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No. 106: Sustainable red deer management

Sunde, P. & Haugaard, L. 2014. Bæredygtig krondyrforvaltning. Populationsbiologiske analyser af krondyrbestandene på Oksbøl og Djursland med reference til jagtlig forvaltning. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 76 s. - Videnskabelig rapport fra DCE - Nationalt Center for Miljø og Energi nr. 106. http://dce2.au.dk/pub/SR106.pdf

Summary

In Denmark, red deer (Cervus elaphus) are legal quarry (subject to no quotas) throughout the hunting season (1 September – 31 January) for anyone holding a valid license to hunt on their own land (more than 1 hectare) or on rented ground (larger than 5 hectares). As a consequence, in most parts of Denmark, multiple land owners and hunters on rented ground compete for the same individuals without any overall plan or coordination of the culling of local populations. The disadvantages of such a lack of management (for instance, the apparent deficit of mature stags because of high hunting pressure before reaching maturity) have been debated for decades.  To contribute factual information to this debate, the demographic composition of two Danish red deer populations with contrasting land owner structure and hunting regimes were analysed and compared, from Djursland (hunting seasons 2008/9-2012/13) and Oksbøl red deer reserve (1985/86-2012/13). Djursland (1417 km2, the easternmost tip of Jutland) represents a ‘typical’ Danish landscape, comprising multiple owners of small or larger estates each of which run their own hunting practices. In this area, 1-year old males were protected in an effort to increase the proportion of mature stags in the population. During a five-year period, hunters on Djursland voluntarily delivered jaws from hunted red deer for age determination and morphometric information. The population on the Oksbøl red deer reserve (163 km2, south western Jutland) is managed by the Danish Nature Agency, with the aim of maintaining a stable red deer population with a high proportion of mature stags through a deliberate harvesting policy. On Oksbøl, the age of all harvested and deer found dead had been estimated on the basis of teeth wear (although the validity of this locally developed age estimation method had never been tested on independent data).

The aims of this analysis were (i) to test and calibrate the wear-based age estimation method used on Oksbøl against independent data, (ii) to describe the demographic composition of the two populations from age at death distributions, and (iii) to establish population models from this information. Since data was also available on the number of points on antlers, weight, pregnancy and lactation rates, relationships between these variables and age, population density and date were also calculated.

In summary, the results are as follows:

  1. From a reference material of 37 individuals of known age (marked before 2 years old), true age correlated closely (R2 = 97%) and accurately  (no bias) with age estimated from the number of incremental lines in teeth cementum layers (‘method 1’), although with a precision of ±2 years for any given individual. Age estimation based on dentin layers can thus be considered as a reliable method to estimate age of Danish red deer.
  2. Age estimated from tooth wear in 15 red deer from Oksbøl correlated closely with age estimated from dental lines (R2 = 92 %). However, the age estimated from the number of dentin lines was on average 49.5 % higher (p< 0.0001) than that estimated from tooth wear, suggesting that the locally developed wear based age determination method systematically underestimated the age of dead red deer. A deer estimated to be 10 years old on the basis of the tooth wear method would, on average, be estimated to be 15 years old from counts of dentin lines.
  3. The demographic composition of the Oksbøl population was constructed based on life tables established from 4278 aged females and 2896 males which died  between 1990/91 and 2012/13. Females had an estimated annual mortality (based on adjusted age distribution from method 2 calculated at the start of the hunting season) of 33% as calves and 15-20% for all older age classes. This was equivalent to a spring population of females which consisted of 19% yearlings/calves from the preceding summer, 13% 2-year olds, 11% 3-year olds and 55% 4+ year olds. Males had an estimated annual mortality of 45% for calves, 35% for 1-3 year olds, and 20% for stags from 4 years of age, equivalent to an male age composition of 26% yearlings, 17% 2-years olds, 11% 3-years olds, 26% 4-7 year olds and 21% of 8 years of age or older. At the start of the rutting season, the population consisted of 2.6 hinds (i.e. females aged 1½ years old or older) for every stag aged 2½ years or more and 5.7 hinds for every stag aged 5½ year or more.
  4. The demographic composition of the Djursland population was constructed based on life tables established from 895 aged females and 622 males which died between 2008/09 and 2012/13. Seventy-four unsexed calves where assumed to represent an even sex distribution and were divided equally within the male and female life tables. Females had an estimated annual mortality (calculated from the start of the hunting season) of 23% as calves and 20% as 1-7 year olds, and 34 of 8 years of age or older. This was equivalent to a spring female population of 22% yearlings/calves from the preceding summer, 18% 2-year olds, 15% 3-year olds and 45% 4+ year olds. Males had an estimated annual mortality of 24% for calves, 4% for yearlings, 26% for 2-year olds, and about 50% for 3-7 years of age. An apparent reduction in annual mortality after 8 years of age (24%) may be an artefact caused by a few old stags escaped from captivity. The estimated age distribution of males in spring thus consisted of 29% yearlings, 28% 2-year olds, 21% 3-year olds, 11% 4 year olds and 11% of 5 years of age or older. At the start of the rutting season, the population consisted of 1.8 hinds for every stag aged 2½ year or more, and 8.1 hinds for every stag aged 5½ year or more.
  5. The population effect of protecting 1-year old males from hunting on Djursland could not be estimated rigorously because of the lack of reference material (because protection was implemented throughout the sampling period) but could be cautiously estimated as a difference in mortality between yearlings and calves and 2-year old stags of about 20%. Accordingly, without the protection of yearling males from hunting, the number of all older age groups of males would probably be 20% lower than observed.
  6. Male body weight increased with age until the 5th year on Djursland and 10th year at Oksbøl. In both populations, female body weights increased with age until their 3rd year. Females from Djursland weighed on average 12% more than females from Oksbøl (methodological differences precluded comparisons of stags between populations). In both populations, calf weights increased by 6-7 kg for both sexes during the first 2 months of the hunting season. During the same period the weight of adult stags decreased because of rutting activities. Calves from the Oksbøl population weighed 8-9% less in those years (mid 1990s) when population density peaked at almost twice the size as in the 1980s and after 2000. In 1- and 2-year old males, there was a 12% difference in body weight between cohorts born during the highest and lowest population densities.
  7. The number of antler points increased until 8-15 years of age, reaching an average of 11 but with considerable (±3) individual variation. In both populations 2/3 of all males had at least 10 points at 5 years of age and 80-90% had at least 10 points at 8 years of age. The proportion of stags with 14 points or more peaked at c.25% during 12-15 years of age.
  8. In both population more females than males were reported amongst the grown individuals (Djursland: 60% females, Oksbøl: 58% females) even though the sex ratio amongst calves was close to unity. In Oksbøl, the female: male ratio varied from 50:50 for individuals from cohorts born during lowest population densities to 65:35 for cohorts born during greatest densities. Since the sex-ratio of (dead) calves did not vary significantly as a function of population density, the female biases sex ratio of adults is likely to reflect increased emigration of stags under dense population conditions. On Djursland, the apparent female-biased sex ratio in the population may be the result of a campaign aimed at ceasing the growth of the population by targeting hinds and/or lower reporting frequencies for stags than for hinds.
  9. Data on pregnancy and lactation rates were available from Djursland only. Here, 88% of all hinds were pregnant, divided between 64% amongst 1-year old and 91% of older hinds. At the start of the hunting season 79% of all hinds were lactating (i.e. being with calf), based on 57% of the 2-old and 82% of older hinds.
  10. On the basis of estimated age specific mortalities for each population and observed fecundity on Djursland (extrapolated to Oksbøl), population growth rates were estimated to be l=1.02 at Oksbøl and 1.00 at Djursland. Since the population at Oksbøl remained about the same from the 1980ies to 2012/13, the predicted positive growth rate for the Oksbøl population might be the result of the substituted fecundity rates in the model (taken from the Djursland population) exceeding the true rates of the Oksbøl population. A population model incorporating a fecundity rate estimated from Djursland and no hunting mortality (99% annual survival until 8th years of age, 80% annual survival after that age), predicted an annual population growth rate of l=1.21, which is equal to a doubling of the population size in four years. Estimates are given for population growth rates under different combinations of female age specific annual survival.
  11. The estimated population patterns for the Oksbøl and Djursland populations are discussed in relation to five dominant (but partly opposing) population objectives to: (i) maximise the number of harvested individuals relative to the population size (‘game ranch model’), (ii)  maximise the number of harvested mature stags relative to the size of the population (‘trophy model’), (iii) maintain a ‘high’ proportion of mature stags in the living population, (iv) attain a ‘sustainable’ harvesting (several definitions), and (v) sustain an intended ‘natural’ demographic composition.
  12. In terms of number of harvested individuals relative to the size of the winter population, the populations at Oksbøl and Djursland resulted in a yield of 37 and 32 harvested individuals/ 100 individuals in the winter population. This was 55-60% of the maximum possible number of harvested individuals in a population managed for that purpose (game ranch model: 62/100), about the same number as harvested in a population managed to maintain an intended natural demographic composition (34/100) and half the number harvested in a population managed to maximize the number of harvested stags of at least 8 years of age.
  13. On Djursland, 0.4 mature stags (defined as at least 8 years of age) were harvested per 100 individuals in the spring population. This constitutes 4% of the harvest from a population managed to maximize the harvest of mature stags (trophy model: 8.7/100), 15% of the harvest from a population with an intended natural demographic composition (2.6/100) and one third the number of mature stags harvested at Oksbøl (1.2/100) or in a population managed in order to optimize meat harvest (game ranch model). Without protection of 1-year old males, the yield of mature stags would probably have been 20% lower than observed. In addition to providing hunters with a very low yield of mature stags, in an evolutionary perspective, the low average longevity of stags on Djursland creates a real risk of selecting for earlier maturity and smaller body size.
  14. Even though the harvest policy on Oksbøl resulted in a three times higher harvest of mature stags relative to the population size than on Djursland and a less biased ratio between the numbers of hinds and mature stags, a change in age-specific culling towards a higher harvest of calves and individuals older than 8 years compared to present would double the harvest of mature stags as well as their proportion of the live population.  This would also be in line with the general mortality pattern observed in naturally regulated ungulate populations elsewhere.
  15. If managers aim to increase the number of mature stags in the population as well as in the overall hunting yield, the most efficient tool would be to protect all immature stags from hunting until they have reached the required size for harvest and mating.