By Rebecca Morelle Science reporter, BBC News

How to count identical penguins and other audit tips

During the next few weeks they will carry out the painstaking job of counting each and every animal that lives in their collection - from gorilla to gecko, frog to flamingo, shark to spider.

British and Irish Association of Zoos and Aquariums (Biaza) director Miranda Stevenson says: "Once a year, zoos around the country need to check that the number of animals that they have on their computers is the same as the number of animals in their collection."

Creature accountancy In pictures

This annual "stock-take" falls under the Zoo Licensing Act, legislation that governs all zoos and aquariums.

But of course, Dr Stevenson adds, some animals are more tricky to tot up than others.

Dr David Gibson is director of husbandry at The Deep, an aquarium in Hull. He thinks counting schools of identical fish poses the ultimate challenge.

"Some of the animals we have in the aquarium are really simple to count - a 3.5m-long [12ft] shark, for example. But the problem comes when you have the schooling species that look identical," he says.

The job requires a keen eye and a lot of patience.

I would be fibbing if I said it wasn't hard work for some species Warren Spencer, Bristol Zoo

Dr Gibson says: "A member of staff will be assigned to a particular species, like the horse-eye jack, and they will stand at the window of the tank and use clicker counters to count the number of fish in the school that they see at any one time.

"They'll do this six, seven or eight times until they get a consistent number."

The technique is surprisingly accurate.

"We know we have 208 horse-eye jacks", Dr Gibson says with confidence.

Birds wear identification bracelets with unique numbers on

Warren Spencer is somebody else facing some challenging counting.

He is the curator of invertebrates at Bristol Zoo Gardens, looking after the 67 different species that live in Bug World.

Mr Spencer says: "We love these animals and these censuses are very very important, but I would be fibbing if I said it wasn't hard work for some species, because it is."

Luckily groups of insects such as ants or locusts are counted as a single colony rather than requiring the laborious task of being checked off as individuals.

"But for others like stick insects and beetles we do have to count them one by one," Mr Spencer adds.

The number-crunching is complicated further still by having to determine the tally of the insects at their various life-stages, separating them into adults, larvae and eggs.

But, says Mr Spencer, although audits are tough work they are vital.

He explains: "We need to have very specific records of all of our animals.

All hands are going to be on deck David Field, ZSL London Zoo

"And although we do this annual census, there are lots of audits taking place throughout the year - for us the job of counting our invertebrates is ongoing and continuous."

David Field, zoological director at ZSL London Zoo and Whipsnade, adds that keeping stock of the animals is key for conservation.

He says many zoos' inventories will be submitted to an international database called the International Species Information System (Isis), which gathers information about animals held in zoological institutions around the world.

He said: "It is a real priority to make sure our figures are correct so we can best manage conservation.

Animal numbers are submitted to an international database

"Some of the species we keep here, like Socorro doves, are now extinct in the wild, so the only populations exist in zoos.

"So when zoos submit data to these central databases, it means that we can run detailed and scientifically based breeding programmes."

Mr Field admits that the next few days are going to be busy for those working in zoos around the country.

"All hands are going to be on deck," he says.

"It is going to be a challenging job, but once we have done stock-take and produced the whole inventory for the year, we are going to be able to sit back and see what we have been able to breed and to see what we have been able to achieve during the year."