Recapture rate, how low can it be?

[claudiapenaloza]

In 16 years over 300,000 tagged yearling turtles have been released.
About 300 have been recaptured.
Only one has been recaptured more than once.

Can I still use these data to estimate survival with MARK?

Thank you,
cp

[cooch]

In 16 years over 300,000 tagged yearling turtles have been released.
About 300 have been recaptured.
Only one has been recaptured more than once.

Can I still use these data to estimate survival with MARK?

Thank you,
cp

Sorry to be the bearer of bad news, but with encounter rates that low, there is precious little you'll ever be able to do (meaning, you won't be able to derive much of anything useful from your data since, conditional on having encounters of marked individuals, and with only one individual seen more than once) there is essentially no data, despite the impressive numbers originally tagged. Your situation is analogous to tagging young for many species with high juvenile dispersal rates - which is why juvenile survival is generally poorly estimated for those species - which is also why studies of such 'nasty creatures' often resort to other, technological solutions (e.g., telemetry, etc).

[Eric Janney]

I think Evan is correct that your best bet is to try to use technology to get more recaptures. I had a similar problem doing a fish survival study in a large lake. We tagged and released around 5,000 fish a year but recapture rates were poor (0.02 - 0.10). We tried increasing sampling effort, but the improvement in recapture probabilities was marginal. So, we started using remote underwater PIT tag antennas placed in strategic locations during the spawning season. The improvement in recaptures has been dramatic. We now have recapture probabilities between 0.50 and 1.0. The key to using remote PIT tag technology is to find locations where the animals congregate or migrate through.

[cooch]

I think Evan is correct

I generally like to assume such is true (at least on average).

that your best bet is to try to use technology to get more recaptures. I had a similar problem doing a fish survival study in a large lake. We tagged and released around 5,000 fish a year but recapture rates were poor (0.02 - 0.10). We tried increasing sampling effort, but the improvement in recapture probabilities was marginal. So, we started using remote underwater PIT tag antennas placed in strategic locations during the spawning season. The improvement in recaptures has been dramatic. We now have recapture probabilities between 0.50 and 1.0. The key to using remote PIT tag technology is to find locations where the animals congregate or migrate through.

There is a basic rule of thumb that if your encounter rate is <0.10, you're not going to be able to do much that will be of any use (<0.05 and some folks would suggest you're wasting your time, and probably some else's money). Technology isn't always a solution but can often be (example as described).

Basically, the 'big law' says do everything you can to increase encounter rate. Better fewer individuals you have a better chance encountering than a lot of individuals you'll never see again.