ABSTRACT Despite recent local reintroductions of the Eurasian lynx _Lynx lynx_ in central and north-eastern Poland, the increase in its population was not followed by its westward expansion.

To address this problem and restore the lynx population in north-western Poland, 61 captive-born individuals of Baltic population origin were released in the province of Western Pomerania

in 2019–2021. Prior to their release, all the animals underwent an adaptation training phase. They were then set free according to a hard-release protocol and subsequently monitored by means

of GPS telemetry. In order to assess the short-term reintroduction success, the survival and causes of death of the released individuals were studied as a function of sex, age, training

the older the lynx was when released, the better its survival changes. In contrast, the longer the training time, the poorer were the chances of survival. There was no evidence of any effect

of sex, month of release or place of release. Based on these results, recommendations were made for the planning of further releases and measures to manage the restored population. SIMILAR

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LYNX (_LYNX LYNX_) IN THE WESTERN CARPATHIANS Article Open access 16 September 2024 INTRODUCTION The Eurasian lynx _Lynx lynx_, L. 1758 (henceforth—lynx) is the largest felid in Europe and

the third largest representative of the order Carnivora on the continent, after the brown bear and the grey wolf1,2. The species used to be widely distributed throughout Europe, but by the

early twentieth century it had become extirpated from most of the areas it had occupied in western and central Europe. By that time, the lynx’s range had contracted to a historical

minimum4,5. The rapid decline in its populations was caused by deforestation, loss of prey, habitat fragmentation and overhunting3,4,5,6. At present, the European range of the lynx covers

approximately 813,400 km2, i.e. the area occupied by five autochthonous populations in Fennoscandia (Scandinavian and Karelian), the south-eastern Baltic catchment area, the Carpathians and

Balkans, as well as six restored populations in central and western Europe (Dinaric, Bohemian-Bavarian, Alpine, Jura, Vosges-Palatinian and Harz Mountain)6,7. At present, lynx are found in

23 European countries6. Some 9000–10,000 individuals are estimated to be persisting in the wild in continental Europe, excluding Russia and Belarus7. The situation of the lynx over the

centuries in Poland is a good illustration of its predicament in central Europe: deteriorating ecological conditions for its existence coupled with negative human attitudes led to its

disappearance, first from the lowlands and later from mountain areas, by the end of the nineteenth century2,8,9. Already by the late eighteenth century the lynx had become extinct west of

the River Vistula, and further disappearances took place farther east in subsequent decades9. The lynx was eradicated from north-western Poland between the fifteenth and eighteenth

centuries9. By the mid-nineteenth century, therefore, the Baltic and Carpathian populations had become isolated from each other9. The situation worsened over the next hundred years, so that

by the start of the Second World War only two populations remained—one in the Białowieża Primeval Forest and the other in the Carpathian Forest9. In the post-war years, however, the

situation started to improve, and the lynx’s population in Poland slowly rose9,10,11, culminating in some 600 animals in the 1980s8. Thereafter, it started to decline, and by the beginning

of the twenty-first century the population had fallen to an estimated 200 individuals, scattered in local natural populations in north-eastern and south-eastern Poland, with just a few

dispersing animals occasionally recorded in north-western Poland and the Sudety Mountains5,10,11,12. At the end of the 2000s, there were an estimated 285 lynx in the country13. The

approximate extent of the lynx’s current range in Poland is 10,800 km2 12, but this population increase has not been matched by a corresponding range expansion in the country, even after it

had been granted strict legal protection in 199511. At the European scale, the decline of the lynx population and the shrinkage of its distribution were both arrested in the mid-twentieth

century5. A combination of several species-enhancing factors, including containment of large-scale deforestation, improvement of the situation of the roe deer population—the most important

prey species, a significant reduction in the scale of lynx hunting, as well as granting the species legal protection, has enabled the lynx to recolonize parts of its former range3,5,6.

Consequently, the overall situation of the species has much improved recently, which is reflected in its categorization as “least concern” (worldwide) by the International Union for

Conservation of Nature (IUCN) Red List of Threatened Species14. Nevertheless, some local populations in Europe are endangered because of demographic instability, limited ecological

connectivity and the consequent restricted gene flow, as well as anthropogenic mortality (i.e. illegal kills)6,15,16,17,18,19,20. In order to further improve the lynx’s situation in Europe

and restore its local populations, 15 reintroduction projects, involving over 170 individual animals, were carried out across eight countries from the 1970s to the 2000s3. Five of these

attempts have been successful3. Three reintroduction programmes have so far been carried out in Poland. In 1993–2000 a small, viable population was formed from captive-born individuals in

central Poland in the Kampinos National Park8,21. Between 2003 and 2006 another reintroduction of captive-born lynx took place in the Pisz Forest in north-eastern Poland3. In 2012–2015, a

combination of zoo-originated and translocated wild animals were released in the Pisz Forest and in the neighbouring Napiwoda-Ramuki Forest22. The greatest potential barriers to the lynx’s

range expansion in Poland, mainly towards the west, are habitat fragmentation by non-forested and urban areas, and transportation infrastructure23,24. In addition, the genetic variability of

the autochthonous population in north-eastern Poland is lower than that of the populations in Estonia and Latvia; the former being isolated, it suffers from limited gene flow. This implies

the need for further active conservation measures to restore ecological connectivity, support natural expansion and increase the viability of native and reintroduced populations18,25,26. The

current reintroduction project has been conducted since 2017 by the West Pomeranian Nature Society, in cooperation with the Mammal Research Institute of the Polish Academy of Sciences in

Białowieża, the Cultural Center in Mirosławiec and the World Wide Fund for Nature (WWF) Poland. The project aims to restore the population of the lowland lynx in north-western Poland and to

ensure appropriate conditions for its development27. The very limited westward expansion, if any, of the lynx into the western lowlands of Poland, and often the lack of appropriate

monitoring and evaluation of reintroduction programmes justify the properly planned, continuous tracking of the fate of the reintroduced animals. This will enable the short- and long-term

evaluation of current reintroductions and inform future projects of this kind3,8,11. The IUCN defines post-release monitoring as “the means to measure the performance of released organisms

against objectives, to assess impacts, and provide the basis for adjusting objectives or adapting management regimes or activating an exit strategy”28. Of key importance is the assessment of

demographic performance, including the monitoring of population growth and spread in order to estimate individual survival, reproduction and dispersal28. In addition, such monitoring serves

to coordinate conservation measures for the species, including the ongoing management of its populations, in the whole region. To address these issues, we assess here the short-term

reintroduction success of the “Return of the Lynx to north-western Poland” project, based on post-release survival indicators. The aim of this study was to evaluate the longevity and

adaptation to life in the wild of the released animals, assuming that greater survival (and independence of humans) is expressed by the effectiveness of hunting, and subsequent mating that

ends in parturition and the rearing of kittens. The effects of sex, age, time spent in captivity, release date and location were determined. The causes of lynx deaths were also reported and

analysed. Based on the conclusions drawn, recommendations were made for the pre-release, as well as for further stages of reintroduction and management measures of the restored lynx

population in north-western Poland. All the analyses were performed for the data as of 30 September 2021, as the reintroduction project is being continued. We hypothesized that traffic

collisions were the main mortality factor29. Data on post-release migration and spatial distribution, as well as reproductive success will be published in a separate paper. RESULTS FATE OF

THE RELEASED ANIMALS Sixty-one Eurasian lynx—35 males and 26 females—were released between January 2019 and July 2021. Thirty-seven of these 61 animals, including one female caught after

release and re-released in another location, were alive in the wild as of 30 September 2021. Field observations, conducted as part of the post-release monitoring, indicate that these animals

have settled well in the wild, are efficient hunters and are only very occasionally sighted by people. Two males were caught soon after being released and were placed permanently in the

breeding centre. In one case this was due to the need for treatment, and in the other because the animal would persistently remain near human settlements, sometimes even entering farmhouses.

In the period in question, ten certain cases of reproduction of released individuals were recorded: six females had one litter and two others gave birth twice in two consecutive years.

Fifteen of the released animals died in the wild. The fate of seven others is not known because there was no GPS contact (telemetry collar malfunction or, in one case, its loss). Figure 1

summarizes the fate of all the individuals released, separately for males and females, while Table 1 sets out details of the individuals, i.e. sex, date of birth, age, time spent in the

adaptation enclosure and release centre, date and place of release, date of death, survival time and fate. The mean age of the released males is 36.6 months, and females 35.1 months, while

the mean training time of males and females is 566.1 and 532.5 days, respectively. SURVIVAL RATE The overall survival rate (covering the period from the first release, i.e. 23.01.2019, to

30.09.2021) of the released animals (N = 52) was 71.15%; the figure for males (N = 28) was 64.29% and for females (N = 24) 79.17%. There was no significant difference between the sexes in

this respect (chi-squared test, _P_ = 0.24). Assuming that the fate of lynx from which telemetry data are no longer received is known, this parameter would be 74.58% for all released

individuals (N = 59), 69.7% for males (N = 33) and 80.77% for females (N = 26) if these animals were still alive, or 62.71% for all animals (N = 59), 54.55% for males (N = 33) and 73.08% for

females (N = 26) if these animals were dead. The median survival time (N = 15) was 202 days (males—186.5 days, females—202 days). No significant differences were found between the sexes

(Mann–Whitney _U_-test for comparisons of median values, _P_ = 0.95), but there was much greater variability in life expectancy in the wild for females (Fig. 2). Most mortality was recorded

during the first 300 days following release (Figs. 3 and 4). The overall probability of surviving beyond three, 12, 18, 24 and 30 months was equal to 92.2%, 73.8%, 70.1%, 65.4% and 60%,

respectively (N = 52). The survival time for males declined to a plateau of about 60% in 680 days, while the fall in female survival was slower, declining to a plateau of about 55% in 740 

days. Taking into account the age of sexual maturity (3 years for males and 2 years for females5), the released animals survived in the wild for 1.65 mating periods on average, during which

they had the opportunity to mate and breed (N = 20). For males (N = 8) this average was 1.75, and for females (N = 12) 1.58 mating periods (Fig. 5). COX PROPORTIONAL HAZARD ANALYSIS

According to the Cox proportional hazard regression analysis (Table 2), the following variables were significantly related to survival—age at release (P = 0.0002, Hazard ratio = 0.718),

training time (P = 0.0003, Hazard ratio = 1.011), defined as the number of days spent in adaptation enclosure and release centre, and year of release (P = 0.0117, Hazard ratio = 0.197) (N = 

50). No evidence was found for any effect of sex, month of release or place of release. CAUSES OF DEATH The cause of death could not be determined in 6 out of 15 cases of mortality among the

released animals recorded in 2019–2021 (Fig. 6). The majority of deaths (55.6%) were attributed to environmental factors, i.e. scabies (sarcoptic mange), and four (44.4%) were caused by

humans (animals hit by a road vehicle or, in two cases, by a train). There are indications that one of the animals may have been killed by a poacher, one may have died of an unknown

infectious disease and one may have been killed by a predator (possibly another lynx). However, there is no hard evidence to substantiate these suspicions. There was a statistically

significant difference in the life expectancy of lynx in the wild from the day of release to the day of confirmed death caused by a collision with a road vehicle or train and by other causes

of death, most probably infection with environmental factors, like scabies (Mann–Whitney _U_-test, _P_ = 0.015). This indicates that the limiting value to which death was caused by road and

rail collisions is 200 days after release. After this period, environmental factors (scabies) was the main cause of death of animals in the wild (Fig. 7). DISCUSSION The success of a

long-term reintroduction programme is assured when a viable, sustainable, ecologically and genetically functional population has been restored in the wild. The effectiveness of measures

implemented to achieve this goal can be assessed with the aid of post-release monitoring, defined by indicators of short-term reintroduction success, i.e. survival of released animals and

breeding by released animals30. The very effectiveness of these measures depends on the animals’ hunting efficiency (adaptation to life in the wild conditioned by pre-release training) and

the avoidance of human-carnivore conflicts (appropriate reintroduction programme design and conflict management in the later stages), with subsequent mating that ends in parturition and the

rearing of kittens (reproductive success)31,32. On average, seven out of every ten lynx released in 2019–2021 in north-western Poland, whose fate is known, were alive as of 30 September

2021. This tallies with survival rates between 63 and 95% for this species reported by other authors (as revised by Franz and Romanowski8). The mean survival time (from release to recorded

death) of animals released during the above-mentioned three-year period was 275 days (5–870 days). However, one male—still alive—has so far spent 981 days in the wild. This mean value is

comparable with that for the reintroduced population in the Kampinos National Park in Poland (272 days)8. The average age of lynx released in north-western Poland is, as of 30 September

2021, slightly above 37 months. The Eurasian lynx is reported to live up to 17 years in the wild, but the average age of resident animals in a population is only about 4–5 years5. At the

same time, it is estimated that under natural conditions, only 24% of lynx lived for three years in the Białowieża Forest (Poland)33. A study carried out in the Bohemian Forest, on the

Czech-German border, indicated that individuals up to 4 years of age made up 64% of the entire population and that the mean generation time for resident reproducing females was 2.64 years34.

Meanwhile, the probability of reaching the age of 2.5 years for lynx reintroduced to north-western Poland was 51.1%. The sex of the animals had no effect on the probability of survival,

which suggests no effect of sexual dimorphism. Differences in the probabilities of male and female survival have been found among other carnivores, both in species with (e.g. European mink

_Mustela lutreola_35) and without sexual dimorphism (e.g. swift fox _Vulpes velox_36). When assessing the reproductive success of the reintroduced lynx, one has to assume that the number of

mating seasons during which they could reproduce is comparable with the number of litters, i.e. mating seasons during which animals successfully reproduced. In the case of the lynxes covered

by the “Return of the Lynx to north-western Poland” project, ten litters were recorded in 2019–2021, which translates into 30.3% of the chances of mating and successful breeding. On the

other hand, the fact that eight females had a litter at least once means that 80% of the sexually mature reintroduced animals (83.3% of females) reproduced at least once in the wild in

2019–2021. For comparison, only 35% of the released females in the population reintroduced to the Kampinos National Park were found to be breeding37. The effect of the covariates on the

survival curve was shown to be strong and significant for the age at release and the time spent in the adaptation enclosure and release centre (training time), while year of release had less

significance. The hazard ratios of covariates, explaining the multiplicative effects on the ratio of death, indicate in particular that being older when released augurs well for survival in

the wild, whereas a longer training time is linked with poor survival prospects. The results therefore suggest that the post-release survival of younger reintroduced individuals is lower,

probably because they have a lower potential for coping with the stress associated with translocation than older ones38. The relationship between the survival rate of reintroduced animals

and the year in which they were released is governed by stochastic meteorological factors (obviously independent of humans) and therefore cannot be mitigated in the hard-release scenario. To

overcome interannual variation as regards climate and infrequently occurring natural disturbances, IUCN recommends releasing individuals over several years28. It should also be noted that

the number of releases in a given year may distort the interpretation of the results, as more releases in later years translates directly into fewer days spent in the wild by late-released

animals. There is no evidence of any effect of sex, month of release and the place where the animals were released on the life expectancy of reintroduced lynx in the wild. Both, sex of

released individuals and the place of release (analysed in terms of linear distance dependence on the training centre) makes no statistically significant contribution to the difference in

the hazard ratio after adjusting for age, sex, month, year and training time. In the case of lynx reintroduced to north-western Poland, releasing the animals in the earlier months of the

year did not appear to favour their greater survival. Such a dependence was demonstrated by the results of an experiment demonstrating that the survival of released least weasels _Mustela

nivalis_ improved when releases took place during a season with abundant food resources39. The Canada lynx _Lynx canadensis_ reintroduction protocol also recommends releasing them in the

spring months, so to ensure the highest annual abundance of prey40. The insufficient amount of data probably determined the lack of significance of survival in relation to a place of

release, determined by the distance from the training centre. Despite some tendencies reported by practitioners, this issue requires further investigation. According to Maran et al.35 the

following factors potentially influence the survival of released animals: biological variables (age, sex), pre-release factors (maintenance conditions, pre-conditioning, experience with

humans), release methodology (hard or soft release), release site characteristics (availability of suitable shelter and habitat, abundance of predators, availability of food resources, level

of disturbance). The biological variables include the genotype and thus the origin of the animals. The use of wild animals in mammalian carnivore restoration is sometimes claimed to be more

successful than the use of captive-born ones38,39. In contrast to the above statement, the use of captive-breed Eurasian lynx is recommended for reintroductions41. The lynx recruited for

the reintroduction programme in north-western Poland were of captive origin and belonged to the Baltic population. Two aspects favour this approach. Firstly, the reintroduction carried out

in the Harz Mountains (central Germany) demonstrated that zoo-born individuals were able to adapt to life in the wild, and secondly that a considerable advantage of captive-bred animals over

wild ones is the possibility of conditioning their genetic profile and health status before release41. An analysis of previous restitutions of mammalian carnivores points out that far too

little attention is paid to sociological and organizational variables20,42. The latter include pre-release treatment, behavioural conditioning (training) and pre-release housing conditions

(active adaptive management)28,35,43. As it turns out, the previously mentioned claim regarding the greater survival chances of reintroduced wild-born animals is based largely on the fact

that they are better able to tolerate translocation stress38. At the same time, many authors emphasize the possibility of a positive impact of appropriate pre-release training (learning

effect) on shaping the behaviour of individuals intended for reintroduction (e.g. inducing development of “natural” behaviours, familiarity with the natural habitat and avoidance of danger),

increasing their post-release chances of survival and life span8,28,30,38,44,45. The results indicate that pre-release training (enclosure effect) significantly influences the survival of

reintroduced animals, but in a rather non-obvious way: remaining too long in the adaptation enclosure and the release centre is negatively correlated with the number of days that the

released animal survives in the wild. While the training itself is undoubtedly of great importance in adapting to post-release life in the wild, too much of it can produce a side effect in

the form of pre-release habituation to captivity, to humans and, generally, to stimuli that may pose a danger after release, as has been shown, for example, in the case of released swift

foxes in North America44. The mortality of reintroduced animals is a derivative of biological and sociological variables. It is commonly believed that most lynx mortalities in Europe are

caused by humans and result from recreational harvesting, poaching and vehicle collisions5,8,20,46,47. In the reintroduced population in north-western Poland, the only confirmed causes of

animal death were environmental factors (scabies) and traffic collisions. Moreover, the numbers of the former, i.e. death from a natural cause, were slightly higher. Likewise, in a strictly

protected area in the Bohemian Forest, lynx were not found to be strongly affected by human-related mortality34. By contrast, a study in the Swiss Alps and the Jura Mountains (Switzerland)

showed that natural causes accounted for 44.9% of all established causes of death in this region and that infectious diseases accounted for 26.5% of them. Scabies alone accounted for only

6.1% of deaths from a known cause48. Sarcoptic mange, caused by _Sarcoptes scabiei_, is a highly contagious epizootic skin parasitosis, distributed worldwide and observed most frequently in

mammals49. Occasional cases of scabies as the cause of death of lynx in Europe have been reported from Norway50, Sweden51, Switzerland52 and Poland49. The source of infection in lynx is

likely to be the red fox _Vulpes vulpes_, in which case _S. scabiei_ must be endemic in some parts of Europe52,53. The recently upsurge in reports of outbreaks in new locations suggest that

this parasite is expanding across the continent53. One of the most interesting conclusions from the results of the present study is the clear distinction in life expectancy of reintroduced

lynx that died as a result of traffic collisions and as a result of environmental factors, i.e. scabies infection. These results indicate that the former is the most serious cause of death

in newly released animals (up to 200 days in the wild), while the latter represents the main known cause of mortality in animals surviving more than 200 days in the wild. This fact may be of

great practical importance in terms of modifying pre-release training, but the topic needs to be further explored. Even if “quick learners” cannot avoid the natural, objective threat of

scabies, their ability to avoid traffic accidents does enhance their chances of survival in the wild. Numbers of observations being limited, it is recommended that further data should be

collected on lynx mortality and that analyses in this respect should be continued. It should also be borne in mind that appropriate pre-release training to avoid danger can have a

considerable effect on lynx survival, as well as the implementation of conservation measures improving the safety of wild animals vis-à-vis road and rail infrastructure, such as under- and

overpasses, reduced speed zones, and fencing8. The effectiveness of such an approach has been demonstrated in reducing the effect of road mortality on the Iberian lynx _Lynx pardinus_ in

Spain54, and the need for vehicle strike mitigation in the case of the bobcat _Lynx rufus_ in Ohio (U.S.A.)55. The results obtained for north-western Poland (in particular, the lack of

significant human-lynx conflict cases revealed by the three-year post-release monitoring) augur well for the possible coexistence between lynx and humans, also reported from other parts of

Europe, thus indicating that they can share the same landscape7,56,57,58. Cases of felids capable of persisting even in heavily human-modified landscapes (_Panthera tigris_, _Panthera

pardus_, _Puma concolor_) were reviewed by Bouyer et al.56. In conclusion, although the present study was to some extent limited by the sample size, the short-term results of the Eurasian

lynx reintroduction programme in north-western Poland provide the most comprehensive and substantial scientific basis to date for planning the restoration of this felid in the Southern

Baltic Lake District, and also inform future releases of this felid in similar conditions. The following conclusions and recommendations can be formulated, based on the results obtained and

literature review: * 1. In order to ensure better survival, it is preferable that mature animals be released. * 2. The pre-release training of animals should not last too long and should

focus on arousing fear in them of road vehicles and trains. * 3. As the first 300 days after release are critical for the release operation (the period with the highest mortality), any

interventions supporting the survival of the released individuals should focus on this period. * 4. Telemetry collars make it possible to precisely monitor the fate of the released animals,

thus enabling a comprehensive assessment of the short-term effectiveness of the reintroduction measures. * 5. Monitoring of the reproduction, movements and behaviour of the released animals

should be continued (telemetry collars replacement and maintenance) in order to assess the long-term effectiveness of the reintroduction project. It should be also noted, that despite the

promising results regarding the survival and reproduction rate of the released animals, it seems advisable to continue the reintroduction project of the Eurasian lynx in north-western Poland

at least until the number of kittens born in the wild begins to compensate for the losses due to sub-adult and adult mortality. Termination of the project at the present stage would mean

the gradual disappearance of the population hitherto created, as shown by the experience from central Germany19. However, further research into the reproductive success of the reintroduced

population is necessary. METHODS STUDY AREA The area selected for reintroduction (Ińsko Lakeland, Drawsko Military Compound and Mirosławiec Forestry; Fig. 8) is situated in the Southern

Baltic Lakeland (province of Western Pomerania). It is a young post-glacial landscape, well forested (44% of the area is woodland), with minimal fragmentation (a dense network of forested

ecological corridors) and large numbers of roe and red deer (720 kg/km2)26. Tree stands more than 100 years old account for c. 10% of the forest area59. The dominant tree species is pine

(nearly 60% of all the forest trees), and the amount of harvested timber is 572.1 m3 per 100 ha of forest59. The average annual air temperature is 10.8 °C, with extremes of − 30 and 37.8 °C.

The total annual precipitation is 705 mm, and the averages of wind speed, insolation and cloudiness are 3.5 m/s, 1.976 h and 4.9 octants, respectively59. 21.8% of the area consists of land

subject to various forms of legal protection (national parks, nature reserves, landscape parks, etc.)59. The region is predominantly rural (93.8%), with an average population density of 74

inhabitants per km2. At the same time, 68.4% of the population lives in urban areas59. The densities of main roads, secondary roads and railways are 0.08, 0.10 and 0.05 km/km2,

respectively26,59. There are 0.78 road vehicles per inhabitant59. It is estimated that the project area can host at least 80 lynx, guaranteeing a survival of 100 years at the level of 57% if

isolated26,60. ORIGIN OF THE RELEASED ANIMALS The Eurasian lynx reintroduction programme in north-western Poland involved 61 captive-born individuals originating from the Baltic population,

imported from European zoos and enclosures (not associated with the European Association of Zoos and Aquaria, EAZA), and two animals born in the Dłusko breeding centre (Table 1)26,27.

Individuals kept in this breeding centre derive from the EAZA breeding programme, for which a European studbook (ESB) for Northern lynx _L. l. lynx_ was established. Only individuals that

genetically did not differ significantly from wild lynx within the Baltic population were included in the breeding and reintroduction programme (each individual was subjected to genetic

testing prior to release). RELEASE METHODOLOGY The reintroduction was carried out within the framework of the EU-financed project “The Return of the Lynx to north-western Poland”. The

conditions for reintroduction were laid down in the decisions of the General Directorate for Environmental Conservation (DZP-WG.6401.08.10.2017.JR.bp of 2017, DZP-WG.6401.08.2019.bp of 2019,

and DZP-WG.6401.08.46.2019.TŁ of 2020) and followed IUCN guidelines28. All experiments were performed in accordance with the Polish nature protection law, regulating the proceedings

(restrictions and prohibitions) with wild animals subject to legal protection in the territory of the Republic of Poland—Act of April 16, 2004 on nature protection (Journal of Laws of 2004,

No. 92, item 880). After quarantine and acclimatization, during which the lynx were carefully monitored (photo-traps, CCTV system, direct observation during feeding), they were assessed for

the next pre-release steps. The period of adaptation was individually adapted to the behaviour of each animal. Most lynx required only a short, 1–2 week training period before release, but

some others needed more training and stayed longer in the semi-natural environment of the adaptation enclosure in Dłusko. The Dłusko facility has six enclosures from 0.5 to 0.8 ha in size,

located within a fenced area (enclosure) of 90 ha. Careful observation made it possible to select lynx for breeding, which were then moved to the breeding centre in Jabłonowo (facility of

the West Pomeranian Nature Society) because of their low degree of anthropophobia. Secretive animals, exhibiting shy, cautious behaviour and avoiding contact with humans, were intended for

release and trained for hunting wild prey. It should be noted, however, that the lynx's strong hunting instinct does not require reinforcement and the main behaviours to be enhanced

were avoidance of humans and arousing fear of humans26,27. Their contacts with keepers were thus reduced to a minimum and restricted to unavoidable husbandry. During the entire pre-release

training period, the animals were fed exclusively with the meat of their natural prey. Lynx intended for release were moved to the outside enclosures of the release centres in Mirosławiec

and Drawsko (situated in forests), where they were initially fed, after which feeding was limited and the lynx were left outside the enclosure, usually after 1 week. During adaptation period

animals were kept in natural enclosures, with trees, bushes, fallen trees, branches, stones and several hiding places (in the case of the adaptation enclosures in Dłusko, and release

centres in Mirosławiec and Drawsko; Fig. 8). Related individuals were released in different locations, except for mothers and their kittens. Releases took place in different months

throughout the year, in forested areas away from human settlements, and followed a hard-release protocol. The age, sex, release date and place, and time spent in the adaptation enclosure and

release centre were recorded for every individual released. Due consideration was given to animal welfare at all stages of the reintroduction process. RADIO-TRACKING AND POST-RELEASE

MONITORING All released lynx were tracked using GPS telemetry. Each released individual was fitted with a GPS/GPRS/VHF telemetry collar weighing from 280 to 320 g, matched to the animal’s

size. The collars, manufactured according to an original design by the West Pomeranian Nature Society, have a GPS locator that records a lynx’s position every 3 h, a GPRS/GSM communication

module that transmits data to telemetry servers twice a day, and a continuously operative VHF transmitter with an independent power supply, enabling the animal in question to be sought in

the field. The maximum estimated working time of the GPS module is 15 months, whereas the VHF transmitter can work for up to 3 years. The collar is equipped with a mechanical dropper, which

allows a lynx to lose its collar after about 2–2.5 years. Collars with dead batteries were replaced with new ones. All the antennas are concealed in the telemetry device case and collar

strap, as a result of which the collar has no protruding components. With telemetry data one can determine with a high degree of probability the sites where hunting has been successful. In

the first period after release, all the sites where prey was caught were checked to assess the degree of independence of the released animals and the type of catch. In the following months,

all lynx that exhibited atypical movements or were inactive (stationary) for a suspiciously long time were tracked and monitored in the field. Where necessary, on site observations enabled

the cause of unusual behaviour or death to be discovered. Individuals lost to radio-tracking and not subsequently recaptured were assumed to have died (designated as "unknown

status" and excluded from further analysis). Post-mortem examination was conducted by a qualified veterinarian, in accordance with routine veterinary medicine protocols and post-mortem

procedures for wildlife veterinarians61, on all dead lynxes found in the field, in order to determine a proximate cause of death. The examination included a systematic necropsy, except in

cases where obvious traumatic traffic accidents caused the death (X-ray imaging performed if needed), and was focused on the following etiological factors: congenital, infectious,

nutritional, toxicological, traumatic, other potential reasons founded on generalized etiologic principles (politraumatic events, multi-raility of limbs, fractures, traumatic haemorrhage).

Scabies was diagnosed based on a visual observation of skin lesions and confirmed by detecting the mites in skin scrapings with microscopy. Mites were collected by scraping affected skin

areas with a sterile surgical blade, following by placing samples on a glass plate, dripped with liquid paraffin and consequently covered with a glass cover and then examined microscopically

under 100–400 × magnification. Mites were morphologically identified as _S. scabiei_ using a diagnostic reference62. The submission of tissue samples for further diagnostics was based on

necropsy findings when additional insight was warranted to confirm or determine the cause of death, and included microbiological (bacteriological culture), biochemical (c-reactive protein)

and toxicological (coumarin and coumarin derivatives, ethylene glycol, strychnine, organophosphorus insecticides, bromethalin, petroleum, toxin alkaloids, cicutotoxin, organochlorine

insecticides and mycotoxins) analyses commissioned to reference laboratories. During the necropsy examination all samples were taken in rigorous asepsis conditions and transported for the

further analyses as soon as possible. All post-mortem examinations were documented (descriptively and photographically). SURVIVAL ANALYSES The survival rate was calculated as the percentage

of individuals remaining alive as at 30 September 2021 in the pool of animals released into the wild and not caught and placed in the breeding centre, and whose fate in the wild was known (N

 = 52). Lynx whose fate in the wild was not known were included in the analysis of hypothetical survival rates calculated on the assumption that these animals were alive or dead (N = 59).

Life expectancy was calculated for those animals whose death was recorded after release into the wild (N = 15), to exclude the effect lowering the value of this indicator, resulting from the

analysis of the situation for the selected time point (30 September 2021), after which a significant percentage of the released animals are still alive. In addition, the average number of

mating periods survived in the wild by reintroduced sexually mature individuals was calculated. Overall (including plot as a function of place and year of release) and sex-dependent

cumulative survival probability curves were plotted by the Kaplan–Meier method63 and visualized using the ‘Survival’ v. 3.2-1364, ‘Survminer’ v.0.4.965 and ‘SurvMisc’ v. 0.5.566 packages for

R67. Two males caught after being released and placed permanently in the breeding centre (Nos. 1 and 2 in Table 1) were excluded from the survival analyses (N = 59). Statistica v.13.0 PL

software (TIBCO Software Inc.) was used to assess the statistical significance of the difference in overall survival times between males and females (Mann–Whitney _U_-test for comparisons of

median values)62 and differences in life expectancy of lynx in the wild from the day of release to the day of confirmed death caused by a traffic collision or by environmental factors, i.e.

scabies (Mann–Whitney _U_-test)68. Statistical significance was at P ≤ 0.05. MEASURE OF DEPENDENCE AMONG CO-VARIABLES Multivariate Cox proportional hazard regression69,70 was applied to

investigate the association between the survival time (life expectancy) of released individuals and the following predictor variables: age (at release), sex, month of release, year of

release, training time and place of release. For this, the ‘Survival’ v. 3.2-13 R package was used64. The significance of the proposed model was assessed by the Likelihood ratio test, the

Wald test and the Score (logrank) test71,72,73. Two males placed permanently in the breeding centre (Nos. 1 and 2 in Table 1), two females caught after release and re-released in other

locations (Nos. 11 and 28 in Table 1), as well as seven individuals of unknown status (Nos. 4, 25, 30, 37, 39, 41 and 53 in Table 1) were excluded from the analysis of the above-mentioned

co-variables. All data used in the Cox analysis concerning live individuals at the time of closing the analysis were treated as a right-censored data. Sex data, of a binary nature, were

coded as: males = 1, females = 2. In the case of data on a release site, the model uses the code 1 for animals released in an immediate vicinity of the training site, while the following

numbers encode the two remaining release sites, the sequence of which is determined by an increasing distance to the training site: adaptation enclosure in Dłusko = 1, release centre in the

Drawsko Forest District = 2, release centre in the Mirosławiec Forest District = 3 (Fig. 8). ETHICAL STATEMENT The research presented in this study was conducted on the basis of a permit

issued by the General Directorate for Environmental Conservation (DZP-WG.6401.08.10.2017.JR.bp of 2017, DZP-WG.6401.08.2019.bp of 2019, and DZP-WG.6401.08.46.2019.TŁ of 2020). All

experiments were performed in accordance with the Polish nature protection law, regulating the proceedings (restrictions and prohibitions) with wild animals subject to legal protection in

the territory of the Republic of Poland—Act of April 16, 2004 on nature protection (Journal of Laws of 2004, No. 92, item 880). DATA AVAILABILITY All data generated or analysed during this

study are included in this published article. REFERENCES * Sunquist, M. E. & Sunquist, F. C. Family Felidae. In _Handbook of the Mammals of the World_ Vol. 1 (eds Wilson, D. E. &

Mittermeier, R. A.) 54–170 (Lynx Editions, 2009). Google Scholar  * Breitenmoser, U. _et al._ Action plan for the conservation of the Eurasian Lynx (_Lynx lynx_) in Europe. _Nat. Environ._

112, 1–70 (2000). Google Scholar  * Linnell, J. D. C., Breitenmoser, U., Breitenmoser-Würsten, C., Odden, J. & von Arx, M. Recovery of Eurasian lynx in Europe: What part has

reintroduction played? In _Reintroduction of Top-Order Predators_ (eds Hayward, M. W. & Somers, M. J.) 72–91 (Blackwell Publishing, 2009). Chapter  Google Scholar  * Schmidt, K.,

Ratkiewicz, M. & Konopiński, M. K. The importance of genetic variability and population differentiation in the Eurasian lynx _Lynx lynx_ for conservation, in the context of habitat and

climate change. _Mammal Rev._ 41, 112–124 (2011). Article  Google Scholar  * von Arx, M. _et al._ Status and conservation of the Eurasian lynx (_Lynx lynx_) in Europe in 2001. _KORA Bericht_

19, 1–330 (2004). Google Scholar  * Kaczensky, P. _et al_. Status, management and distribution of large carnivores—Bear, lynx, wolf and wolverine in Europe. Part 1 - Europe summaries.

_Report: 1–72. A Large Carnivore Initiative for Europe Report prepared for the European Commission_ (2013). * Chapron, G. _et al._ Recovery of large carnivores in Europe’s modern

human-dominated landscapes. _Science_ 346, 1517–1519 (2014). Article  ADS  CAS  PubMed  Google Scholar  * Franz, K. W. & Romanowski, J. Revisiting the reintroduced Eurasian lynx

population in Kampinos National Park Poland. _Eur. Zool. J._ 88, 966–979. https://doi.org/10.1080/24750263.2021.1968046 (2021). Article  Google Scholar  * Bieniek, M., Wolsan, M. &

Okarma, H. Historical biogeography of the lynx in Poland. _Acta Zool. Cracov._ 41, 143–167 (1998). Google Scholar  * Jędrzejewski, W., Nowak, S., Schmidt, K. & Jędrzejewska, B. Wilk i

ryś w Polsce: Wyniki inwentaryzacji w 2001 roku. _Kosmos_ 51, 491–499 (2002). Google Scholar  * Mysłajek, R., Kwiatkowska, I., Diserens, T., Haidt, A. & Nowak, S. Occurrence of Eurasian

lynx in western Poland after two decades of strict protection. _CATnews_ 69, 12–13 (2019). Google Scholar  * Schmidt, K. _Program ochrony rysia Lynx lynx w Polsce – Project. Strategia

ochrony Rysia Warunkująca Trwałość Populacji Gatunku w Polsce_ (Warsaw University of Life Sciences, 2011). Google Scholar  * Kaczensky, P. _et al_. Status, management and distribution of

large carnivores—Bear, lynx, wolf and wolverine in Europe. Part 2: Country Species Summaries. _Report: 1–200. A Large Carnivore Initiative for Europe Report prepared for the European

Commission_ (2013). * Breitenmoser, U. _et al_. Lynx lynx (errata version published in 2017). _The IUCN Red List of Threatened Species 2015: e.T12519A121707666_. Accessed 30 Oct 2021 (2015).

* Vandel, J.-M., Stahl, P., Herrenschmidt, V. & Marboutin, E. Reintroduction of the lynx into the _Vosges mountain massif_: From animal survival and movements to population development.

_Biol. Conserv._ 131, 370–385. https://doi.org/10.1016/j.biocon.2006.02.012 (2006). Article  Google Scholar  * Zimmermann, F., Breitenmoser-Würsten, C. & Breitenmoser, U. Importance of

dispersal for the expansion of a Eurasian lynx _Lynx lynx_ population in a fragmented landscape. _Oryx_ 41, 358–368. https://doi.org/10.1017/s0030605307000712 (2007). Article  Google Scholar

  * Schmidt, K., Kowalczyk, R., Ozolins, J., Mannil, P. & Fickel, J. Genetic structure of the Eurasian lynx population in north-eastern Poland and the Baltic states. _Conserv. Genet._

10, 497–501. https://doi.org/10.1007/s10592-008-9795-7 (2009). Article  Google Scholar  * Ratkiewicz, M. _et al._ Long-range gene flow and the effects of climatic and ecological factors on

genetic structuring in a large, solitary carnivore: The Eurasian Lynx. _PLoS ONE_ 9, e115160. https://doi.org/10.1371/journal.pone.0115160 (2014). Article  ADS  CAS  PubMed  PubMed Central 

Google Scholar  * Port, M. _et al._ Rise and fall of a Eurasian lynx (_Lynx lynx_) stepping-stone population in central Germany. _Mammal Res._ https://doi.org/10.1007/s13364-020-00527-6

(2020). Article  Google Scholar  * Drouilly, M. & O’Riain, J. M. Rewilding the world’s large carnivores without neglecting the human dimension: A response to reintroducing the Eurasian

lynx to southern Scotland, England and Wales. _Biodivers. Conserv._ 30, 917–923. https://doi.org/10.1007/s10531-021-02112-y (2021). Article  Google Scholar  * Böer, M., Smielowski, J. &

Tyrala, P. Reintroduction of the European lynx (_Lynx lynx_) to the Kampinoski National Park/Poland field experiment with zooborn individuals. Part I: Selection, adaptation and training.

_Der Zool. Garten_ 70, 304–312 (1994). Google Scholar  * Jakimiuk, S. (ed.). Aktywna ochrona populacji nizinnej rysia w Polsce. 1–144 (WWF, Poland, 2015). * Huck, M. _et al._ Habitat

suitability, corridors and dispersal barriers for large carnivores in Poland. _Acta Theriol._ 55, 177–192 (2010). Article  Google Scholar  * Niedziałkowska, M. _et al._ Environmental

correlates of Eurasian lynx occurrence in Poland: Large scale census and GIS mapping. _Biol. Conserv._ 133, 63–69. https://doi.org/10.1016/j.biocon.2006.05.022 (2006). Article  Google

Scholar  * Schmidt, K., Kowalczyk, R., Ozolins, J., Männil, P. & Fickel, J. Genetic structure of the Eurasian lynx population in north-eastern Poland and the Baltic states. _Conserv.

Genet._ 10, 497–501. https://doi.org/10.1007/s10592-008-9795-7 (2009). Article  Google Scholar  * Tracz, M. _et al._ The return of lynx to northwestern Poland. _CATnews_ 14, 43–44 (2021).

Google Scholar  * The Return of Lynx to north-west Poland. http://www.rysie.org/en/rysie-strona-glowna. Accessed on 31 Oct 2021. * IUCN/SSC._ Guidelines for Reintroductions and Other

Conservation Translocations. Version 1.0. 1–57_ (IUCN Species Survival Commission, 2013). * Rueda, C., Jiménez, J., Palacios, M. J. & Margalida, A. Exploratory and territorial behavior

in a reintroduced population of Iberian lynx. _Sci. Rep._ 11, 14148. https://doi.org/10.1038/s41598-021-93673-z (2021). Article  ADS  CAS  PubMed  PubMed Central  Google Scholar  * Gusset,

M. A framework for evaluating reintroduction success in carnivores: Lessons from African wild dogs. In _Reintroduction of Top-Order Predators_ (eds Hayward, M. W. & Somers, M. J.)

307–320 (Blackwell Publishing, 2009). Chapter  Google Scholar  * Breitenmoser, U. & Haller, H. Patterns of predation by reintroduced European Lynx in the Swiss Alps. _J. Wildl. Manage._

57, 135–144 (1993). Article  Google Scholar  * Drouilly, M. & O’Riain, M. J. Rewilding the world’s large carnivores without neglecting the human dimension. _Biodivers. Conserv._ 30,

917–923 (2021). Article  Google Scholar  * Jędrzejewski, W. _et al._ Population dynamics (1869–1994), demography, and home ranges of the Lynx in Białowieza Primeval Forest (Poland and

Belarus). _Ecography_ 19, 122–138 (1996). Article  Google Scholar  * Palmero, S. _et al._ Demography of a Eurasian lynx (_Lynx lynx_) population within a strictly protected area in Central

Europe. _Sci. Rep._ 11, 19868. https://doi.org/10.1038/s41598-021-99337-2 (2021). Article  ADS  CAS  PubMed  PubMed Central  Google Scholar  * Maran, T., Põdra, M., Põlma, M. &

Macdonald, D. The survival of captive-born animals in restoration programmes: Case study of the endangered European mink _Mustela lutreola_. _Biol. Conserv._ 142, 1685–1692 (2009). Article 

Google Scholar  * Moehrenschlager, A. & Macdonald, D. W. Movement and survival parameters of translocated and resident swift foxes _Vulpes velox_. _Anim. Conserv._ 6, 199–206 (2003).

Article  Google Scholar  * Böer, M., Reklewski, J., Śmiełowski, J. & Tyrała, P. Reintroduction of the European Lynx to the Kampinoski Nationalpark/Poland: A field experiment with zooborn

individuals. Part III: Demographic development of the population from December 1993 until January 2000. _Der Zool. Garten_ 70, 304–312 (2000). Google Scholar  * Jule, K. R., Leaver, L. A.

& Lea, E. G. L. The effects of captive experience on reintroduction survival in carnivores: A review and analysis. _Biol. Conserv._ 141, 355–363 (2008). Article  Google Scholar  *

Hellstedt, P. & Kallio, E. R. Survival and behaviour of captive-born weasels (_Mustela nivalis nivalis_) released in nature. _J. Zool._ 266, 37–44 (2005). Article  Google Scholar  *

Devineau, O. _et al._ Evaluating the Canada lynx reintroduction programme in Colorado: Patterns in mortality. _J. Appl. Ecol._ 47, 524–531 (2010). Article  Google Scholar  * Lengger, J.,

Breitenmoser, U. & Sliwa, A. EAZA breeding programmes as sources for lynx reintroductions. _CATnews_ 14, 76–77 (2021). Google Scholar  * Reading, P. R. & Clark, T. W. Carnivore

introductions: An interdisciplinary Examination. In _Carnivore Behavior, Ecology and Evolution_ (ed. Gittleman, J. L.) 296–336 (Cornell University Press, 1996). Google Scholar  * McCarthy,

M. A., Armstrong, D. P. & Runge, M. C. Adaptive management of reintroduction. In _Reintroduction Biology: Integrating Science and Management_ (eds Ewen, J. G. _et al._) 256–289

(Wiley-Blackwell, 2012). Chapter  Google Scholar  * Bremner-Harrison, S., Prodohl, P. A. & Elwood, R. W. Behavioural trait assessment as a release criterion: Boldness predicts early

death in a reintroduction programme of captive-bred swift fox (_Vulpes velox_). _Anim. Conserv._ 7, 313–320 (2004). Article  Google Scholar  * Harrington, L., Põdra, M., Macdonald, D. &

Maran, T. Post-release movements of captive-born European mink _Mustela lutreola_. _Endanger. Species Res._ 24, 137–148 (2014). Article  Google Scholar  * Andrén, H. _et al._ Survival rates

and causes of mortality in Eurasian lynx (_Lynx lynx_) in multi-use landscapes. _Biol. Conserv._ 131, 23–32 (2006). Article  Google Scholar  * Heurich, M. _et al._ Illegal hunting as a major

driver of the source-sink dynamics of a reintroduced lynx population in Central Europe. _Biol. Conserv._ 224, 355–365 (2018). Article  Google Scholar  * Schmidt-Posthaus, H., Breitenmoser,

Ch., Posthaus, H., Bacciarini, L. & Breitenmoser, U. Causes of mortality in reintroduced Eurasian lynx in Switzerland. _J. Wildl. Dis._ 38, 84–92 (2002). Article  PubMed  Google Scholar

  * Kołodziej-Sobocińska, M., Zalewski, A. & Kowalczyk, R. Sarcoptic mange vulnerability in carnivores of the Białowieża Primeval Forest, Poland: underlying determinant factors. _Ecol.

Res._ 29, 237–244 (2014). Article  Google Scholar  * Holt, G. & Berg, C. Sarcoptic mange in red fox and other wild carnivores in Norway. _Nor Veterinaertidsskr_ 102, 427–432 (1990).

Google Scholar  * Mörner, T. Sarcoptic mange in Swedish wildlife. _Rev. Sci. Tech. Off. Int. Epiz._ 11, 1115–1121 (1992). Article  Google Scholar  * Ryser-Degiorgis, M. P. _et al._ Notoedric

and sarcoptic mange in free-ranging lynx from Switzerland. _J. Wildl. Dis._ 38, 228–232 (2002). Article  PubMed  Google Scholar  * Soulsbury, C. D. _et al._ The impact of sarcoptic mange

_Sarcoptes scabiei_ on the British fox _Vulpes vulpes_ population. _Mam. Rev._ 37, 278–296 (2007). Google Scholar  * Garrote, G., Fernández-López, J., López, G., Ruiz, G. & Simón, M. A.

Prediction of Iberian lynx road–mortality in southern Spain: A new approach using the MaxEnt algorithm. _Anim. Biodivers. Conserv._ 41, 217–225 (2018). Article  Google Scholar  * Bencin, H.,

Prange, S., Rose, Ch. & Popescu, V. Roadkill and space use data predict vehicle-strike hotspots and mortality rates in a recovering bobcat (_Lynx rufus_) population. _Sci. Rep._ 9,

15391 (2019). Article  ADS  PubMed  PubMed Central  CAS  Google Scholar  * Bouyer, Y. _et al._ Tolerance to anthropogenic disturbance by a large carnivore: The case of Eurasian lynx in

south-eastern Norway. _Anim. Conserv._ https://doi.org/10.1111/acv.12168 (2014). Article  Google Scholar  * López-Bao, J. V. _et al._ Eurasian lynx fitness shows little variation across

Scandinavian human-dominated landscapes. _Sci. Rep._ 9, 8903 (2019). Article  ADS  PubMed  PubMed Central  CAS  Google Scholar  * Cimatti, M. _et al._ Large carnivore expansion in Europe is

associated with human population density and land cover changes. _Divers. Distrib._ 27, 602–617 (2021). Article  Google Scholar  * Wegner, M. (ed.). _Statistical Yearbook of

Zachodniopomorskie Voivodship_. 1–213 (Statistical Office in Szczecin, 2020). * Górny, M., Schmidt, K. & Kowalczyk, R. Analiza przydatności środowiska dla reintrodukcji rysia w

północno-zachodniej Polsce oraz prognoza i perspektywy funkcjonowania populacji. _Expert study under the project POIS.02.04.00–0143/16 "Return of the lynx to northwestern Poland”_.

1–25. * Woodford, M. H., Keet, D. F. & Bengis, R. G. _Post-mortem Procedures for Wildlife Veterinarians and Field Biologists_. 1–55 (IUCN Species Survival Commission (SSC) &

Veterinary Specialist Group, Care for the Wild International, World Organisation for Animal Health (OIE), 2000). * Fain, A. Ѐtude de la variabilitѐ de _Sarcoptes scabiei_ avec une rѐvision

des Sarcoptidae. _Acta Zool. Pathol. Antverp_ 47, 1–196 (1968). Google Scholar  * Kaplan, E. L. & Meier, P. Nonparametric estimation from incomplete observations. _J. Am. Stat. Assoc._

53, 457–481 (2012). Article  MathSciNet  MATH  Google Scholar  * Therneau, M., Lumley, T., Atkinson, E. & Crowson, C. _Survival Analysis. R Package Version 3.2-13_.

http://CRAN.R-project.org/package=survival (2021). * Kassambara, A., Kosinski, M., Biecek, P. & Scheipl, F. _survminer. Drawing Survival Curves using 'ggplot2'. R package

version 0.4.9_. http://CRAN.R-project.org/package=survminer (2021). * Dardis, C. _survMisc. Miscellaneous Functions for Survival Data. R package version 0.5._5.

http://CRAN.R-project.org/package=survMisc (2018). * R Core Team. _R: A language and environment for statistical computing_ (R Foundation for Statistical Computing).

https://www.R-project.org (2021). * Snedecor, G. W. & Cochran, W. G. _Statistical Methods_ 7th edn. (Iowa State University Press, 1980). MATH  Google Scholar  * Cox, D. R. Regression

models and life tables (with discussion). _J. R. Stat. Soc. B._ 34, 187–220 (1972). MATH  Google Scholar  * Bradburn, M. J., Clark, T. G., Love, S. B. & Altman, D. G. Survival Analysis

Part II: Multivariate data analysis: An introduction to concepts and methods. _Br. J. Cancer._ 89, 431–436 (2003). Article  CAS  PubMed  PubMed Central  Google Scholar  * Wald, A. Tests of

statistical hypothesis concerning several parameters when the number of observations is large. _Trans. Am. Math. Soc._ 54, 426–482 (1943). Article  MATH  Google Scholar  * Aitchison, J.

& Silvey, S. D. Maximum likelihood estimation of parameters subject to restraints. _Ann. Math. Stat._ 29, 813–828 (1958). Article  MathSciNet  MATH  Google Scholar  * Mantel, N.

Evaluation of survival data and two new rank order statistics arising in its consideration. _Cancer Chemother. Rep._ 50, 163–170 (1966). CAS  PubMed  Google Scholar  Download references

ACKNOWLEDGEMENTS The research reported in this manuscript was supported by the project "The Return of the Lynx to north-western Poland", co-financed from the Cohesion Fund under

the Operational Programme Infrastructure and Environment 2014-2020 (Grant No. POIS.02.04.00-00-0143/16-00). AUTHOR INFORMATION AUTHORS AND AFFILIATIONS * Institute of Marine and

Environmental Sciences, University of Szczecin, 71-415, Szczecin, Poland Jakub Skorupski & Przemysław Śmietana * West Pomeranian Nature Society, 71-415, Szczecin, Poland Jakub Skorupski,

 Magdalena Tracz, Maciej Tracz & Przemysław Śmietana * Polish Society for Conservation Genetics LUTREOLA, 71-784, Szczecin, Poland Jakub Skorupski & Przemysław Śmietana Authors *

Jakub Skorupski View author publications You can also search for this author inPubMed Google Scholar * Magdalena Tracz View author publications You can also search for this author inPubMed 

Google Scholar * Maciej Tracz View author publications You can also search for this author inPubMed Google Scholar * Przemysław Śmietana View author publications You can also search for this

author inPubMed Google Scholar CONTRIBUTIONS Conceptualization—J.S., P.Ś., Mag.T. and Mac.T., methodology—J.S. and P.Ś., software—P.Ś., validation—J.S., P.Ś., results analysis and

investigation—J.S., P.Ś., Mag.T. and Mac.T., data curation—J.S., writing—J.S., review and editing—J.S., P.Ś., Mag.T. and Mac.T., visualization—J.S. and P.Ś., funding acquisition—Mag.T. and

Mac.T. All the authors have read and agreed to the published version of the manuscript. CORRESPONDING AUTHOR Correspondence to Jakub Skorupski. ETHICS DECLARATIONS COMPETING INTERESTS The

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CITE THIS ARTICLE Skorupski, J., Tracz, M., Tracz, M. _et al._ Assessment of Eurasian lynx reintroduction success and mortality risk in north-west Poland. _Sci Rep_ 12, 12366 (2022).

https://doi.org/10.1038/s41598-022-16589-2 Download citation * Received: 12 January 2022 * Accepted: 12 July 2022 * Published: 20 July 2022 * DOI: https://doi.org/10.1038/s41598-022-16589-2

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