regional populations on the east, west, and south coasts of the South Island (Hamner et al. 2012). The two subspecies are recognized, in part, based on a diagnostic distinction in mitochondrial (mt) DNA haplotypes (Baker et al. 2002). Since a concerted effort to collect samples began in 1988, the Maui's dolphin has been characterized by a single unique mtDNA control region haplotype (G), as compared to the 20 mtDNA haplotypes currently found among the Hector's dolphin subspecies around the South Island (Hamner et al. 2012). The only potential exceptions were three historical museum samples reportedly collected on the North Island, which had haplotypes otherwise found only in Hector's dolphins (J in the Bay of Islands ca. 1870, N in Waikanae in 1967, and J in Oakura in 1988; note: the latter two are corrected from Baker et al. 2002 to match Pichler 2001). However, doubts about the reported collection location of one specimen and potential for postmortem drift of the other two recovered carcasses, as well as evidence that their skeletal measurements were more similar to those of Hector's dolphins, led Baker et al. (2002) to exclude them from the analyses used to define the two subspecies.

image

Figure 1. Mitochondrial control region haplotypes (360 bp) sampled from the coastal distributions of the Maui's dolphin and three regional populations of Hector's dolphins between 1988 and 2012 (Hamner et al. 2012; current work). Haplotypes based on 576 bp sequences are indicated next to the icons for the six Hector's dolphins found on the North Island, but are not available for all samples in the reference data set at this time.

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The current genetic isolation of the two subspecies is likely maintained by the relatively large geographic distance between them and the small average home ranges of these dolphins (Hamner et al. 2012). Previous studies on the movements of Hector's dolphins suggest that individuals are not likely to regularly move across distances larger than approximately 60 km, with only rare movements in excess of 100 km (Bräger et al. 2002, Stone et al. 2005, Rayment et al. 2009). This limited movement is consistent with the limited gene flow observed within the Hector's dolphin subspecies, among the East Coast, West Coast, and southern Te Waewae Bay and Toetoe Bay populations (Fig. 1; Hamner et al. 2012).

Genetic monitoring provides a framework for assessing changes in the demographic and genetic status of a species by establishing a baseline genetic assessment from an initial sampling event, followed by the continued collection and analysis of samples over time (Schwartz et al. 2007). Here we report the unexpected natural dispersal of four Hector's dolphins detected between 2010 and 2012 through the genetic monitoring of the Maui's dolphin along the northwest coast of the North Island (Oremus et al. 2012). We also report two additional Hector's dolphins that were sampled in 2005 and 2009 on the southwest coast of the North Island, outside the known distributions of either subspecies. The northward dispersal of Hector's dolphins into the distribution of the Maui's dolphin could lead to the genetic enhancement of Maui's dolphins and promote the preservation of the species as part of the west coast North Island marine ecosystem.

Materials and Methods

  1. Top of page
  2. Abstract
  3. Materials and Methods
  4. Results
  5. Discussion
  6. Acknowledgments
  7. Literature Cited
  8. Supporting Information

As part of an on-going collaborative program for monitoring the abundance and genetic diversity of Maui's dolphins, small skin samples were collected via dart biopsy (Krützen et al. 2002) during boat-based surveys conducted from 4 February to 2 March 2010 and from 14 February to 10 March 2011 (Fig. 1; see Oremus et al. 2012). Additionally, a dart biopsy sample collected from a single dolphin sighted in Wellington Harbour in 2009, and skin samples collected from all Maui's or Hector's dolphins recovered as beachcast or entangled carcasses through 25 April 2012 were provided to us by the New Zealand Department of Conservation and archived in the New Zealand Cetacean Tissue Archive at the University of Auckland (Thompson et al. 2013).

All samples were stored in 70% ethanol at −20°C prior to tissue digestion with proteinase K followed by total cellular DNA extraction using a standard phenol:chloroform protocol (Sambrook et al. 1989) as modified for small samples by Baker et al. (1994). We assembled DNA profiles for each sample, including genetic sex identification, mtDNA control region haplotype and 21-locus microsatellite genotypes. Existing DNA profiles previously reported for the Maui's dolphin baseline samples collected in 2001–2007 (Baker et al., in press) and for the samples collected in 2010–2011 (Oremus et al. 2012) were built upon to complete the extended DNA profiles described here.

Sex was identified using a multiplexed PCR protocol to amplify fragments of the sry and ZFX/ZFY genes according to Gilson et al. (1998). A fragment of approximately 700 bp from the 5′ end of the maternally-inherited mtDNA control region was amplified and sequenced according to Hamner et al. (2012). Sequences were aligned and edited using Geneious Pro v5.5.2 (BioMatters). Haplotypes were initially assigned based on the 360 bp reference sequences of the 22 haplotypes previously identified for Hector's and Maui's dolphins (Pichler et al. 1998, Pichler and Baker 2000, Pichler 2002, Hamner et al. 2012), however several of these haplotypes were further resolved based on alignment with longer 576 bp sequences.

All samples were genotyped for 21 microsatellite loci using published cetacean primers (Table 1). For the “SGUI” loci and TtruGT48, each 10 μL PCR reaction contained 1 ×  PCR II buffer, 2.5 mM MgCl2, 0.04 μM of the forward primer with M13 tag, 0.4 μM reverse primer, 0.4 μM fluorescent label with M13 tag, 0.2 mM dNTP, 20 mg/mL bovine serum albumin (BSA), 0.25 units Platinum Taq (Invitrogen) and 10–20 ng/μL DNA template, and were amplified using the thermocycling profile of Cunha and Watts (2007) with modifications to the annealing temperature specified in Table 1. For all other loci, each 10 μL PCR reaction contained 1 ×  PCR II buffer, 2.5 mM MgCl2, 0.4 μM each primer, 0.2 mM dNTP, 0.125 units Platinum Taq (Invitrogen) and 10–20 ng/μL DNA template, and were amplified using the following thermocycling profile: 93°C for 2 min; (92°C for 30 s, TA for 45 s, 72°C for 50 s) × 15; (89°C for 30 s, TA for 45 s, 72°C for 50 s) × 20; 72°C for 3 min, with the annealing temperatures (TA) stated in Table 1. Products were run on an ABI 3130XL DNA Analyzer and allele peaks were binned and visually verified using GENEMAPPER v.3.7 (Applied Biosystems). To minimize genotyping error, each amplification and sizing run included a negative control to detect contamination and 10 internal control samples to ensure comparable allele sizing across all runs and to estimate genotyping error. A genotyping error rate was calculated by dividing the number of incongruent allele calls by the total number of alleles compared for the samples that were genotyped twice (Bonin et al. 2004).

Table 1. Microsatellite loci genotyped for the individual and subspecies identification of Maui's and Hector's dolphins sampled on the northwest coast of New Zealand's North Island between 2010 and 2012 (= 76 samples, representing 44 individuals), and on the southwest coast of the North Island in 2005 and 2009 (= 2 samples, representing two individuals)
LocusF primer labelTAb (ºC)n samples (individuals)# allelesP(ID)cP(ID)sibd
  1. a

    Ten loci were also genotyped previously for Hector's dolphins sampled on the South Island (Hamner et al. 2012) and used for subspecies assignment.

  2. b

    TA = annealing temperature.

  3. c

    P(ID) = probability of identity.

  4. d

    P(ID)sib = probability of identity for siblings.

  5. e

    Schlotterer et al. 1991.

  6. f

    Valsecchi and Amos 1996.

  7. g

    Bérubé et al. 2000.

  8. h

    Hoelzel et al. 1998.

  9. i

    Hoelzel et al. 2002.

  10. j

    Krützen et al. 2001.

  11. k

    Rosel et al. 1999.

  12. l

    Cunha and Watts 2007.

  13. m

    Rooney et al. 1999.

  14. n

    Caldwell et al. 2002; forward primer modified to include an M13 tag.

415/416aeHEX4575 (43)20.4640.680
EV1afHEX4576 (44)11.0001.000
EV14afVIC6078 (46)40.3040.581
EV37fHEX4570 (41)50.4400.675
EV94afFAM5578 (46)60.2630.552
GT211gFAM5076 (44)40.2290.504
GT23agVIC5578 (46)30.3730.617
GT575agFAM5078 (46)20.8200.906
KWM12aahVIC5577 (45)110.2140.525
KWM9baiFAM5077 (45)60.1720.459
MK5ajTET5578 (46)30.2360.511
MK6jNED5074 (42)30.8090.901
PPHO110akFAM5077 (45)40.2740.545
PPHO130 kNED5578 (46)30.8160.905
PPHO142 kNED5578 (46)20.3930.615
SGUI06lM13-VIC5772 (41)40.8950.946
SGUI07lM13-NED5776 (44)30.7740.882
SGUI16lM13-VIC5776 (44)20.4450.664
SGUI17lM13-NED6075 (43)30.3720.597
TexVet5mFAM5076 (44)20.9250.962
TtruGT48nM13-VIC5576 (44)30.4930.710
Overall  78 (46) 3.7 × 10−83.1 × 10−4

Genotypes were compared to identify replicate samples of the same individual using CERVUS v. 3.0 (Kalinowski et al. 2007). The probability of identity (P(ID)) and probability of identity for siblings (P(ID)sib) for each locus and across all loci were calculated in GenAlEx v. 6.1 (Peakall and Smouse 2006). To avoid false exclusion, initial matching allowed for up to five mismatching loci, and we examined each of these “relaxed matches” for potential allelic dropout or processing error, and repeated them as needed for confirmation. Sex and mtDNA haplotypes were subsequently compared to support our confidence in correctly identifying replicate samples. After review and replication for correction or confirmation of the relaxed matches, we accepted samples with matching genotypes as resamples of the same individual.

For individuals found to have Hector's dolphin haplotypes (“putative Hector's dolphins”), as opposed to the characteristic G of the Maui's dolphin (see 'Results'), the subspecies was confirmed and populations of origin were identified using the Bayesian assignment procedures in the programs Structure v2.3.2 (Pritchard et al. 2000, 2010) and GeneClass2 v2.2.2 (Piry et al. 2004). For this, we used a reference data set of genotypes from 10 microsatellite loci in linkage equilibrium for Maui's dolphins (= 87 individuals) and Hector's dolphins (= 176 individuals) from across the three regional populations (Hamner et al. 2012). Although several loci showed slight departures from Hardy-Weinberg equilibrium (Hamner et al. 2012), none were significant across all populations. Simulations by Cornuet et al. (1999) suggest that such slight departures from Hardy-Weinberg equilibrium are not likely to influence the result of assignment tests. In Structure, no population information was included for the putative Hector's dolphins and the “UsePopInfo” option assuming no admixture and correlated allele frequencies was applied to the reference samples to run 106 Markov Chain Monte Carlo (MCMC) replicates following a burn-in of 105 for = 4 populations. A membership coefficient (q) ≥ 0.900 was used as the threshold for confidently identifying the population of origin. This threshold has been accepted as sufficient evidence for prosecution in wildlife poaching cases (i.e., Lorenzini et al. 2011), and is considered more appropriate for management cases given the lower rate of false exclusion of the true identity than the more stringent qi = 0.999 threshold required by other wildlife forensic cases (Manel et al. 2002, Millions and Swanson 2006). In GeneClass2, the Bayesian method of Rannala and Mountain (1997) was implemented to assign the putative Hector's dolphins to the reference data set described above, using an alpha of 0.01 as evidence of origin. Additionally, Paetkau et al.'s (2004) permutation procedure was implemented with 1,000 simulated individuals and a threshold of < 0.01 to exclude populations as an individual's origin, as is used in other wildlife applications (Berry and Kirkwood 2010, Drewry et al. 2012).

Results

  1. Top of page
  2. Abstract
  3. Materials and Methods
  4. Results
  5. Discussion
  6. Acknowledgments
  7. Literature Cited
  8. Supporting Information

A total of 76 samples were collected within the Maui's dolphin distribution on the northwest coast of the North Island between 2010 and 2012. Of these, 73 were collected from living dolphins during the 2010 and 2011 surveys (Oremus et al. 2012), and 3 were provided to us from recovered dolphin carcasses: Chem10NZ06 collected on 20 November 2010 floating off Raglan, Che11NZ06 collected on 26 October 2011 at Clark's Beach in Manukau Harbour, and Che12NZ02 collected on 25 April 2012 at Opunake, Taranaki. Our work also considers two dolphins sampled outside of the known distributions of either the Hector's or Maui's dolphin subspecies: a neonate found just before death on Peka Peka Beach in 2005 (Che05NZ20) and a single dolphin biopsy sampled during a rare sighting in Wellington Harbour in 2009 (Che09WH01; Fig. 1).

The DNA profiles identified 46 individual dolphins from the 78 samples described above, with a combined microsatellite P(ID) = 3.7 × 10−8 and P(ID)sib = 3.1 × 10−4 (Table 1). No contamination was detected by the negative controls, and a genotyping error rate due to allelic dropout was estimated to be 0.4% based on the repeated genotyping of the 10 control samples (252 alleles). However, the error rate in the final data set is likely to be lower than this, as genotypes of relaxed matches were also replicated to either correct allelic dropout or confirm the genotype.

The mtDNA control region sequence of 40 individuals matched the G haplotype that has been diagnostic of the Maui's dolphin population since the collection of contemporary samples began in 1988 (Pichler and Baker 2000). However, four individuals sampled within the Maui's dolphin distribution (CheNI10-03, CheNI10-24, Che11NZ06, Che12NZ02) and the two sampled on the southwest coast of the North Island (Che05NZ20, Che09WH01) represented haplotypes found only in Hector's dolphins: C, H, I, and J (360 bp; Fig. 1; Table S1), and were considered putative Hector's dolphins. These are the four most common Hector's dolphin haplotypes (Hamner et al. 2012), which have now been resolved into three to four subtypes each when using longer 576 bp sequences (RMH, unpublished data). Based on these longer sequences, the six dolphins of interest each have a different haplotype: CheNI10-03, Ib; CheNI10-24, Jb; Che11NZ06, Cb1; Che12NZ02, Hb; Che05NZ20, Ia; and Che09WH01, Ca; GenBank Accessions: KC492580-KC492585). These six haplotypes differ from the G haplotype (also extended to 576 bp; GenBank Accession: KC492586) at two to six sites each. However, as not all samples in the reference data set of Hector's dolphin haplotypes have the longer sequences, we are unable to examine their relative frequencies in the different Hector's dolphin populations at this time.

To confirm the subspecies and likely population of origin, the genotypes of the putative Hector's dolphins were compared to baseline samples described by Hamner et al. (2012). The Structure analysis clearly assigned the six putative Hector's dolphins to the Hector's dolphin subspecies, while all other samples collected on the North Island clearly assigned to the Maui's dolphin (Fig. 2). Two females sampled alive within the Maui's dolphin distribution assigned strongly to the population of Hector's dolphins on the west coast of the South Island (CheNI10-03 = 0.9790, CheNI10-24 = 0.9783; Fig. 2), however, the other four dolphins showed ambiguous assignment to the Hector's dolphin populations (highest  0.6; Fig. 2). The GeneClass2 analysis further supported the evidence that these six individuals were Hector's dolphins by excluding the Maui's dolphin as the population of origin for each of them with high certainty ( 0.003; Table 2). However, the only Hector's dolphin population that could be excluded as a source was the South Coast for one of the dolphins, Che12NZ02 (= 0.002). As in the Structure results, GeneClass2 assigned CheNI10-03 and CheNI10-24 to the West Coast South Island with high likelihoods (Table 2). GeneClass2 also provided high assignment likelihoods for Che12NZ02 to the West Coast South Island, and for Che09WH01 and Che11NZ06 to the East Coast South Island population, although they did not exceed the high confidence threshold of 0.01 (Table 2). Again similar to the Structure results, Che05NZ20 showed a more ambiguous assignment among the Hector's dolphin populations with a moderate likelihood of 0.6189 to the West Coast, followed by 0.2353 to the East Coast.

Table 2. Likelihood of individuals originating in the Maui's dolphin or East Coast, West Coast or South Coast Hector's dolphin population calculated using Rannala and Mountain's (1997) assignment method (highest likelihood in bold) and the probability of exclusion (P < 0.01, shaded gray) using Paetkau et al.'s (2004) permutation procedure in GeneClass2 based on 10-locus microsatellite genotypes and the reference data set of Hamner et al. (2012)
IndividualMaui's dolphinHector's dolphin
East CoastWest CoastSouth Coast
LikelihoodPLikelihoodPLikelihoodPLikelihoodP
CheNI10-030.0000<0.00010.00150.6890.99510.9920.00340.455
CheNI10-240.0000<0.00010.00000.1780.99950.7320.00050.034
Che11NZ060.0000<0.00010.98620.5000.01090.1070.00290.017
Che12NZ020.0000<0.00010.06100.0840.93740.0950.00150.002
Che05NZ200.0000<0.00010.23530.6910.61890.6610.14590.384
Che09WH010.00070.00300.97540.3890.00650.0780.01740.020
image

Figure 2. Identification of six Hector's dolphins using a Structure v.2.3.2 analysis of 10-locus microsatellite genotypes and a reference data set of Maui's dolphins (= 87) and Hector's dolphins from the East Coast (= 93), West Coast (= 51), and South Coast (= 32) South Island populations (Hamner et al. 2012). Four of the Hector's dolphins (CheNI10-03, CheNI10-24, Che11NZ06, Che12NZ02) were sampled within the Maui's dolphin distribution on the northwest coast of the North Island and two (Che05NZ20, Che09WH01) were sampled between the distributions of either subspecies on the southwest coast of the North Island.

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Discussion

  1. Top of page
  2. Abstract
  3. Materials and Methods
  4. Results
  5. Discussion
  6. Acknowledgments
  7. Literature Cited
  8. Supporting Information

Our findings demonstrate the fundamental concept of genetic monitoring—observing changes in demographic and genetic parameters over time. The genetic monitoring of the Maui's dolphin resulted in the unexpected discovery of four Hector's dolphins within the Maui's dolphin distribution on the northwest coast of the North Island between 2010 and 2012. The presence of these Hector's dolphins would not have been evident without the extensive baseline of genetic diversity initiated by Pichler (2002) and updated by Hamner et al. (2012) to include individuals sampled between 1988 and 2007. This reference sample set was intentionally time-limited so as to minimize the potential for generational changeover, assuming an estimated 20 yr maximum lifespan of Hector's and Maui's dolphins (Slooten and Lad 1991), while maximizing the number of contemporary samples across the distribution of the species.

In light of the unexpected discovery of the Hector's dolphins among Maui's dolphins, we reexamined genotypes of two Hector's dolphins sampled on the southwest coast of the North Island in 2005 and 2009. These dolphins sampled at Peka Peka Beach (Che05NZ20) and Wellington Harbour (Che09WH01), were found between the distributions of the two subspecies. Although the sample from Peka Peka Beach (Che05NZ20) was collected in 2005, within the 1988–2007 time period used for the baseline, it was excluded from the genetic baseline as an outlier, given that it was a neonate found beachcast in an area extralimital to the known distribution of either subspecies. However when considered together, the six Hector's dolphins sampled on the North Island pose several nonexclusive scenarios: (A) several independent events occurred where one or more dolphins dispersed from known population(s) on the South Island to the North Island; (B) a single stochastic event occurred, where several Hector's dolphins dispersed together as a group from a known population on the South Island to the North Island; or (C) a small population of previously unsampled Hector's dolphins exists along the southern North Island or northern South Island, several of which dispersed into the Maui's dolphin distribution. We consider a combination of either A and C or B and C to be most consistent with the results of the population assignment analyses.

The discovery of these Hector's dolphins on the North Island calls for reconsideration of three historical samples described by Baker et al. (2002). These three samples were reportedly collected on the North Island, but did not have the characteristic G haplotype of the Maui's dolphin. Baker et al. (2002) excluded them from the analyses used to classify the subspecies due to doubts about the actual collection location of one specimen and the potential for postmortem drift of the two that were found beachcast in advanced states of decomposition. Unfortunately, we have no additional information from these bone and tooth samples to support or refute the provenance of these dolphins or to confirm their subspecies, so cannot determine if they represent historical mtDNA diversity that has been lost from the Maui's dolphin or if they were in fact migrant Hector's dolphins.

In any case, the dispersal of Hector's dolphins into the distribution of the Maui's dolphin is not likely to have been a frequent occurrence. Using a binomial distribution probability function (Swofford and Berlocher 1987), the chance of detecting a Hector's dolphin haplotype in the baseline of 96 Maui's dolphin samples collected from 1988 to 2007 (Hamner et al. 2012) is 93.3% for a Hector's dolphin haplotype at a frequency of 5%, and 61.9% for a Hector's dolphin haplotype at a frequency of 1%. More importantly, no genetic admixture between Hector's and Maui's dolphins has been detected in any of the 269 individuals from both subspecies that were sampled and genotyped between 1988 and 2012 (Hamner et al. 2012; current work). Furthermore, the BayesAss analysis presented by Hamner et al. (2012), estimated negligible migration rates between the two subspecies, ranging from 0.006 to 0.014 dolphins per generation.

Our findings are the first contemporary evidence of Hector's and Maui's dolphins cooccurring in the same area. Although four Hector's dolphins have now been documented within the geographic range of the Maui's dolphin, it is premature to raise concerns about the validity of the subspecies. To date, we have not detected evidence of interbreeding between the Hector's and Maui's dolphins, and there are no examples from captivity to assess this potential. The number of documented dispersal events at this time is low. However, if further dispersal of Hector's dolphins occurs and the subspecies are shown to interbreed, it could lead to a loss of the genetic and morphological distinctiveness that was used to support their classification as subspecies (Reeves et al. 2004, Perrin et al. 2009).

The minimum distance of 400 km required for Hector's dolphins to travel from the West Coast South Island population to the central northwest coast of the North Island was surprising given previous observations of restricted home ranges. Despite over 25 yr of research on Hector's dolphins, the maximum distance of dispersal observed was just over 100 km and most observed movements have been within a home range of 30–60 km (Bräger et al. 2002, Rayment et al. 2009). The deep water of Cook Strait was thought to deter these dolphins from moving between the North and South Islands, consistent with most observations of Hector's dolphins occurring in depths less than 39 m (Bräger et al. 2003, Rayment et al. 2011) and the rarity of sightings in the Fiordland area where depths can exceed 300 m (Cawthorn 1988). However, our identification of two Hector's dolphins from the West Coast South Island confirm that movements between the islands do occasionally occur, even if it is not known whether the dolphins are crossing the deeper waters at the narrowest point of Cook Strait or perhaps following an offshore corridor of shallower water to the northwest.

The ambiguous assignment of four dolphins to the Hector's dolphin populations, suggests the potential for a previously unsampled population of Hector's dolphins that is not included in our baseline reference data, or perhaps an area of interbreeding between the East and West Coast Hector's dolphin populations. Therefore, the potential for a small and elusive resident population of Hector's dolphins along the southern part of the North Island, outside the current range of the Maui's dolphin, or along the northern part of the South Island between the East and West Coast populations of Hector's dolphins should be investigated.

The protection of habitat and removal of anthropogenic threats are crucial if the Maui's dolphin is to survive (Currey et al. 2012). The New Zealand government has recognized this by establishing the West Coast North Island Marine Mammal Sanctuary and placing restrictions on seabed mining, acoustic seismic surveys, and fishing activities (New Zealand Department of Conservation 2008, New Zealand Ministry of Fisheries 2012). However, with the known distance of individual movement greatly increased to at least 400 km and the confirmation that these dolphins will at least occasionally disperse from the South Island to North Island, there is the possibility that genetic exchange between the subspecies will also benefit the Maui's dolphin and promote the survival of the species on the west coast of the North Island. If protected corridors connecting the Maui's dolphin on the North Island and Hector's dolphin populations on the South Island are not maintained, then such natural dispersal events are less likely to occur.

Rare natural dispersal events similar to the one described here for Hector's dolphins have been beneficial for improving the genetic diversity and fitness of wolves in Scandinavia (Vila et al. 2003) and Isle Royal National Park (Adams et al. 2011), and perhaps other cases overlooked by a narrow definition of genetic rescue (Hedrick et al. 2011). The genetic rescue of the Isle Royale wolves is thought to be the cause for a slight increase in the population at a time when space and prey were limiting (Adams et al. 2011). Unfortunately, in this case the benefits seem to have been short-lived due to deteriorating environmental conditions and recent stochastic events, which reduced the population to 14 males and 2 females in 2011 (Vucetich et al. 2012). This illustrates the importance of continued monitoring and the need to mitigate all known threats to a population if its chances for surviving stochastic events are to be maximized.

Although the Hector's dolphin migrants have the potential to enhance the genetic diversity of the Maui's dolphin, there is also the potential for outbreeding depression to occur if the Maui's dolphin has undergone selection or specialization making it better adapted to its North Island habitat. Outbreeding depression occurs when “hybrid” offspring do not inherit local adaptations, causing them to be less fit than individuals whose parents originate from the same locally adapted population. Although difficult to document in wild populations, this was observed when migrants naturally entered the otherwise isolated song sparrow population on Mandarte Island (Marr et al. 2002). The possibility of local adaptations and outbreeding depression for Hector's and Maui's dolphins could be assessed by applying a genomic approach to assess functional genetic divergence between the two subspecies (Allendorf et al. 2010).

Our findings highlight the value of genetic monitoring, particularly for cryptic subspecies or populations, as such discoveries cannot be made by other means, but have important conservation implications. During the time period of our study, one additional dolphin mortality was reported by a commercial fisherman who found it entangled in his set net off Cape Egmont in January 2012 (New Zealand Department of Conservation 2012). Unfortunately, no sample was taken for genetic analysis to confirm the subspecies before the fisherman followed the protocol in place at the time and returned the carcass to the sea.

Only time and continued genetic monitoring will reveal if the living Hector's dolphin migrants remain permanent North Island residents and if they are successful at contributing to the diminished gene pool of the Maui's dolphin. Available evidence suggests that the dispersal may be permanent, as CheNI10-24 was sampled in both 2010 and 2011 (Oremus et al. 2012; Table S1). If the female migrants breed with Maui's dolphins, their relative breeding success can be tracked by monitoring the frequencies of their distinctive maternally inherited mtDNA haplotypes. Additionally, biparentally inherited microsatellite genotypes can be used to detect potential evidence of admixture between the subspecies and genetic rescue of the Maui's dolphin.

Acknowledgments

  1. Top of page
  2. Abstract
  3. Materials and Methods
  4. Results
  5. Discussion
  6. Acknowledgments
  7. Literature Cited
  8. Supporting Information

Our research was funded by the New Zealand Department of Conservation (DOC), as well as a Mamie Markham Research Award, Ted Thorgaard Student Research Awards, Oregon Lottery Scholarships, and Oregon State University Laurels Scholarships to RMH. Publication of this paper was supported, in part, by the Thomas G. Scott Publication Fund. Many thanks to everyone at DOC who assisted in the field and with sampling of carcasses, especially C. Duffy, G. Hickman, K. Hillock, C. Lilley, K. MacLeod, and B. Williams; to N. Gibbs for collecting the Wellington Harbour biopsy sample; to those involved with the current and baseline labwork: A. Alexander, E. Carroll, D. Heimeier, S. Lavery, F. Pichler, K. Russell, D. Steel, K. Thompson, and M. Vant; and to V. Ward for the Hector's dolphin drawing. We are grateful for support from local iwi and DOC Area Offices and Conservancies. We also thank M. Schwartz and three anonymous reviewers for comments to improve the manuscript. Biopsy samples were collected under permit RNW/HO/2009/03 issued to CSB from DOC and the protocol AEC/02/2008/R658 approved by the University of Auckland Animal Ethics Committee.

Literature Cited

  1. Top of page
  2. Abstract
  3. Materials and Methods
  4. Results
  5. Discussion
  6. Acknowledgments
  7. Literature Cited
  8. Supporting Information

Supporting Information

  1. Top of page
  2. Abstract
  3. Materials and Methods
  4. Results
  5. Discussion
  6. Acknowledgments
  7. Literature Cited
  8. Supporting Information
FilenameFormatSizeDescription
mms12026-sup-0001-TableS1.pdfapplication/PDF61KTable S1. Four Hector's dolphins (Cephalorhynchus hectori hectori) sampled within the distribution of the Maui's dolphin (C. h. maui) on the northwest coast of New Zealand's North Island and *two Hector's dolphins sampled on the southwest coast of the North Island, outside the previoulsy known distribution of either subspecies.

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