What is Capture-Recapture Sampling?
Estimating population size is a fundamental objective of many wildlife management programs. The conservation and management of wild animal populations requires knowledge of their dynamics and the associated environmental influences.1 Management often requires periodic estimates of population size and methods for projecting the consequences of management actions for subsequent state dynamics.2
It can be very difficult to draw conclusions about things such as system state and dynamics for natural animal populations and communities however, the primary challenges being the tendency of animal densities to vary substantially over space and that sampling methods can produce counts which represent unknown segments of the animals in sampled locations.1
Questions about the conservation and management of natural resources also requires information about the spatial distribution of individual animals. For threatened or endangered species, the size of the species’ range is also used to assign species status as threatened or endangered.1,3 For invasive species and disease organisms, the dynamics of range expansion are relevant to control efforts to protect vulnerable species.1
Capture-Recapture Sampling Defined
Capture-recapture sampling is often used to estimate population size via the repeated sampling of uniquely marked animals within populations/areas. Capture-recapture sampling (which is also referred to as "capture-mark-recapture sampling" or "mark-release-recapture sampling") is a method used to estimate the unknown size of a population, because it is often impractical or impossible to manually count every individual element in a population. In many situations, capture-recapture sampling can produce a statistically valid estimate of a population size in a more efficient and timely manner than a census.1
How is Capture-Recapture Sampling Performed?
The most basic form of capture-recapture sampling consists of two stages. The first stage involves capturing a random sample of individual animals from a population of unknown size. The individuals are then marked or tagged and released back into the population. The second stage consists of capturing a second random sample of individual animals from the population. The second set will necessarily contain both marked (previously captured) and unmarked (not previously captured) individuals.
The ratio (proportion of marked to unmarked animals) is used to estimate the population’s size.
The size of populations of invertebrates or small mammals in an area can be estimated using capture-recapture sampling. This technique is particularly useful for animals with shells, such as snails and limpets or invertebrates with exoskeletons such as woodlice.
There are several assumptions that are made when estimating the size of population via capture-recapture sampling, some of which by nature have a tendency to skew results:
- The population must be closed (there is no migration of individuals into or out of the population).
- All members of the have an equal chance of capture (e.g., they mix randomly, which is often not the case based on sexual behaviors, age, etc.).
- Identifying materials (e.g., tags, paint) are not lost between samplings.
- The identifying material does not harm the animal (e.g., compromise its health or make it more vulnerable to disease, predators).
- The identifying materials do not affect chances of recapture.2
It has been widely argued that capturing and marking animals can be cost prohibitive and affect animal behaviors, which can bias population estimates.2 However, capture-recapture has long been used and considered reliable by a majority of biologists and wildlife managers. Additionally, multiple new approaches, including the use of digital photography, digital sound recording and DNA sampling are becoming available for conducting capture-recapture sampling without physically capturing animals.4
Interested in learning more about safe capture? The San Diego Zoo now offers courses in safe capture techniques and best practices. Learn reliable, safe, and effective techniques for the species you work with and the scenarios you encounter!
1Lavrakas, P. J. (2008). Encyclopedia of survey research methods (Vols. 1-0). Thousand Oaks, CA: Sage Publications, Inc.
2Lee, K., et. al. (2014). To catch or to sight? A comparison of demographic parameter estimates obtained from mark-recapture and mark-resight models. Biodiversity and Conservation. 23. 2781–2800.
4Manning, J., Goldberg, C. Estimating population size using capture–recapture encounter histories created from point-coordinate locations of animals. In: J. British Ecological Society, 12 November 2010.
About NexGen Pharmaceuticals
NexGen Pharmaceuticals is an industry-leading veterinary compounding pharmacy, offering sterile and non-sterile compounding services nationwide. Unlike other veterinary compounding pharmacies, NexGen focuses on drugs that are difficult to find or are no longer available due to manufacturer discontinuance or have yet to be offered commercially for veterinary applications, but which still serve a critical need for our customers. We also specialize in wildlife pharmaceuticals, including sedatives and their antagonists, offering many unique options to serve a wide array of zoo animal and wildlife immobilization and anesthesia requirements.
Our pharmacists are also encouraged to develop strong working relationships with our veterinarians in order to better care for veterinary patients. Such relationships foster an ever-increasing knowledge base upon which pharmacists and veterinarians can draw, making both significantly more effective in their professional roles.
Disclaimer
The information contained in this blog post is general in nature and is intended for use as an informational aid. It does not cover all possible uses, actions, precautions, side effects, or interactions of the medications shown, nor is the information intended as medical advice or diagnosis for individual health problems or for making an evaluation as to the risks and benefits of using a particular medication. You should consult your veterinarian about diagnosis and treatment of any health problems. Information and statements have not been evaluated by the Food and Drug Administration ("FDA"), nor has the FDA approved the medications to diagnose, cure or prevent disease. Medications compounded by NexGen Pharmaceuticals are prepared at the direction of a veterinarian. NexGen Pharmaceuticals compounded veterinary preparations are not intended for use in food and food-producing animals.
NexGen Pharmaceuticals, LLC does not recommend, endorse or make any representation about the efficacy, appropriateness or suitability of any specific dosing, products, procedures, treatments, services, opinions, veterinary care providers or other information that may be contained in this blog post. NEXGEN PHARMACEUTICALS, LLC IS NOT RESPONSIBLE NOR LIABLE FOR ANY ADVICE, COURSE OF TREATMENT, DIAGNOSIS OR ANY OTHER INFORMATION, SERVICES OR PRODUCTS THAT YOU OBTAIN THROUGH THIS BLOG POST.