SC/58/PFI2

An examination of the plausibility of different stockstructure hypotheses of North Pacific Bryde’s whale basedon the available information
LUIS A. PASTENEThe Institute of Cetacean Research, Toyomi 4-5, Chuo-ku, Tokyo 104-0055, JapanABSTRACTThis paper examines the plausibility of the four stock structure hypotheses agreed during the ‘FirstIntersessional Workshop’ for North Pacific Bryde’s whale Implementation, based on the information
presented and discussed at that Workshop. The following kind of information was available to theWorkshop: allozymes, mtDNA, microsatellites, sighting distribution, catch distribution, external body
proportion, biological parameters, mark-recapture and age distribution. Evidence from this informationin favour or against the hypotheses was summarized. Based on this evaluation it is proposed thefollowing rank of plausibility: Hypothesis 1: High, Hypotheses 2 and 3: Low-no sufficient data,
Hypothesis 4: Low.
KEYWORDS: BRYDE’S WHALES, IMPLEMENTATION, WESTERN NORTH PACIFIC, STOCKSTRUCTURE HYPOTHESES, PLAUSIBILITYINTRODUCTION
Following the schedule for an Implementation and subsequent Implementation Review recommended in2004 (IWC, 2005a), the Scientific Committee (SC) completed the pre-implementation assessment ofwestern North Pacific Bryde’s whale during a Workshop carried out in early 2005 (IWC, 2005b). TheWorkshop considered all the aspects required under the ‘Requirements and Guidelines’ for
Implementations to complete a pre-implementation assessment (IWC, 2005a), specifically a)abundance estimates; b) catches; c) stock structure hypotheses; d) dispersal rates; and e) data forconditioning. Based on the results of the examination, the Workshop recommended to the SC that it
considered the pre-implementation process completed. The SC accepted the Workshop report andagreed that the pre-implementation assessment for western North Pacific Bryde’s whales wascompleted and recommended that Implementation can commence (IWC, 2005c).
With regard to stock structure of western North Pacific Bryde’s whales, the pre-implementationassessment Workshop had identified five alternative hypotheses, which considered sufficiently
inclusive that collection of new data during the Implementation process is unlikely to suggest a newstock hypothesis (IWC, 2005b). The Workshop had agreed that the various stock hypotheses were notequally plausible, but did not assign plausibility weights to these hypotheses as this is scheduled for the‘First Annual Meeting’.
The ‘First Intersessional Workshop’ of North Pacific Bryde’s whale was conducted in October 2005.
The primary objective of this Workshop was to develop an appropriate Implementation Simulation
Trials structure and to specify the associated conditioning so that it can be carried out before the ‘First
Annual Meeting’. A review of the hypotheses on stock structure was carried out during the Workshop
with the aim to eliminate any hypotheses that are inconsistent with the data. Of the five hypotheses
agreed in the pre-implementation assessment, four hypotheses were retained during the ‘FirstIntersessional Workshop’.
The objective of this paper is to assign plausibility to the hypotheses on stock structure derived fromthe ‘First Intersessional Workshop’ taking into account all the available scientific information
presented and discussed at that Workshop.

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DESCRIPTION OF THE HYPOTHESES ON STOCK STRUCTURE
The four hypotheses on stock structure retained during the ‘First Intersessional Workshop’ aresummarized in Figure 1.
Hypothesis 1: this is a single stock hypothesis under which only one stock of Bryde’s whale is found inthe area from 130°E-155°W (excluding the area in which East China Stock is found) and there are nosub-stocks.
Hypothesis 2: Different stocks in sub-areas 1 and 2 and there are no sub-stocks.
Hypothesis 3: Same as Hypothesis 2. This hypothesis differs from Hypothesis 2 in that the stock foundin sub-area 1 is also found in sub-area 2.
Hypothesis 4: Same as Hypotheses 2 and 3, except that there are two sub-stocks which mix in sub-area
1.
EVALUATING PLAUSIBILITY OF THE STOCK STRUCTURE HYPOTHESESNo quantitative method has been agreed in the SC for evaluating plausibility. During the 2002 SC
meeting Japanese scientists presented the results of an AIC (Akaike’s Information Criterion)-based
evaluation of the plausibility of baseline stock scenarios for North Pacific common minke whales.
These were based exclusively on mtDNA data. However there was no agreement at the SC on thevalidity of using AIC for evaluating plausibility.
Here a qualitative analysis is conducted by listing up all the scientific evidences (genetics and nongenetics) pro and again different hypotheses and then assigning rank of low, medium and high (Table1). The scientific evidences are the same as those discussed during the ‘First Intersesional Workshop’.
Notations at the bottom of Table 1 were extracted from the report of the ‘First Intersessional
Workshop’ (IWC, 2005d). Original references can be found in that report. The final assignment ofranks is according the criterion of the author.
According to this evaluation Hypothesis 1 is assigned a high rank, Hypothesis 2 and 3, low rank (itshould be emphasized that there is no sufficient data to evaluate this hypothesis) and Hypothesis 4 alow rank.
REFERENCESInternational Whaling Commission. 2005a. Report of the Scientific Committee. J Cetacean Res.
Manage. 7 (Suppl.): 1-391
International Whaling Commission. 2005b. Report of the Workshop on the pre-implementationassessment of western North Pacific Bryde’s whales. Document SC/57/Rep3 (30pp).
International Whaling Commission. 2005c. Report of the Scientific Committee. J. Cetacean Res.
Manage. (Suppl.00): 00-00.
International Whaling Commission. 2005d. Western North Pacific Bryde’s Whale Implementation:
Report of the First Intersessional Workshop. IWC Document SC/58/RepX (44pp).

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Table 1: General summary of the information useful to assess plausibility of alternative stock-structure
hypotheses. A ‘+’ indicates evidence in favour of a hypothesis, ‘-‘ indicates evidence against ahypothesis, ‘(+)’ indicates weak evidence in favour of a hypothesis, ‘(-)’ indicate weak evidence
against a hypothesis. A ‘O’ indicate no sufficient evidence (data). Notations at the bottom of this tablewere extracted from the report of the ‘First Intersessional Workshop’ (IWC, 2005d).
Evidence
Hypothesis 1
Hypothesis 2
Hypothesis 3
Hypothesis 4
aAllozymes(+)

mtDNA+

Microsatellites+Sighting
distribution
Catch distribution+

External body
proportion
Biologicalparameters
Mark-recapture(+)

Age distribution
Rankbbee+ff(+)

g+i(-)
Higha(-)

cO

cOea(-)

cOcO

e-eeff(-)

hOi(-)
Low-insufficient
datad--e-(-)

d-e-

-a(-)

-f(-)

f(-)

hOi(-)
Low-insufficient
dataf(-)f(-)

g-i(+)
LowaThe Workshop agreed that while the power to detect genetic structure might be considered to be low
as only a single locus was analysed, the fact that this had been sufficient to detect structure in thePacific Ocean suggest that if there are multiple stocks differentiated at a level similar to that between
the western North stock and other stocks, examination of this single locus should be sufficient to detect
this.

b

Clustering and hypothesis testing analyses of mtDNA and microsatellites were conducted, and as
noted by the Committee, none of these analyses revealed any significant heterogeneity (and hence
evidence for more than one stock).

cAnalysis of mtDNA and microsatellites have been conducted only for samples available in sub-area 1.
No DNA data are available for sub-area 2.

dMixing of two sub-stocks: There are no genetic data for the breeding grounds so the possibility ofmultiple sub-stocks cannot be excluded. Furthermore if two breeding stocks mix almost completely, it
will be difficult to detect differences using, for example, genetics tests based on comparisons between
data for the west and east of sub-area 1. However while complete mixing may lead to all methods ofdetecting stock structure having low power, the plausibility of this was considered fairly low given thebehaviour of most large whales. Hypothesis tests based on comparisons for approximately the same
area in sub-area 1 found no significant differences among years which suggest that if two sub-stocks
mix in sub-area 1, there is little difference in the distribution proportion among years. Although thedata set encompasses only four years, such a lack of variation in distribution proportions among years
seems unlikely given the known behaviour of whales. In principle evidence for hypotheses 4 could beobtained by testing for deviation from Hardy-Weinberg equilibrium within sub-areas 1W and 1E
because such deviations provide evidence of non-random mating as well as selection or migration, i.e.
when genetically two different populations are being sampled. Analyses of nuclear markers for Bryde’s
whales in sub-area 1 have been conducted and these analyses provide no evidence for the significant
deviations in Hardy-Weinberg equilibrium within sub-areas 1W and 1E which would provide support
for multiple sub-stocks.
Statistical power: The Workshop received a paper, which evaluated the power to detect populationstructure using the chi-square test and Fisher Exact test under an island model in which populationdifferentiation is controlled using a single parameter, Fst. Statistical power of the genetic analysis in
sub-area 1 was found to be high for moderate sample sizes and quite small values for Fst, while it was
higher for microsatellite data than for mtDNA.
e

The Workshop examined the sightings data and agreed that it revealed no evidence of a discontinuity
in distribution within sub-areas 1 and 2. A similar conclusion was reached with respect to catch3

distribution. Discontinuity in the commercial catches identified in earlier meetings merely reflected
operational constraints.
f

External body proportion data (three features) and several biological parameters (body length,pregnancy rate, length at sexual maturity, seasonality in breeding) had been examined using data frompast commercial whaling. Although operational differences (e.g. different minimum length limits forcoastal and pelagic whaling) meant that some comparisons could not be made, the authors of theseanalyses concluded that there were no differences that could not be attributed to operational factors.

gMark-recapture analysis revealed movement of animals within sub-area 1. A very limited number ofmarks were placed in sub-area 2 and while none has been recovered in sub-area 1, the sample sizes forthis sub-area are sufficiently small that even if there is mixing between sub-area 1 and 2, zerorecaptures would not be highly unlikely.

hMark-recapture data are available mainly for sub-area 1.

iOne of the possible explanation for the differences observed in age distribution between sub-area 1Wand 1E+2 is that these differences are real. Other possible explanations were also listed up by theWorkshop.

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Figure 1: Stock structure hypotheses of North Pacific Bryde’s whales retained at the ‘FirstIntersessional Workshop’.

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