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I am trying to determine population size through monthly open population surveys. We only did one marking event - with no subsequent marking thereafter. Would I be able to use a POPAN model even if marking of animals did not continue?

No, that wouldn't work. The essence of POPAN is that it tracks the "arrivals" and "departures" of individuals over time, to estimate the total number of individuals over the span of sampling. By not marking new individuals you lose the ability to estimate the arrival process. It's difficult to comment beyond that, without knowing more about the system you're dealing with and whether abundance at each point time is of interest.

Hi Bill
Thanks so much for the feedback - it makes sense and I just wanted to double check. So, if I did put this into a POPAN model (and it did spew something out) - the N really only provides a trend over time with regards to the animals marked originally. And thus not a population estimate per se.
Would this be suitable to do? And if so, would the other parameters (phi, p and pent) be reliable?
Or would it be best to just leave out any attempt at N estimates and just stick to a CJS approach?
Thanks again for your time and help - I do appreciate it!

Given each of the individuals in your dataset will be captured and tagged in the first sampling period, I don't believe you're going to get anything useful out of POPAN, and will likely be led astray. You don't have the data collection necessary. I would switch to CJS.

Hi Bill
This helps a lot - thanks so much!