What the Heck is DNA Mark & Recapture

by Aug 14, 2019Conservation

In Episode 001 of The Hunter Conservationist Podcast, grizzly bear researcher Clayton Lamb from the University of Alberta said that the DNA mark and recapture method of estimating wildlife populations is the “gold standard” in wildlife population science. What the heck is DNA mark and recapture? Can’t they just go out and count all the animals in the population?

Counting every individual in most wildlife populations is not possible or practical. Reliable population estimates are, however, important in wildlife conservation for a multitude of reasons. Most notability, populations estimates are used to make decisions about the protection of rare species, recovery of endangered species and to ensure hunter harvest levels are not detrimental to the population that the game is being harvested from.

Traditional methods of estimating wildlife populations include directly counting animals which is often done from aircraft. The technique of capture-mark-recapture was developed as another way to estimate wildlife populations. This method involves physically capturing a set of animals, marking them with a number, releasing them and then capturing another set of animals at another time. Based on the proportion of marked animals in the second subset of captured animals, scientists can reliably estimate the total population.

The capture-mark-recapture method is challenging, though, because it requires researchers to capture animals without causing them harm. There is a phenomenon in wildlife research called capture myopathy, which is a condition an animal can develop as a result of the stress caused by chemical immobilization, transport or handling by humans. Some species are very susceptible to capture myopathy, which can lead to their immediate or delayed death after being handled by humans.

Another problem with capturing animals is that some individuals, after being captured and handled, might not go off an behave naturally after being released. Their altered behaviour can bias a scientist’s goal of getting objective information about the population being studied.

In the 1990’s, scientific advancements in the field of genetics allowed wildlife scientists to develop a variation of the capture-mark-recapture technique.  DNA collected from hair, bones, muscle tissue and the cells discarded from the wall of the intestines in feces can be used for genetic-fingerprinting individual animals.  This reveals things like the animals’ sex and who they are related to. DNA mark and recapture, also known as genetic capture-mark-recapture, is a proven reliable scientific technique for estimating wildlife populations.

Genetic capture-mark-recapture has given researchers the ability to estimate population sizes and determine sex ratios, survival, migration, and the ability of individuals to reproduce. This method allows wildlife scientists to assess population changes more easily and more reliably than some traditional wildlife population study techniques. Studying cryptic species (rare and/or elusive species) is now more effective and efficient since researchers only need to find hair or scat samples of a cryptic species in order to fingerprint individuals and estimate their population size. Grizzly bears are one species that were notoriously difficult to study, and it was difficult to confidently estimate their population size until the genetic capture-mark-recapture technique was accepted as a rigorous scientific approach.

The genetic capture-mark-recapture method is considered a non-invasive and ethical form of wildlife research because researchers can collect samples without having to capture or immobilize the individual. In some studies, such as those involving whales, cougars and polar bears, researchers sometimes use biopsy darts to remove a tiny plug of tissue. The genetic capture-mark-recapture technique is like the traditional capture-mark-recapture approach except that scientists don’t need to physically capture and recapture the animals. The genetic analysis of DNA samples tells researchers when they have re-captured the same individual and when they have captured a new individual in the population. The recapture rate is a key element needed to estimate population size.

Several creative ways have been developed to allow researchers to collect DNA samples. Carnivores can be attracted to stinky bait stations where strands of strategically placed barbed-wire are used to snag bits of hair that are later collected by researchers when the toothy critters aren’t around. For bears, barbed-wire can also placed on their rub trees to collect hair samples. Snagging hair at burrow entrances is another technique that is used for ground dwelling animals like badgers. Collecting ungulate poop pellets is a non-invasive technique used to gather DNA samples without harming or disturbing the behaviour of the animals being studied.

Even though genetics is an advanced scientific tool, scientists still take great care in designing genetic capture-mark-recapture wildlife studies because there things that can mess up the study that they need to mitigate. It is paramount that all DNA samples are of high quality. Poor quality DNA samples can cause errors in the genetic analysis that lead to mis-identifying individuals in the population. Scientists must also ensure that all individuals in their study have a reasonable chance of being captured. For carnivores that are being attracted to bait stations, researchers are on the look out for individual animals that are “trap happy” or “trap shy” since both these behaviors affect the capture rate which can lead to over or under estimating the size of a population.

A population is a group of members of the same species that all live in the same geographical area and are capable of interbreeding. In genetic capture-mark-recapture studies, researchers will establish a grid system across their study area from which they collect DNA samples. The grid system is designed to represent the home range of the animals being studied.

Genetic capture-mark-recapture studies can be affected by something called a closure violation. A closure violation occurs when the animals being studied have home ranges larger than the study grid used to collect DNA samples. When this happens, the research project ends up capturing animals moving in and out of the sampling grid area near its boundary which can lead to the over estimation of the population size in the study grid area. Researchers will sometimes combine using GPS collars on some study animals with the genetic capture-mark-recapture technique to improve the delineation of the home range of the study population and the estimate of the population within that home range.

Genetic capture-mark-recapture can be a cost-effective way to make reliable, large-scale population estimates about wildlife for the purpose of measuring the status of endangered species, estimating the size of a population for species with large home ranges and estimating the population size of game animals that need to be managed responsibly for sustenance hunting. As with any kind of wildlife science, it’s not important for hunter conservationists to know exactly how the science is done. Rather it’s more important to understand what the evidence is saying.

The gold standard of all wildlife research lies in the fact that science provides policy makers with objective evidence – hard facts that describe reality. Objective evidence needs to be a trusted way of making conservation decisions about fish and wildlife whether the policy decisions are intended to conserve moose in Newfoundland, black bears in Ontario, mountain caribou in Alberta or steelhead trout and chinook salmon in British Columbia.

As with all rigorous scientific studies, the genetic capture-mark-recapture technique of estimating the size of wildlife populations is a hallmark for the North American Model of Wildlife Conservation. Tenant 6 of the model states that science is the proper tool to use for making wildlife policy decisions.

Whether it’s an estimate of a wildlife population derived from a genetic capture-mark-recapture study that can help establish a safe harvest level for hunting or findings from a wildlife research project that will guide the recovery an endangered species, hunter conservationists are the type of people that always demand that policy makers use science-based evidence in making decisions about conservation.

Cover Image: © doneyanedoman / Adobe Stock


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