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Editor’s note: Conservancy Senior Marine Scientist Alison Green is on an expedition to the Raja Ampat islands in Indonesia — amidst some of the most spectacular and biodiverse coral reef ecosystems in the world. Catch up on all her posts from the expedition.

When Dr. Seuss wrote the iconic children’s book in the 1960s of the above title, the art of coral reef ecology was just developing, and he could not have imagined that counting fish would become such a serious endeavor for science and conservation.

Fifty years later, we have well-established protocols for counting reef fishes, particularly those that occur at safe SCUBA diving depths, and are conspicuous because of their size, coloration and behavior.

How do we do it? Well there are several ways, but I prefer to use underwater visual census methods. This involves divers swimming along the reef slope at a fixed depth (usually 10m) counting and estimating the size of all reef fishes they see within a standard area (e.g. five 50m-long x 5m-wide transects), where the length and width of the transects depends on the species they are counting. (Big fish need bigger transects.)

But it isn’t as easy as it sounds. First of all, you have to know all the species you are counting, and here in the global center of marine diversity that includes 42 families and over 750 species! Just to make things interesting, most species have two or three totally different color phases (e.g. juveniles, adult males and adult females). So we have to learn literally thousands of color patterns before we can start counting fish.

And of course they don’t all just sit still waiting for us to count them. Imagine swimming through a cloud of brightly colored fishes of all shapes and sizes, all swirling around you, and trying to count them! It isn’t easy, but it can be done. It just takes a lot of training and experience.

Purwanto Irawan counting fish in SE Misool. (Photo: Andreas Muljadi)

And that’s one of the reasons I’m here in Raja Ampat — to help train our local monitoring team in methods for monitoring coral reef fishes.  Fortunately they are a very experienced team, so we actually training each other. I’m teaching them some new methods, and they are teaching me the local species. Oh, happy days!

-- Text by Alison Green, Cool Green Science Blog