The sub-group based its deliberations on the computations set out below, which were carried out by Givens following input from sub-group members

Butterworth, Doug S

Givens, Geof H

Heide-Jørgensen, Mads-Peter

Laidre, Kristin

Witting, Lars

English

Cape Town University OpenUCT

148 REPORT OF THE SCIENTIFIC COMMITTEE, ANNEX EThe sub-group based its deliberations on the computationsset out below, which were carried out by Givens followinginput from sub-group members.Data were available on 614 M and 1891 F minke whalescaught between 1987 and 2006. The response variable was1 if the whale was F and 0 if it was M. Predictor variableswere Year (treated as a continuous variable and expressed asyears since 1986), Month (treated as a factor variable withsum contrasts withApril as the reference group), and Region(three regions northwest (NW), central west (CW) andsouthwest (SW) treated as a factor with CW as the referencegroup). Observations from January, February, and March(n=27) were deleted from the dataset.We fit a standard logistic regression using the glm()function in R. Month and Year were allowed to interact withRegion. Model comparisons were made using the likelihoodratio test. For simplicity, we did not fit an overdispersionparameter, but this should probably be investigated later.There was no significant Region:Month interaction howeverRegion:Year interaction was statistically significant, assummarised below:Model 1: Sex ~ Region + Year + MonthModel 2: Sex ~ Region + Year + Month + Region:YearThe best model has main effects for Year, Month, andRegion, and a Region:Year interaction. Here are theestimated model coefficients:Signif. codes: 0’***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ‘ 1(Dispersion parameter for binomial family taken to be 1)Null deviance: 2763.6 on 2477 degrees of freedomResidual deviance: 2717.4 on 2464 degrees of freedomThese results show that the proportion of females in the SWregion (in the observed dataset) has declined as yearsprogress.We also need to know whether the proportion ofsampled animals (in the dataset) has shifted betweenregions over time. To address this question, we fit anotherlogistic regression. In this model, the response variable was1 if the animal is from SW and 0 otherwise. The predictor isYear. The results are:Model 1: SW ~ 1Model 2: SW ~ Yearwith coefficientsThis second analysis shows that sampling effort (forsexed whales in the dataset) has shifted northward as yearsprogress. The analysis results presented above demonstratethat the female ratio in the SW region has declined over timewhile – simultaneously – sampling has shifted away fromSW. These two trends could offset each other, therebyyielding an apparently flat time series of sex ratios that doesnot fully reflect underlying demography.Given these results, a small working group considered theoptions for modifying the modelling and assessmentapproach to accommodate spatially dependent interannualtrends in sex ratios. First, stratification by area wasrecommended, except that the NW and CW areas should becombined since they show no significant sex ratiodifferences. Second, the proportion of the catch in each ofthe three regions was examined with respect to month andyear (see Fig. 1). This analysis showed that the monthlydistribution of in the two strata (CW+NW and SW) did notvary substantially between the first and second decadesrepresented in this dataset. This supports a decision not tostratify assessment modelling/analyses by month.However, the analyses conducted here used only the mostrecent dataset from Greenland (1987-2006). Therefore, theworking group made two suggestions about the remainingcatch data. First, Cherry Allison should investigate theNorwegian catches to determine if the data suggest a similarspatial division i.e., one area north of, and one other areasouth of, 63.0 degrees north; if they exhibit a similarYear:Region interaction; and if they exhibit a lack ofMonth:Region interaction in the catch proportion data (asseen in Fig. 1). Second, the Greenlandic scientists shouldexamine the dataset from the first Greenland fishery periodto: (a) determine if the data are of sufficient quality toinvestigate spatial division, and (b) if they are, examine thesame questions recommended for investigation with theNorwegian data. It was noted that population modellingwould be simplified if the same spatial division for the laterGreenland period could be used for the two earlier periodsas wellAppendix 4INVESTIGATION AND IMPLICATIONS OF SPATIAL AND TEMPORAL PATTERNS IN SEX RATIO DATAFROM WEST GREENLAND MINKE WHALE CATCHESGeof H. Givens, Kristin Laidre, Lars Witting, Doug Butterworth, and Mads-Peter Heide-JørgensenAnnex E:Layout 1 8/5/08  09:30  Page 148J. CETACEAN RES. MANAGE. 10 (SUPPL.), 2008 149Fig. 1. Proportion of catch by month in NW and CW regions (pooled) and SW separately. Solid lines arecatches in 1987-96 and dotted lines are 1997-2006.Annex E:Layout 1 8/5/08  09:30  Page 149

Investigation and implications of spatial and temporal patterns in sex ratio data from West Greenland minke whale catches.

https://open.uct.ac.za/bitstream/11427/18959/1/Givens_2008.pdf

Investigation and implications of spatial and temporal patterns in sex ratio data from West Greenland minke whale catches.

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