Abstract:
This thesis uses molecular genetics as a tool to uncover information about the
population structure and genetic variation in Hector’s dolphin (Cephalorhynchus
hectori), to track population declines and to assess the evolutionary origins and
taxonomic status of this species. A high-resolution genetic analysis of population
structure was considered important for the determination of population boundaries
and delimitation of conservation management units due to potentially unsustainable
fisheries-related mortality.
Population structure and dispersal rates were assessed using 281 samples collected
from individual Hector’s dolphins of ten population groups representing the known
geographic range of this species. Variation among mitochondrial DNA sequences
(ΦST = 0.545) and microsatellite allele frequencies at six loci (RST = 0.252) indicated
the presence of four genetically isolated regional populations, North Island (n = 29),
East Coast South Island (n = 110), West Coast South Island (n = 122) and South
Coast South Island (n = 19). Significant levels of genetic differentiation were not
detected within local sub-populations of the East Coast and West Coast regional
populations. However, the estimated geneflow between these sub-populations fitted a
one-dimensional stepping-stone model (r2 = 0.6225) suggesting a vulnerability of
local populations to fragmentation. A measure of expected mtDNA diversity
(Tajima’s D statistic) suggested decline in eight of the ten populations. Microsatellite
heterozygosity was also lower than expected in the East Coast and North Island
regions, suggesting either further regional sub-structuring (Wahlund effect), loss of
diversity due to population decline or the presence of null alleles.
Examination of all Hector’s dolphin museum specimens of known origin (n = 55)
enabled comparison of historic (1870 - 1987) genetic diversity to contemporary (1988
– 1999) diversity in two regional populations to assess the possibility that these
populations have undergone recent declines. Over the last 20 years the North Island
population has been reduced from at least three lineages (h = 0.41) to a single lineage
(h = 0, p < 0.05). The diversity of the East Coast, South Island population has
declined significantly from h = 0.65 to h = 0.35 (p < 0.05). These results suggest that
the low abundance currently observed is due to recent population declines and that the
North Island population is threatened with extinction in the near future. Based on a
trend analysis of the mtDNA, it can be predicted that the East Coast South Island
population may lose all mtDNA diversity within the next 20 years. Alternatively,
detection of a one dimensional dispersal pattern may indicate that some populations
are at risk of extirpation while others may not be in decline. If this is the case then the
East Coast regional population is at risk of fragmentation.
On a wider evolutionary scale, Hector’s dolphin is one of four species of the genus
Cephalorhynchus, all of which suffer fisheries–related mortality. To describe the
origin and radiation of these species, 485 bp of the mitochondrial DNA control region
was sequenced from 320 individuals (including previously sequenced 200 Hector’s
dolphins) representing nine of the ten species in the sub-family Lissodelphininae. The
hypotheses that either Cephalorhynchus is a monophyletic genus or that the four
species have arisen separately from pelagic Lissodelphine species and have converged
morphologically were tested. The mtDNA phylogeny supported the monophyly of
the genus and suggested that the genus Cephalorhynchus originated in the waters of South Africa and, following the West Wind Drift, colonised New Zealand and then
South America. Secondary radiations resulting in two genetically isolated populations
were found for the Kerguelen Island Commerson’s dolphin and the North Island
Hector’s dolphin.
A comparison of the genetic differentiation between the Commerson’s dolphins of the
Kerguelen Islands (n = 11) and the coast of South America (n = 35), and between the
North Island (n = 14) and South Island (n = 185) Hector’s dolphins, was conducted in
order to assess the conservation and taxonomic status of these populations. A single
fixed substitution in the mtDNA control region was diagnostic for the Kerguelen
Island compared to South America (FST = 0.306, ΦST = 0.602) and the North Island
compared to the South Island (FST = 0.442, ΦST = 0.495). Population differentiation
of four microsatellite alleles (including unique alleles in each of the four populations)
between the Kerguelen Island and South American Commerson’s dolphin (FST =
0.036, RST = 0.0493) and between the North and South Island Hector’s dolphins (FST
= 0.391, RST = 0.3197) indicated restricted nuclear as well as maternal geneflow.
These data, combined with additional evidence of morphological and geographic
isolation, indicated that the Kerguelen Island Commerson’s dolphin and the North
Island Hector’s dolphin are likely to be reproductively isolated from their alternate
con-specific populations. Examination of various species concepts and definitions of
conservation units leads to the conclusion that these four populations should each be
considered unique at the subspecies level for the purposes of management, protection
and evolutionary potential.
These results lead to the conclusion that the Hector’s dolphin consists of highly
subdivided populations. As a result of this and a low reproductive potential, Hector’s
dolphin populations are vulnerable to extirpation through even low levels of human
induced mortality. To manage such populations, it is appropriate to consider each of
the two islands as separate sub-species. Within the South Island, the populations may
be further subdivided into three demographically independent Management Units –
the East, West and South Coasts. The South Coast management unit is vulnerable due
to its low abundance and isolation and requires further investigation. Population
modelling will need to reflect the fact that the local populations within the East and
West coast regions share only limited dispersal with immediately adjacent populations
and are thus susceptible to fragmentation. These results also show that the population
declines of the East Coast South Island and the North Island populations are of recent
origin thus implicating fisheries-related mortality as the principal threat to Hector’s
dolphin. To prevent further decline or fragmentation of South Island populations
more stringent control of inshore gillnet fisheries is required. By contrast, current
decline of the North Island population may be a result of inbreeding depression.
Given the low abundance and rapid decline of the North Island population, it is
imperative to evaluate the potential for inbreeding depression while continuing to
mitigate all human-related threats.