Hi,
While reviewing some trap results, I came across what may be an inconsistency in the CCR report.
I’m looking at project ID 8611421, although I’ll keep the project details private on the public forum.
Trap ID 8645081 is a double-set DOC 200 station. In the Trap Records report, filtering for:
- 8 August 2021 to 7 August 2026
- Rat
- DOC 200
produces 21 recorded kills. I have checked the individual records and can confirm that 21 is correct.
Over five years, a double-set trap has a theoretical maximum of approximately 3,650 trap-nights: 1,825 nights multiplied by two trap mechanisms. On an uncorrected basis, this would produce:
21 ÷ 3,650 × 100 = 0.58 rats per 100 trap-nights
The corrected rate should be slightly higher because one mechanism becomes unavailable after a capture until the next servicing visit.
I then applied the same date, species and trap-type criteria in the CCR report.
The first indication of a possible problem was the sorting. When I sorted by “Catches per 100 nights”, the highest figure shown at the top of the list was 0.45. However, when I sorted by “Catch strikes”, the results included a trap with 21 strikes and a CCR of 0.70, while trap ID 8645081 appeared second in the list with a CCR of 0.89. This suggests that the “Catches per 100 nights” sorting is not working correctly.
There also appears to be a possible issue with the trap-night calculation. The CCR report assigns trap ID 8645081 only 2,355 corrected trap-nights, producing:
21 ÷ 2,355 × 100 = 0.89
That trap was serviced 91 times during the reporting period, at reasonably even intervals. It has never recorded both DOC 200s as triggered during the same interval: all 21 records were single kills. Therefore, after each kill, one trap mechanism remained available while the other was unavailable until servicing.
Assuming captures occurred, on average, halfway between servicing visits, I estimate approximately 3,441 corrected trap-nights:
- Gross trap-nights: approximately 3,650
- Average servicing interval: approximately 20 days
- Estimated lost trap-nights per kill: approximately 10
- Total adjustment: approximately 21 × 10 = 210 trap-nights
- Corrected trap-nights: approximately 3,650 − 210 = 3,440
- Expected CCR: approximately 21 ÷ 3,440 × 100 = 0.61
The precise figure will depend on the actual interval associated with each kill, but I cannot see how the corrected trap-night total could fall as low as 2,355—or the CCR rise to 0.89—unless the calculation is treating the entire double-set station as unavailable whenever one of its two traps is triggered.
Could you please confirm how double-set traps are handled in the CCR calculation? In particular, does each mechanism contribute a separate trap-night, and does a single kill remove availability only from the triggered mechanism?
Cheers
David