Survival Rate of Released Mink Study - Animal Liberation Front

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1688 T. Maran et al. / Biological Conservation 142 (2009) 1685–1692Table 2Mink radio-tracked in 2000–2003. Maximum survival time – known from yearly live-trapping census following the release operation. These data are not included in the analysisfor the sake of consistency in measuring methods. Average T - Number of generations in captivity. Status – Mink with the following status were censored: Lost – signal lost; Alive– alive at the end of radio-tracking; Collar – radio-collar lost.ID Sex Pregnant Birth date Release date Last recordedsignalSurvival time(radio-tracking)Maximumsurvival timeEnclosuretypeAge Average T Status686 Female No 09.06.1999 28.06.2000 7.07.2000 9 9 Small 1 1.500 Lost695 Male No 18/06/1999 28.06.2000 29.11.2000 154 196 Small 1 3.125 Alive679 Female No 04/06/1999 29.06.2000 6.08.2000 38 38 Large 1 1.000 Signal failure680 Female No 04.06.1999 29.06.2000 16.07.2000 17 17 Large 1 1.500 Dead682 Male No 27.05.1999 29.06.2000 23.11.2000 147 630 Small 1 1.500 Alive697 Male No 18.06.1999 29.06.2000 3.08.2000 35 35 Large 1 1.500 Lost681 Female No 04.06.1999 30.06.2000 3.12.2000 156 156 Large 1 1.500 Alive685 Female No 09.06.1999 30.06.2000 16.07.2000 16 16 Small 1 2.250 Dead696 Male No 18.06.1999 30.06.2000 14.07.2000 14 14 Small 1 2.125 Collar844 Male No 2.06.2000 12.06.2001 23.06.2001 11 11 Small 1 6.866 Dead876 Female No 8.06.2000 12.06.2001 27.06.2001 15 15 Small 1 7.092 Collar866 Female No 6.06.2000 13.06.2001 23.06.2001 10 10 Small 1 7.092 Dead886 Male No 10.06.2000 13.06.2001 20.06.2001 7 7 Small 1 7.092 Collar867 Female No 6.06.2000 16.06.2001 3.07.2001 17 17 Small 1 7.092 Dead894 Male No 13.06.2000 16.06.2001 4.07.2001 18 650 Small 1 6.866 Lost694 Female No 18.06.1999 17.06.2001 2.07.2001 15 15 Small 2 7.479 Collar842 Male No 29.05.2000 17.06.2001 18.08.2001 62 62 Large 1 6.205 Collar891 Male No 11.06.2000 17.06.2001 25.08.2001 69 69 Large 1 1.875 Lost872 Female No 8.06.2000 18.06.2001 26.06.2001 8 8 Small 1 1.875 Lost897 Female No 13.06.2000 18.06.2001 26.06.2001 8 8 Small 1 2.250 Dead898 Male No 15.06.2000 18.06.2001 20.08.2001 63 1020 Large 1 2.375 Lost677 Female No 7.06.1999 19.06.2001 15.07.2001 26 26 Small 2 2.375 Lost678 Male No 4.06.1999 19.06.2001 4.07.2001 15 15 Large 2 1.500 Lost689 Female No 13.06.1999 19.06.2001 25.06.2001 6 6 Small 2 1.875 Lost887 Female No 10.06.2000 19.06.2001 5.07.2001 16 16 Small 1 1.750 Collar683 Male No 9.06.1999 20.06.2001 25.08.2001 66 146 Small 2 1.750 Alive856 Female No 5.06.2000 20.06.2001 27.06.2001 7 7 Large 1 1.500 Collar772 Male No 7.06.2000 28.06.2001 1.07.2001 3 3 Small 1 3.250 Collar830 Male No 19.05.2000 28.06.2001 19.07.2001 21 21 Small 1 3.250 Collar862 Female No 5.06.2000 28.06.2001 2.07.2001 4 4 Small 1 2.563 Dead868 Male No 7.06.2000 28.06.2001 19.07.2001 21 21 Small 1 2.563 Dead880 Male No 10.06.2000 28.06.2001 25.08.2001 58 198 Small 1 1.500 Alive832 Female Yes 19.05.2000 4.05.2002 5.05.2002 1 1 Small 2 2.250 Dead852 Female Yes 2.06.2000 4.05.2002 9.05.2002 5 5 Small 2 2.250 Collar881 Female Yes 10.06.2000 4.05.2002 9.05.2002 5 5 Small 2 2.500 Dead904 Female Yes 17.05.2001 4.05.2002 11.07.2002 45 45 Small 1 2.250 Collar853 Female Yes 2.06.2000 15.05.2002 22.05.2002 7 7 Small 2 2.125 Dead906 Female Yes 17.05.2001 15.05.2002 27.05.2002 12 12 Small 1 2.250 Lost560 Female No 25.05.1998 27.05.2002 5.06.2002 9 9 Small 2 1.750 Dead592 Female No 9.06.1998 27.05.2002 15.06.2002 19 19 Small 2 1.750 Dead927 Female Yes 2.06.2001 27.05.2002 21.08.2002 86 305 Large 1 1.938 Alive935 Female No 4.06.2001 27.05.2002 4.07.2002 38 38 Small 1 1.500 Dead952 Female No 14.06.2001 27.05.2002 11.06.2002 15 15 Small 1 2.625 Dead1014 Female No 31.05.2001 27.05.2002 2.06.2002 6 6 Small 2 1.750 Dead911 Female Yes 30.05.2001 27.04.2003 5.05.2003 8 8 Large 2 6.376 Dead980 Male No 7.05.2001 27.04.2003 30.04.2003 3 3 Large 2 7.979 Lostof release). The priority of this project was the successful release ofvaluable specimens of the rare European mink, and of their welfare.This led to some constraints on the experimental design,and we are alert to the resulting partial lack of orthogonality betweenvariables. Although this reduces the power of our final conclusions,we make this explicit and believe that our findingsnonetheless add significantly to the scientific basis for future releasesof this, and similar, species.Kaplan–Meier survival analysis was used (Muenchow, 1986;Pollock et al., 1989a, 1989b; Scheiner and Gurevitch, 2001) to analysethe overall survival curve and the effect of sex and housing onsurvival.The multivariate Cox proportional hazard regression (Muenchow,1986; Scheiner and Gurevitch, 2001; StatSoft, 2007) was appliedto identify the model best fitting our data. The general modelwas reduced stepwise, following the Akaike’s Information Criterion(AIC) and Schwarz’s Bayesian Information Criterion (Schwarz’s BIC;Burnham and Anderson, 2002). To reduce the effect of correlationbetween variables the data subsets with eliminated correlatingvariable were used.Two data subsets were used for modelling:1. Yearlings within 2000–2001(N = 27). The main purpose of themodel was to test the impact of yearly variation on survival.2. Yearlings and adults in 2000–2003 excluding pregnant females(N = 38).Kaplan–Meier survival analysis was applied to three datasets:3. Yearlings and adults of both sexes released in 2000–2003 fromstandard enclosures (N = 28). This dataset eliminated any effectof different housing conditions, and provided an opportunity todistinguish the survival of different sexes. Pregnant femaleswere excluded.4. Datasets of males (N = 17) and females (N = 21) released in2000–2003. These datasets were used to test the effect of housingconditions while controlling for any effect of sex. Pregnantfemales were excluded.5. Females, both pregnant and non-pregnant, released from standardenclosures in 2002 (N = 11). This dataset was used to testfor any effect of pregnancy.
T. Maran et al. / Biological Conservation 142 (2009) 1685–1692 1689The Systat v 12 for Windows (Systat, 2007) was used to analysethe survival distribution and to model our data with Cox proportionalhazard regression.2.6. Co-variables measured (Table 2)2.6.1. Average number of generations in captivityThe number of generations in captivity depends upon thelength of pedigree lineages of all the ancestors of a given individual.The number of generations in captivity is therefore best characterizedby the average length of all these ancestral lineages.These values were calculated with SPARKS v1.55 software (ISIS,2007). The animals were divided into two groups: an average of3 generations in captivity (n = 12). Data for pregnantfemales were omitted.2.6.2. Housing conditions before releaseThe animals were kept in enclosures of two types (data for pregnantfemales omitted):1. Small enclosures: twenty 2 4 m enclosures located in tworows separated by a service corridor. The enclosures were furnishedwith a few stumps and pipes for environmental enrichment.Each enclosure included one pool 30 50 cm in area and25 cm in depth. Adjacent enclosures were separated with doublewelded wire above a 50 cm tall plywood partition at theground level. The animals were fed and the enclosures cleaneddaily. A total of 28 individuals was released from theseenclosures.2. Large enclosures: four 25–50 m 2 pre-conditioning enclosuresconstructed of welded wire net encompassing natural areaswith trees, bushes and stumps. Each enclosure includes a naturalisticpond (approximate area 1.5 1.5–2.5 2.5 m anddepth 1 m). Contact with keepers was minimal. A total of 10individuals was released from these enclosures.3. Results3.1. Overall survivalMost mortality was recorded during the first 30 days followingrelease (Fig. 2). The mean survival time was 54 days and 25% of themortality occurred during the first 16 days (95% Conf. Lower – 10days; 95% Conf. Upper – 22 days).3.2. Cox proportional hazard analysis3.2.1. Dataset 1. Yearlings within 2000–2001 (N = 27)According to the Cox proportional hazard regression analysis(Statistic = 11.391; df = 4; P = 0.023), at least one of the candidatedependent variables was significantly related to survival (generalmodel in Table 3). There is no evidence of an effect of history incaptivity or of a year of release. According to this general model,sex has a strong and significant effect. The reduced model confirmthe effect of sex on survival (Statistic = 9.755; df = 2; P = 0.008; Table4). The omission of the year and the generations from this modelimproved its fit to our data.3.2.2. Dataset 2. Yearlings and adults in 2000–2003. (N = 38)Considering that fitting the model to dataset 1 had shown thatinter-annual differences did not explain significant variation, weexpanded the dataset with results from 2002/2003 and repeatedthe model fitting exercise. The results were similar. The generalmodel showed that at least one of the variables had significant10080Survival curveConf.int. 95%2.6.3. Age classesThe released animals were divided into two age classes (pregnantfemales excluded):1. Yearlings: animals released during the second warm seasonafter birth (n = 29).2. Adults: animals released later than the second warm seasonafter birth (n = 9).2.6.4. SexTwenty-one females and 17 males were included in the CoxProportional Hazard Regression (having excluded pregnantfemales).2.6.5. Pregnant femalesOnly females released from standard enclosures in 2002were included in testing the effect of pregnancy (N = 11). Six ofthese were pregnant and released during their third week ofpregnancy.2.6.6. Year of releaseThe impact of years to survival was tested in two datasets: (a)release of yearlings in 2000–2001 (N = 27) and an expanded datasetfrom 2000–2003 (N = 38).2.7. Causes of mortalityField signs were used to ascertain, as far as possible, the causesof death of radio-tracked mink.Survival (%)60402000 20 40 60 80 100Time (Days)Fig. 2. Overall cumulative survival curve of released European mink (N = 38).Table 3General model for Dataset 1 of yearlings in 2000–2001 (N = 27). Cox proportionalhazard regression analysis (the value of estimate means the covariate increases thehazard, that is – decreases the survival; AIC = 42.218; Schwarz’s BIC = 42.536).Estimate Standard error Z P-valueSex 2.406 1.023 2.351 0.019Year 0.628 1.040 0.604 0.546Housing 1.983 1.331 1.490 0.136No of generations 0.299 0.931 0.321 0.748.
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