Trap-nights measure how long traps have been available to catch pests. Catch rate expresses catches relative to that trapping time, so you can compare trapping activity across periods and projects with different numbers of traps.
Trap-nights measure trapping time
One trap armed for one night counts as one trap-night. For example:
- 50 traps armed for 1,000 nights = 50,000 trap-nights
- 500 traps armed for 100 nights = 50,000 trap-nights
Both represent the same amount of trapping time, although the number of traps differs. On a project dashboard, the total covers your selected period.
Catch rate is catches per 100 trap-nights
Catch rate is the number of catches per 100 corrected trap-nights. TrapScan corrects the trapping time to account for sprung traps and long gaps between checks, as explained under How trap-nights are counted.
If a project records 10 catches over 1,000 corrected trap-nights, its catch rate is:
10 ÷ 1,000 × 100 = 1 catch per 100 trap-nights
This makes catches relative to trapping time easier to compare than catch totals alone. Habitat, season, target species, trap type and previous predator control also affect the result, so catch rates from different projects may not be directly comparable.
Reading a catch rate
A catch rate is a measure of catch activity, rather than a score for how well trappers or projects are performing. A falling rate may reflect successful predator suppression, seasonal changes or differences in pest activity. A high rate may simply indicate more pests in the area.
With many traps operating, trap-nights accumulate quickly. Even a substantial number of catches can therefore produce a low rate.
Catch rate is most useful when you follow it over time alongside trap checks, trap-nights and catches on your dashboard. The National Trapping Network dashboard explains the shared charts and time periods.
How trap-nights are counted
TrapScan calculates trap-nights between recorded checks. The first check provides the starting point and contributes no trap-nights itself.
For a selected dashboard period, a check only adds trap-nights if it falls within that period. If the previous check was before the period began, only the time from the period's start counts; the 90-day limit and sprung-trap correction apply to that shorter interval. Time after the last check in the period isn't counted, even if another check is recorded after the period ends.
Sprung traps count half
When a trap is found sprung, TrapScan can't tell when it went off. It therefore counts half the time since the previous check, whether the trap caught something or was sprung and empty. A sprung trap can't catch another pest until it's reset.
| Check | Days since previous check | What's found | Trap-nights counted |
|---|---|---|---|
| Day 0 | First check | 0 | |
| Day 10 | 10 | Still set | 10 |
| Day 20 | 10 | Caught a rat | 5 |
| Day 30 | 10 | Sprung and empty | 5 |
| Total | 1 catch | 20 |
That's one catch over 20 corrected trap-nights, giving a catch rate of 5 catches per 100 trap-nights. This half-interval adjustment follows DOC's corrected-trap-night method.
The 90-day limit
TrapScan counts no more than 90 days between two checks, even if a trap goes unchecked for months or years. This prevents a long gap from adding years of unverified trapping time.
For example, when a check is recorded three years after the previous check, a trap that's still set earns 90 trap-nights. If the trap is found sprung, TrapScan applies the limit before halving the time, giving 45 trap-nights.
The limit applies regardless of the reason for the gap, including bad weather or difficult access. It doesn't change catches or trap-nights already recorded before the gap.
The 90-day limit matches Trap.nz's default maintenance threshold and the period TrapScan uses to classify traps as Not Reporting.