robust design question

[bschmidt]

Dear all

I have a question about whether we can rearrange a data set such that we can use the robust design for analysis. Using the robust design would be nice because non-random temporary emigration is likely.

We have data from a long-term study on a toad population. Toads were sampled at their breeding site. The breeding site (a pond) was completely encircled with a drift fence and toads were captured on their way to the pond and when moving back to the terrestrial habitat. After capture & marking, toads were released on the other side of the drift fence. Some toads were on their way back to the terrestrial habitat before the last ones migrated to the pond. The drift fence was checked daily and all toads were individually marked.

During each year (=breeding season) toads were capture daily during a period of 2-3 months. We could use each day as a secondary sampling period, but then we would have a huge number of secondary sampling periods with only a few captures within each.

Can we reduce the number of secondary sampling periods? Kendall (2001, Proceedings 2nd International Wildlife Management Congress) provides some advice on pooling secondary sampling periods:
1) if the entire population is present at the first session but begins to leave before the last session, then sessions 2 to last could be pooled.
2) if sampling begins before all animals have arrived, but all are present at the last session, then sessions 1 to (last-1) can be pooled.
In our study, we have a mixture of the two: some toads leave before the last session and sampling begins before all have arrived.

Now, could we pool the data such that we have only two secondary sampling periods? The first is when the toad is captured on the way to the pond and the second is when the toad is captured on the way away from the pond. With such data, we could use the "open robust design" of Schwarz & Stobo where animals are allowed to enter and leave the study area once.

One problem is that secondary sampling periods last for a long period of time. For example, a toad may be captured on the way to the pond on 1 May and leaving the pond on 30 May, whereas another toad is captured on the way to the pond on 1 June and leaving the pond on 15 June. Both toads would have capture history 11 instead of two capture histories 1100 and 0011.

I would be happy if you could offer some help.

Best wishes

Benedikt