(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 it comes to coral reef ecology, size really does matter. For example, some species of reef fish play different ecological roles, depending on their size.

For example, parrotfishes begin life on the reef as carnivores, and only become herbivores when they are large enough to grind plant material in their specialized mouthparts. On the other hand, some unicornfishes feed on algae when they are young and zooplankton when they are adults.

And some fish like groupers even change sex! So the small ones are female and the big ones are male.

So when we count fish, it is important that we estimate their size so we can understand their role in the ecosystem.

How do we estimate the size of fish underwater? Well, that requires a lot of training…and since estimating fish sizes is the greatest source of error in underwater visual censuses, it is important we get it right. So our field teams practice this several times a year to make sure that their estimates are as accurate as possible. To do this, they practice using wooden and plastic models of fish on land and in the water.


This week, Purwanto Irawan, monitoring coordinator with The Nature Conservancy and WWF’s Joint Program in Wakatobi National Park, trained us in estimating fish sizes underwater (see photo above). This is not as easy as it sounds, partly because everything appears larger underwater, and partly because some of the fish are quite large (over 1m long!). Fortunately, Purwanto is a master at this–he has 90% accuracy, so we had a great trainer. 90% accuracy is really good, particularly given that the fish are moving all the time, and some are in large schools!

Now that we have completed this training, the team is ready to start counting fish. Let the survey begin!

( Image 1: Monitoring team practicing fish size estimation underwater, Southeast Misool. Credit: Andreas Muljadi. Image 2: Monitoring team learning how to estimate fish sizes on the beach, Southeast Misool. Credit: Mohammad Syakir.)

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