Gummy Bears and Math

Electronic mark-resight studies: part 1

Aspiring young scientists often joke that they should put “expert counter” on their resume. Counting seals, counting barnacles in a quadrat, counting birds, counting cells… you name it, an intern has counted it. This is because a lot of ecological research and conservation requires knowing how many of something there are in a particular space, or how the number of something changes over time. It is the first step to knowing if populations are doing well or are declining, and is essential for setting management targets (e.g. we first need to have an idea of how many fish are in the sea before we decide how many we can harvest).

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Counting the number of fur seal pups while they are moving and squirming around is definitely ‘advanced counting’.

This often means sitting down in front of thousands of pictures to count seals, or getting knee-deep in marsh mud to count snails–which as you can guess is time-consuming. Additionally, counts alone rarely provide an estimate of population size or survival.

Most science geeks and wildlife population ecologists are familiar with mark-resight studies– a different tool that can be used to estimate vital rate metrics such as population size or age-specific survival rates. Typically, animals are captured, marked with a unique identifier and released, whereafter they redistribute within the wild population.


The basic concept is very simple: Imagine you have a box of red gummy bears (the population). You want to know how many are in the box but can’t just count them all. Instead, you ‘catch’ some, mark them, and put them back in the box (in this case substitute with yellow ones). You then shake the box and resample. From the ratio of yellow to red in your second sample, you can estimate the total number of gummy bears in the box.

If you prefer numbers:

And who said you’d never use your 9th grade algebra in real life? 🙂

As you eat some of the gummy bears and then resample, you can also estimate gummy bear survival by the number of yellow bears you resample in relation to the original number released.

Translating gummy bears to real bears

In a real study on wildlife, we don’t replace animals, but instead mark them. Many different types of identifiers can be used, including paint marks or bleach marks, numbered tags (such as cattle ear tags), tattoos, hot-iron brands or freeze brands. In some fish, fin clipping is used (although this does not constitute a unique identifier), and even non-visible, internal PIT tags have been used. In some species, natural fur patterns or markings are so unique and persistent, that they can be used for mark-resight studies. Examples of such studies include whale fluke photo-identification.

Check out this video from NOAA about using photo-ID to identify individual whales.
Mark-recapture for birds is usually done with rings or bands on their legs that have unique numbers and colors. We can use the rate of resightings of marked birds to non-marked birds in large aggregations like this one to estimate population size. We also can understand more about individuals. The bird in the top right picture is “Wisdom” (Photo credit: Madalyn Riley /USFWS ), a Laysan albatross on Midway Atoll. Wisdom was first banded in 1956, meaning we know she is at least 68 years old and can track her survival from her band!

A critical part of such studies is the determination of detection probability, of which loss of marking may be a component. Not so much in the case of permanent markings such as fur patterns, branding or tattoos, but certainly when something like a plastic cattle ear tag is used, they can break off, be chewed off, or fall off. To determine that likelihood, scientists usually double-tag. From the incidence of double to single flipper tag sightings over the course of multiple sequential resights, the tag loss rate can be estimated.

For most such markings, the resight effort involves direct observation surveys, and that can take a lot of people, time and money, especially in remote places. So we are right back to the same problems with counting!

That is one reason why researchers have also used electronic tags – usually VHF transmitters – combined with automated scanning systems.

This Pacific Sleeper shark has two telemetry tags and is ready to be released. One tracks diving, light and temperature (the bouyant one with the antenna), and a Vemco acoustic tag (the grey cylinder with the white cap) that emits a signal that can be detected by automated scanning stations.

VHF transmitters or PIT tags are examples of electronic marks that can be used in mark resight studies to infer population size, or age specific survival rates. Satellite-linked transmitters that use the Argos system are a common tool in the study of wild animals. However, they are not typically used for mark resight studies. In part, that is probably due to their cost – much greater than that of VHF transmitters.

However, if these satellite tags are used for other reasons, why not also use them to estimate survival?

Photo Credit Dana Angus © The Marine Mammal Center.

Can Argos satellite transmitters be used for electronic mark-resight studies?

The short answer: probably yes.

It’s one question we are seeking answers to in conjunction with a collaborative study led by Dr. Michelle St. Martin with the US Fish & Wildlife Service.

Tune in Friday to hear more about this research and how we are hoping to learn more about sea otter populations with this technology!

Written by: Markus Horning, PhD and Amy Bishop PhD

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