Comparison of deer abundance and

rates of increase (2018–2019) in 1080-

poisoned and unpoisoned areas on

Molesworth Station

OSPRI – R-10810-02

Prepared for: OSPRI

March 2019

Comparison of deer abundance and rates of increase

(2018–2019) in 1080-poisoned and unpoisoned areas on

Molesworth Station

Contract Report: LC3435

Grant Morriss, Ivor Yockney, Graham Nugent

Manaaki Whenua – Landcare Research

Reviewed by:

David Latham

Scientist

Manaaki Whenua – Landcare Research

Approved for release by:

Chris Jones

Portfolio Leader – Managing Invasives

Manaaki Whenua – Landcare Research

Disclaimer

While every effort has been made to ensure the accuracy of the information provided in this report, no

warranty or representation is provided regarding the accuracy of such information, and Manaaki Whenua –

Landcare Research does not accept liability for any losses or damage arising directly or indirectly from

reliance on the information.

© OSPRI 2019

This report has been produced by Landcare Research New Zealand Ltd for OSPRI. All copyright in this report

is the property of OSPRI and any unauthorised publication, reproduction, or adaptation of this report is a

breach of that copyright and illegal.

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Contents

Summary ................................................................................................................................................................. v

1 Introduction ............................................................................................................................................... 1

2 Background ................................................................................................................................................ 1

3 Objective ..................................................................................................................................................... 1

4 Methods ...................................................................................................................................................... 1

4.1 Operational background and survey design ...................................................................................... 1

4.2 The survey ........................................................................................................................................................ 2

5 Results .......................................................................................................................................................... 5

5.1 Deer counts ..................................................................................................................................................... 5

5.2 Deer detectability .......................................................................................................................................... 6

5.3 Chamois counts ............................................................................................................................................. 6

6 Discussion ................................................................................................................................................... 9

6.1 Deer counts ..................................................................................................................................................... 9

6.2 Rate of deer population recovery ........................................................................................................... 9

6.3 Chamois counts .......................................................................................................................................... 10

7 Recommendations................................................................................................................................ 10

8 Acknowledgements .............................................................................................................................. 11

9 References ............................................................................................................................................... 11

10 Appendix: Transformed mean counts per block. ...................................................................... 12

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Summary

Project and client

• Manaaki Whenua − Landcare Research, Lincoln, was commissioned by OSPRI to

repeat a 2018 comparison of the relative abundance of red deer in part of Molesworth

Station that had been subject to aerial 1080 baiting for possum control in spring 2017

and their abundance in a nearby unbaited part of the station. The work was

undertaken between February and March 2019.

Objective

• To assess the change between 2018 and 2019 in the relative abundance and rate of

increase of red deer in parts of Molesworth Station poisoned in October 2017 and in

adjacent unpoisoned areas.

Methods

• In February 2018, the operational areas were divided into 9 km2 blocks, and a random

selection of these was surveyed (unpoisoned: 11 blocks; poisoned: 13 blocks), totalling

c. 20% of the whole study area. The same blocks were resurveyed in February 2019.

• In each of the 9 km2 blocks an observer and pilot in a helicopter searched the area at

low altitude and recorded any live deer and (incidentally) chamois seen. Searches

were carried out during the first 2 and last 2 hours of daylight, when deer are most

active and visible. Mean counts per block were compared between areas and/or years.

Results

• In total, 193 deer were counted in 2018 and 220 in 2019. The difference in relative

abundances between the poisoned and unpoisoned areas was consistent over the two

surveys and there was no significant area–year interaction.

• In the poisoned area only, the transformed counts of stags did not differ between

years but the threefold higher mean transformed counts of hinds and fawns in 2019

approached statistical significance.

• In both years, the percentage of stags amongst the deer counted was significantly

higher in the unpoisoned area than in the poisoned area. For both years combined,

the number of fawns equated to 92% of the number of hinds in the poisoned area

compared to 83% in the unpoisoned area but the difference was not significant.

• The 2019 counts of hinds and fawns in each poisoned area block were inversely

correlated to the distance from the block centre to the nearest unpoisoned area.

• Low numbers of chamois were seen in 2019, with 18 counted in the poisoned area

and one in the unpoisoned area, much the same as in 2018 (17 and three in the

poisoned and unpoisoned areas, respectively).

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Discussion

• The resurvey confirmed that relative deer abundance was much lower in the poisoned

area than in the unpoisoned area (88% lower in 2018 and 83% lower in 2019).

• In addition to some visible deer being missed, some deer will also have been

obscured by tall cover. Extrapolation from the back-transformed 95% confidence

intervals for ‘all-deer’ counts suggests there were 518–1362 ‘countable’ deer present

in the unpoisoned area in February 2019, plus those missed or obscured by cover. The

equivalent range for the poisoned area is 80–261 countable deer.

• There was no significant change in the overall numbers of deer across both blocks,

making it difficult to confidently assess the rate of recovery over the 2018–2019 year.

Assuming exponential increase at the typical annual rate for red deer, deer abundance

in the poisoned area could increase to the current level in the unpoisoned area within

6-7 years of the poison operation. If there is substantial immigration, the recovery

time would reduce, whereas any hunting offtake would slow the recovery.

• Chamois were counted in much the same numbers and in much the same places in

2019 and in 2018. As 89% of the chamois counted were recorded in the poisoned

area, there is no indication that the 2017 poison operation affected chamois.

Recommendations

• If OSPRI wishes to avoid hunter angst, bad publicity and/or reduction in goodwill,

deer-repellent bait should be used when the unpoisoned area is subjected to 1080

baiting (as is currently scheduled for 2020).

• The aerial deer counts reported here for the unpoisoned area form baseline measures

of deer abundance that could be used to assess the repellency to deer of current or

new formulations of deer-repellent bait in the unpoisoned area.

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1 Introduction

Manaaki Whenua − Landcare Research, Lincoln, was commissioned by OSPRI to repeat a

2018 comparison of the relative abundance of red deer in part of Molesworth Station that

had been subject to aerial 1080 baiting for possum control in spring 2017 and that in a

nearby unbaited part of the station. The work was undertaken between February and

March 2019.

2 Background

In October 2017 OSPRI conducted an aerial 1080-baiting operation over 62,000 ha of

semi-arid mountainous shrubland and grassland on Molesworth Station using a toxic

sowing rate somewhat higher than standard OSPRI practice in an effort to quickly break

the TB cycle in possums. An unexpected outcome was a high incidental by-kill of red deer,

first indicated by observation of large numbers of dead deer (and very few live deer)

during an aerial survey undertaken by the Marlborough branch of the New Zealand

Deerstalkers Association (Pinney in prep.).

In early 2018 OSPRI commissioned a more quantitative comparison of the relative

abundances of red deer in the area that had been poisoned and an adjacent similar area

that had not. Under the (unverifiable) assumption that the deer densities in the two areas

had been similar before the 1080 operation, that survey indicated an 87.6% reduction in

deer density in the poisoned area (Morriss et al. 2018).

In response to hunter concerns about the loss of a substantial hunting resource, OSPRI

commissioned a re-survey of the two areas a year later in order to assess the likely

recovery rate of the deer population. That re-survey is reported here, with results

compared with the 2018 counts.

3 Objective

To assess the change between 2018 and 2019 in the relative abundance and rates of

increase of red deer in parts of Molesworth Station poisoned in October 2017 and in

unpoisoned areas.

4 Methods

4.1 Operational background and survey design

The survey design comprised aerial counts of deer in two sets of 9 km2 sampling units,

with the same areas surveyed in both 2018 and 2019. The survey areas (Figure 1)

comprised the East Acheron zone (62,000 ha east of the Acheron Road), which was

poisoned in October 2017 (hereafter referred to as the poisoned area); and Bush Gully and

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Tarndale (totalling 56,155 ha, west of Acheron Rd (hereafter referred to as the unpoisoned

area).

Permission to carry out an aerial deer survey was granted by the joint land managers of

Pāmu Farms of New Zealand (Landcorp Farming Ltd) and the Department of Conservation

in early February 2019.

4.2 The survey

A 1 km buffer inside the area boundaries was excluded to try to minimise edge effects (i.e.

deer moving between the poisoned and unpoisoned areas), and the remaining core areas

were divided into 3 × 3 km (9 km2) blocks. A random selection of these was surveyed (13

in the poisoned area and 11 in the unpoisoned area), comprising 18.8% and 17.6% of the

two areas, respectively.

In each of the 9 km2 blocks an observer and pilot in a Hughes 500E (in 2018) and a

MD520N (in 2019) helicopter searched the area at low altitude, travelling at 60 km/h, and

recorded any live deer seen. In the first survey a third person was in the back seat of the

helicopter recording animal sightings by the pilot and front observer. A Tracmap GPS

guidance unit, with the observation swath width set at 200 m, was used to record the area

surveyed and to provide an estimate of search coverage. The searchers aimed to fully

search each 9 km2 block, initially searching optimal deer habitat, and then searching the

remainder of the block in a way they considered should maximise their chance of seeing

deer in suboptimal habitats (Figure 2).

Searches were carried out only during the first 2 and last 2 hours of daylight, when deer

are most likely to be active and visible. The start times and end times of each survey in

each block were recorded, along with a subjective estimate of the percentage of tall

vegetation cover in the block and the ambient temperature. Tall vegetation cover was

defined simply as being tall enough to obscure deer from observers.

In 2019 an attempt was made to assess the observed proportion of deer present. For this,

in 22 of the blocks the recorder in the back of the helicopter (on the same side as the

observer) also counted deer (using a Pulsar Helion XP thermal monocular as well as visual

searches in 16 blocks, and visual search only in six blocks when the terrain was too hot for

the thermal monocular to differentiate animal body heat). This potentially enabled

assessment of the numbers of deer seen only by the front observer, the number seen only

by the rear observer, and the number seen by both.

This double search method, in principle, enables estimation of deer detectability and, from

that, the actual number of deer, but one of the primary requirements for the approach

(independence between the two observers) was unavoidably violated when large numbers

of deer were seen, so the theoretical robustness of the approach was greatly undermined.

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Figure 1. Blocks surveyed for live deer in Molesworth Station, February 2018 and 2019. Each

block was 9 km2. The area poisoned in spring 2017 is delineated by the easternmost pink

boundary line (East Acheron), while the unpoisoned area is delineated by the western pink

lines encompassing the Bush Gully (bottom) and Tarndale (top) operational areas.

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Figure 2. An example survey block in the poisoned area, including Mt Jackson and part of the

Tweed River, with the Clarence River visible in the background. The yellow line is the

flightline of the helicopter searching for deer (and chamois) in 2018. Two chamois and no

deer were seen in this block when surveyed on 8 February 2018, and seven chamois and two

stags were seen a year later on 11 February 2019.

Surveys were conducted over 6–14 February 2018 and 7–13 February 2019. Deer sightings

were classified by sex and age class, as follows: stag, hind, and fawn. Because hinds and

fawns were almost always associated with each other, the counts of these two age-sex

classes were combined. The overall numbers of hinds and fawns in each area were

compared and the number of fawns was expressed as a percentage of the number of

hinds. Chamois sightings were also recorded. The number of deer counted in each block

provided an estimate of the relative abundance of deer in poisoned versus unpoisoned

areas, and across years.

Mean counts were compared between areas and/or years using two-factor repeated

measures ANOVA or t-tests. The relationships between the counts per block and ambient

temperature, percentage of tall cover, and distance from the nearest unpoisoned area

were investigated by simple correlation. As the data was highly skewed, particularly by the

large number of zero counts in the poisoned area, the data were square-root transformed

prior to these analyses. The distance from the nearest unpoisoned area was measured for

each of the poisoned area blocks from the centre of the block.

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5 Results

5.1 Deer counts

In total, 193 deer were counted in 2018 and 220 in 2019 (Table 1). For both years

combined, the mean transformed counts per block differed between the poisoned and

unpoisoned areas (F2,22 = 38.0 P < 0.001; two-factor repeated measures ANOVA). However,

the overall difference between years was not significant (P = 0.17) and there was no

significant area–year interaction (P = 0.31).

Focussing on the poisoned area only, the transformed counts of stags did not differ

between years but the three-fold higher mean transformed counts of hinds and fawns in

2019 (Table 1) approached statistical significance (paired t-tests, t = 1.03, df = 12, P =

0.313 for stags and t = 1.76, df = 12, P = 0.052 for hinds and fawns, compared to a

Bonferroni adjusted significance level (α = 0.025)).

Table 1. Deer counts recorded during an aerial survey on Molesworth Station in February in

2018 and 2019, showing the number of blocks searched, the area searched, the total number

of deer sightings in each area, the number of sightings per km2 of search area, and the back-

transformed (square root) mean counts per block (by the front observers only) and

associated confidence limits (based on assumed normality) for all deer, for stags, for hinds

and fawns combined, and chamois. The untransformed mean counts used in the analyses are

presented in the Appendix.

Poisoned Unpoisoned Total

2018 2019 2018 2019 2018 2019

No. of blocks searched 13 13 11 11 24 24

Total area searched (km2) 117.6 117.6 99.6 99.6 217.2 217.2

Total deer seen 24 38 169 182 193 220

Deer sightings/km2 0.20 0.32 1.70 1.83 0.89 1.01

Mean counts per block

Poisoned Unpoisoned Total

Mean ± 95% CL Mean ± 95% CL Mean ± 95% CL

All deer 2018 1.85 1.44 15.18 6.83 7.96 4.18 2019 2.92 1.20 16.55 7.03 9.17 4.26

Hinds & fawns 2018 0.62 0.55 12.45 6.74 6.04 3.90

2019 1.31 0.69 13.36 6.24 6.83 3.75

Stags 2018 1.23 1.26 2.73 2.81 1.92 1.49

2019 1.62 0.97 3.18 4.31 2.33 2.07

Chamois 2018 1.31 1.18 0.27 0.36 0.83 0.69

2019 0.49 0.27 0.01 0.03 0.18 0.10

In 2018, the percentage of stags amongst the deer counted was significantly higher in the

poisoned area than in the unpoisoned area (67% vs 18% respectively; Morriss et al. 2018).

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That significant difference remained in 2019 (55% vs 19%; Pearson 2 = 25.5, df = 1,

P<0.001).

In 2019, totals of nine hinds and eight fawns were counted in the poisoned area,

compared to four hinds and four fawns in 2018. In the unpoisoned area, the respective

total counts were 76 hinds and 61 fawns in 2018 and 79 hinds and 69 fawns in 2018.

Across both years, the number of fawns in the poisoned area equated to 92% of the

number of hinds there, compared to 83% in the unpoisoned area, but that difference was

not significant (Pearson 2 = 0.05, df = 1, P = 0.8).

The 2019 counts of hinds and fawns in each poisoned area block were inversely correlated

to the distance from the block centre to the nearest unpoisoned area (Figure 4; r = -0.57,

df = 11, P = 0.04). For stags, there was no correlation (Figure 4; r = 0.46, df =11, P = 0.11).

5.2 Deer detectability

In 2019, the 22 counts conducted by the rear observer were closely correlated with the

combined count by the pilot and front observer, but about 25% lower (Figure 5; r = 92, df

= 20, P < 0.0001). In the 16 blocks in which a thermal monocular was used, the rear

observer saw 123 deer, but only 31 (25%) with the thermal monocular and only one of

those was definitely not seen by the front observer and pilot.

There was no significant effect of temperature on the counts per block in the unpoisoned

area in either 2018 (r = 0.24, df = 9, P= 0.47) or 2019 (r = 0.17, df = 9, P= 0.29).

There was also no effect of the percentage of tall vegetation on the 2018 counts in the

unpoisoned area (r = 0.00, df = 9, P= 0.99), but a negative relationship in 2019 (r = -0.69,

df = 9, P = 0.04) (Figure 4).

5.3 Chamois counts

Low numbers of chamois were seen in 2019 (Table 1), with 18 counted in the poisoned

area and one in the unpoisoned area, much the same as in 2018 (17 and three in the

poisoned and unpoisoned areas, respectively). Chamois were highly localised, with two-

thirds of the chamois seen in each year being in just the same four (of 24) blocks).

Chamois were classed by age and sex in 2018 and comprised three bucks, 12 does and

four kids.

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Figure 3: Relationship between the transformed 2019 counts for each poisoned area block

and the distance of the block centre from the nearest unpoisoned area, shown separately for

stags and hinds & fawns. Antlered yearling males (spikers) were classified as stags. There was

no relationship for stags, but a significant negative relationship for hinds & fawns (see text)

0

1

2

3

0.0 1.0 2.0 3.0 4.0 5.0 6.0

Transformed 2019

counts

Distance from unpoisoned area (km)

Stags

y = -0.37x + 2.09r = -0.57

0

1

2

0.0 1.0 2.0 3.0 4.0 5.0 6.0

Transformed2019

counts

Distance from unpoisoned area (km)

Hinds & Fawns

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Figure 4: Transformed counts for each block in the unpoisoned area in 2018 and 2019 in

relation to subjective estimates of the percentage of the block covered with tall vegetation

that could obscure deer from view. The cover estimate is the average of the 2018 and 2019

estimates.

y = -0.04x + 5.46

r = -0.7

0

2

4

6

8

0 20 40 60 80 100

Transformed

deer

count

Percentage of block with tall (>2m) cover

2019

0

2

4

6

8

0 20 40 60 80 100

Transformed

deer

count

Percentage of block with tall (>2m) cover

2018

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6 Discussion

6.1 Deer counts

The resurvey confirmed that deer relative abundance (counts) was much lower in the

poisoned area than in the unpoisoned area. In 2019, the mean counts in the poisoned area

were 83% lower than in the poisoned area, compared to being 88% lower in 2018. In an

aerial survey undertaken soon after the 1080 poisoning, 92 dead deer were seen

compared to only 2 live deer, and the density of dead deer was estimated to be 1.54

deer/km2 (Pinney in prep.). That density is similar to the density of deer sightings in the

unpoisoned area in this study (Table 1), suggesting that much of the current difference in

deer counts between the poisoned and unpoisoned areas can be attributed to 1080 by-kill

in 2017.

The sighting of a few deer by the rear-seat observer that were confirmed as not having

been seen by the two forward observers (eight in 2018; Morriss et al. (2018) and one in

2019) indicates some visible deer were missed. However, our efforts to quantify the

percentage of visible deer not counted proved to be impractical.

It seems certain that in addition to some visible deer being missed, some deer will also

have been obscured by tall cover. That is supported by the negative relationship between

the transformed 2019 counts and the average of the percentage of tall cover estimated for

each block in each year (Figure 4), but not by the absence of any such relationship in the

2018 counts. The reasons for the difference between years are not known. If the 2019

relationship is real (as logic suggests is likely, given that deer are extremely difficult to see

under tall cover) and accurate, the mean amount of tall cover in the poisoned blocks (38%)

suggests the possibility that up about one third of deer may not have been visible.

However, we stress that it is extremely unlikely that the 2019 relationship between tall

cover and the counts solely reflects deer visibility, particularly given there was no

relationship in 2018, and given that cover is likely to affect habitat choice (deer may prefer

living in open areas, for example). Nonetheless, we believe the recorded counts should be

viewed as conservative estimates of the minimum numbers of deer present.

Extrapolation from the back-transformed ‘All deer’ counts in the Appendix (and assuming

no double counting), the 95% confidence interval suggests there were 518-1362

‘countable’ deer present in the unpoisoned area in February 2019, plus those missed or

obscured by cover. The equivalent range for the poisoned area is 80-261 countable deer.

6.2 Rate of deer population recovery

There was no significant change in the overall numbers of deer across both blocks, making

it difficult to confidently assess the rate of recovery over the 2018–2019 year. However, the

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threefold increase in the number of hinds and fawns approached significance and is

consistent with a high rate of increase. As hinds each produce, at most, only one fawn per

year, the tripling of the counts cannot be attributed to reproduction alone, suggesting the

possibility that there had been some immigration of females into the poisoned area. That

possibility is supported by the negative correlation between the 2019 counts of hinds and

fawns and the distance from the block centre to the nearest unpoisoned area (Figure 3).

Animal populations unlimited by food or predation tend to increase exponentially

according to the formula Nt = N0 * exp (rmt), where N0 is number of animals at time zero, Nt

is the number after t years, and rm is the maximum or intrinsic exponential annual rate of

reproduction (Caughley 1977). For red deer, Forsyth et al. (2010) summarise seven

published studies in which estimates of rm varied between 0.18 and 0.38. the average was

0.30, which equates to a finite annual increase of 35%. However, there are examples of

much higher rates of increase in deer numbers. In one Canterbury example from the

1960s, a red deer population in the Harper–Avoca Valley was reduced to very low levels by

hunting, but then showed extremely high rates of increase (relative to rm) in both females

(r = 2.33) and males (r = 1.61) in the first 2 years after hunting ceased (Forsyth et al. 2010).

The high rates of increase were attributed to immigration.

For the poisoned area in this study, and applying an exponential increase with rm set at

0.30 to the back-transformed 2019 mean of 158 deer, the population could reach the

current mean in the unpoisoned area (890 deer) within about 6-7 years of the poison

operation. If net immigration is occurring (as suggested by Fig. 3) and continues to do so

for another year or two, the recovery time would reduce somewhat. Conversely, any

hunting offtake would obviously slow the rate of recovery.

6.3 Chamois counts

Chamois were counted in much the same numbers and in much the same places in 2019

and in 2018. As 89% of the chamois counted were in the poisoned area, there is no

indication that the 2017 operation affected chamois.

7 Recommendations

• We reiterate last year’s recommendation (Morriss et al. 2018) that if OSPRI wishes to

avoid hunter angst, bad publicity and/or reduction in goodwill, then deer-repellent

bait should be used when the unpoisoned area is subject to 1080 baiting (as is

currently scheduled for 2020).

• The aerial deer counts reported here for the unpoisoned area form baseline measures

of deer abundance that could be used to assess the repellency to deer of current or

new formulations of deer-repellent bait in the unpoisoned area.

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8 Acknowledgements

We thank Mark Watson, Henry Coulter and Lynda Harrap from Wyndon Aviation for

assistance with the survey. Thanks to Molesworth Station farm manager Jim Ward, and to

Chris Wooton and Phil Bradfield from DOC Wairau/Renwick, for access and permission to

carry out the survey. We also thank Dave Latham for reviewing a draft of the report, and

Ray Prebble for editing.

9 References

Caughley G. 1977. Analysis of vertebrate populations. John Wiley and Sons, London. 234

pp.

Forsyth DM, Allen RB, Marburg AE, MacKenzie DI, Douglas MJ 2010. Population dynamics

and resource use of red deer after release from harvesting in New Zealand. New

Zealand Journal of Ecology 34(3): 277–287.

Morriss G, Yockney I, Nugent G 2018. Comparison of deer abundance in 1080-poisoned

and unpoisoned areas on Molesworth Station. Landcare Research Contract Report

LC3159 for TBfree New Zealand.

Pinney K. (In preparation). Aerial survey of red deer following a pest control operation on

Molesworth Station 2017. Prepared for the Marlborough Branch of the New Zealand

Deerstalkers association.

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10 Appendix: Transformed mean counts per block.

Square root transformed mean counts per block

Poisoned Unpoisoned Total

Mean ± 95% CL Mean ± 95% CL Mean ± 95% CL

All deer 2018 0.88 0.56 3.70 0.72 2.17 0.72 2019 1.51 0.43 3.78 0.89 2.55 0.65

Hinds & Fawns 2018 0.43 0.36 3.17 0.92 1.69 0.72

2019 0.83 0.42 3.28 0.95 1.96 0.69

Stags 2018 0.58 0.51 0.92 0.81 0.73 0.47

2019 0.96 0.46 1.05 0.85 1.00 0.46

Chamois 2018 0.72 0.48 0.22 0.28 0.49 0.31

2019 0.70 0.52 0.09 0.17 0.42 0.31