Individual identification: Photo-identification and mark-recapture

This is the third post about cetacean research methodology and we are going to talk about the individual identification. In concret, we are going to explain the photo-identification and the mark – recapture.

The individual identification consist on the identification of organismes using natural, permanent distinctive marks.

PHOTO-IDENTIFICATION

The use of pictures is very useful for studies about population size, population viability, habitat use, survival, movements and reproduction. It’s important to point that photo-identification isn’t a study by itself, but it’s a tool for other studies.

The photo-identification has some adventages: it’s not necessary to catch the animals; it’s cheap and it allows to creat a regional catalogue (and, thus, establish a migration patern or stay), to study the group fidelity, to do mark-recapture studies (and, thus, estimate the population size)  and to get reproductive information (sexual maduration age, nursing period, length of parental care and lonvegity).

But it has some limitations: it’s necessary to have a good experience and to know the behaviour of the species.

Some important things of photo-identification are:

1- Approach to animals. The approach is different between species, depending of its behaviour. In general, in coastal dolphins the approach is done for its back and laterally and in humpback whales and sperm whale is done for its back.

2- Equipment. It’s good to use a camera with 35 mm, with lightly zoom, that can resist the water and salt.

3- Notes. It’s recommendable to prepare a document where you can write the date, hour, place, number of animals, species, number of pictures, time spent, group fidelity and behaviour.

4- Pictures. The pictures has to be of one individual, except in cases of mum – young, and perpendicular to animal, with the best ilumination and writing the side of the animal (right or left). It’s recommendable to do 5 – 10 pictures of each animal and then you have to choose the best of them (animal type). CIRCE has written some specific instructions (in Spanish): http://www.circe.biz/files/Comohacerfotosdecetaceos.pdf

5- Picture analysis. It’s important to associate different features (scars, pigmentation patron…) to easy the individual identification.  There is different methdos to classify and identify this features. Defran et al (1990) use the dorsal fin ratio with animals with two slots at dorsal fin and Whitehead (1990) use a digital system for the edge of caudal fin in sperm whales.

MARK – RECAPTURE

It’s useful for populations that group in specific places every year. It can be applied in closed populations or in opened populations.

Lincoln-Petersen estimate is for closed populations (either death nor births and either immigration nor emigration). In addition, all the animals can be found with the same probability and the marks can’t be lost. It consist on mark a part of M animals of the population of unknown size N, this animals are returned to system and then you capture C animals of the population, where R animals are marked (they are the recaptured animals). Thus, we can calculate the population size: N = (M·C) / R

Jolly-Seber estimate is for opened populations. It assumes that each animal in the population has the same probability to be catch, each marked animal has the same probability to survive, the marks can’t be lost and all the samplings are short.

Population size. The population size can be studied with mark-recpature methodology. Each record of recapture are compilated on capture histories, that can be used to estimate the population size. It has been used on bottlenose dolphin (Tursiops truncatus), killer whale (Orcinus orca), humpback whale (Megaptera novaeangliae) and blue whale (Balaenoptera musculus). Somtimes, it can be used with DNA studies.
Survival and reproduction parameters. The marked animals are a cohort that can be followed and, thus, the recaptures in the next observations give infromation about survival and reproduction. It has been used in humpback whales (Megaptera novaeangliae), gray whales (Eschrichtius robustus), killer whales (Orcinus orca) and bottlenose dolphins (Tursiops truncatus). It can be useful to know the age of the first birth, the period of time between births and reproduction rates.

If you want more information, you can look for it in this referencees:

ANILAM, RESEARCH AND CONSERVATION, Métodos de investigación de cetáceos: http://www.alnilam.info/index.php/es/investigacion/inv-metodos

AUSTRALIAN GOVERNMENT, DEPARTMENT OF THE ENVIRONMENT, Non-lethal research techniques for studying whales: http://www.environment.gov.au/coasts/species/cetaceans/publications/fs-techniques.html

TRUJILLO, F & DIAZGRANADOS, M. C., Curso de técnicas de estudio de mamíferos acuáticos: manual básico, La Isla de los Delfines – Fundación Omacha, 2005

 

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