How to code probable but uncertain mortalities?

questions concerning analysis/theory using program MARK

How to code probable but uncertain mortalities?

Postby jbauder » Tue Dec 05, 2017 3:34 pm

Hello,

This is an ongoing discussion I have had with colleagues and was wondering if I get some guidance from the MARK community. I have worked with several data sets on snakes where some individuals receive surgically implanted radio transmitters. Often, we either find a transmitter by itself or a snake doesn't emerge from the den in the spring. Usually, researchers (including myself) have assumed these represent moralities, yet we technically don't know. Occasionally a snake may expel a transmitter although mortality is more likely then expelling a transmitter. Hence the "tradition" of considering these probable mortalities as true mortalities.

But I am wondering about how these "probable" mortalities may affect (i.e., may bias) model-based estimates of survival. In any model where known-fates can be incorporated (e.g., known-fate, Burnham's joint recovery model), is it better to code "probable" mortalities as known mortalities, or does this risk under-estimating survival? Or can I even assume than wrongly assigning mortalities would under-estimate survival (maybe it doesn't with high recapture probabilities)? Would a more conservative approach be to censor such individuals after their last confirmed encounter?

Thanks,
Javan
jbauder
 
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Re: How to code probable but uncertain mortalities?

Postby Eurycea » Wed Dec 13, 2017 6:20 pm

Hi Javan,

I think you are correct in assuming that if you incorrectly interpret transmitter loss as a mortality, then your estimates of survival will be biased low. However in my limited experience with transmittered (that a word?) snakes, it's hard for me to see how they would lose one that has been surgically implanted without dying shortly thereafter. I think two far more likely problems with general interpretations are with transmitters you cannot find (is this mortality or emigration?) and how survival of telemetered snakes differs from ones that aren't (the surgery, added weight, etc.).
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