The National Parks and Wildlife Service has released supplementary information and the raw data collected as part of the 2023 horse population survey in Kosciuszko National Park.
The supplementary statement summarises how the raw data is analysed to generate a population estimate for Kosciuszko National Park.
Copies of the raw data sheets will be made available on request. Please email your request to npws.wildhorses@environment.nsw.gov.au
Please note that in reviewing the raw data and conducting additional analysis to inform 2024 operations, a small adjustment to the previously published 2023 population estimate has been made because:
- 18 horses (and 5 clusters) recorded during the survey were not included in the original analysis as a result of a data entry error
- a small GIS discrepancy in the area of the northern block has been corrected.
The adjustment has resulted in an updated population estimate of 12,797 to 21,760 horses (95% confidence interval [CI]) with a best estimate of 17,393 horses. The adjustment to the best estimate is less than 0.5%.
Please note the supplementary statement below is intended to provide a general overview of how the survey was conducted and the raw data used to generate a population estimate. It does not seek to address detailed technical aspects of the survey, nor the statistical analysis.
How the 2023 survey data were collected and analysed
The 2023 survey generated a population estimate of 12,797 to 21,760 feral horses (95% CI), with a best estimate of 17,393 horses.
- It is important to note the survey generates an estimate of the horse population for Kosciuszko National Park based on sampling part of the park. It is not a census because counting every horse in such a large, rugged landscape is impossible.
- The methodology employed – distance sampling – is best practice. The results were peer-reviewed by experts at CSIRO and the Queensland Department of Agriculture and Fisheries.
Horses occur across approximately 365,000 hectares, or 53%, of Kosciuszko National Park.
The 2023 horse population survey was conducted in 4 survey blocks totalling 267,500 hectares, or approximately 39% of the park. That is, the survey produces a population estimate for 39% of the park:
- because feral horses are known to occur in an additional 14% of the park, the population estimate from the survey is an underestimate of the true population in Kosciuszko National Park
- there are several reasons the additional 14% of the park is not surveyed – the primary reason is the steep topography makes it unsafe to survey the area using helicopters
- a map of the horse distribution in Kosciuszko National Park and the 4 survey blocks is provided below (see map 1).
Within the 4 survey blocks, horses were counted across 58,440 hectares, or 21.85%, of the total survey block area. The proportion of each block where horses were counted varies from 19% in the northern block to over 40% in the Snowy Plains block (see table 1).
The survey involved flying a total length of 1,948 km of transects in a helicopter, with trained observers counting horses they observed on either side of the helicopter within 150 metres (300 metres wide in total). Maps 2 and 3 show the transects in the northern and southern survey blocks.
The location of transects follows a systematic random sampling design – that is, the starting point for the transects in each survey block was randomly generated, with layout of the transects then following a systematic pattern. This means that the sampling procedure was unbiased.
In total, 1,926 horses were counted by the observers. This means 1,926 horses were recorded across 58,440 hectares or in 21.85% of the total survey area. Table 1 shows the raw number of horses seen (counted) in each survey block and the proportion of each block that was sampled via transects.
To produce a population estimate for the total survey block area, or 39% of the park, the raw number of horses observed (1,926) needs to be adjusted in 2 ways.
The first adjustment is to reflect the fact that the transects in which horses were recorded covered only a proportion of each survey block area.
- Because the sample in each block was unbiased, the best science means that we assume a similar density across the remaining area of each of the 4 survey blocks.
- This first adjustment is a simple multiplication: the raw number of horses seen in each survey block is adjusted for the proportion of that block that was sampled. In the northern block, this means 1,441 horses are multiplied by 5.2 (to convert 19.4% to 100%). This would generate a population estimate in the northern block of around 7,435 horses.
- This simple adjustment for the proportion of each sampled block provides a total estimate across all 4 survey blocks of around 9,620 horses.
However, this calculation assumes all horses occurring within the 300-metre-wide transect were seen by observers. A second adjustment is required because, in fact, not all horses in the 300-metre-wide transects were seen – that is, there were more than 1,926 horses in the transect area.
- There are several reasons some horses within the transect area were not seen by observers in the helicopter, including vegetation cover and the distance away from the observer.
- Distance sampling modelling is used to estimate the proportion of horses on the transect that observers saw. A different 'detection probability' – the proportion of horses seen – was determined for each survey block. However, on average, across all 4 survey blocks, it was estimated that a little more than half of all horses in the 300-metre-wide transects were seen by observers.
- Once an adjustment is made for the proportion of horses within the transects that were seen by observers, the total population estimate for the 4 survey blocks is 17,393 horses.
- Recognising there is some variability when estimating the level of detection by observers, the 95% CI for the population is 12,797 to 21,760 horses.
- We can be 97.5% confident that the horse population in 39% of the park was at least 12,797 horses at the time of the 2023 survey.
The number of horses removed in the initial aerial shooting program in southern Kosciuszko National Park is consistent with the 95% CI generated by the survey results.
Table 1: A summary of the raw counts of horses in the 4 survey blocks
Survey block | Raw count | % of survey block sampled |
---|---|---|
Northern | 1,441 | 19.4% |
Snowy Plains | 82 | 43.4% |
Cabramurra | 8 | 33.8% |
Southern | 395 | 20% |
Total: Kosciuszko National Park | 1,926 | 21.85% |
Table 2: The population estimates (N) for each of the 4 Kosciuszko National Park survey blocks with confidence interval and coefficients of variation (cv%).
Given also with these estimates are the areas surveyed, including the total area of the 4 survey blocks.
Survey block | Area (km2) | N | Confidence interval (95%) | CV % |
---|---|---|---|---|
Northern Kosciuszko | 1,229 | 13,212 | 8,895 to 16,842 | 16.3 |
Snowy Plain | 161 | 363 | 119 to 743 |
41.2 |
Cabramurra | 139 | 49 | 0 to 106 | 58.9 |
Southern Kosciuszko | 1,146 | 3,769 | 2,337 to 5,720 | 23.7 |
Total: Kosciuszko National Park | 2,675 | 17,393 | 12,797 to 21,760 | 13.8 |
Download
- Map 1: horse distribution across survey blocks October 2023 (PDF 412KB)
- Map 2: southern survey bock with transects October 2023 (PDF 1.6MB) (High points in the landscape are indicated by orange triangles)
- Map 3: northern survey block with transects October 2023 (PDF 1.3MB) (High points in the landscape are indicated by orange triangles)