Raja Ampat 2012: Does Fish Length Matter?

This is the fourth in a multi-part series chronicling the 2012 trip to monitor the health of coral reefs in Raja Ampat, Indonesia. This year, Conservancy scientists are traveling to Raja Ampat alongside colleagues from CI and WWF.

This post was authored by Purwanto, the Conservancy’s technical advisor in the Raja Ampat region, and is cross-posted on CI’s blog.

Using plastic models of fish, the monitoring team has been regularly practicing an important skill: identifying — and estimating the length of — the species we encounter in the wild.

Fish length is calculated by measuring from the tip of the head to the center of the tail. The fish’s weight can then be extrapolated from the length, which is useful for calculating the total biomass we’re observing in the waters we survey. So, it’s important to  be precise when we make our estimations, and practice makes perfect.

The length of the fish can also tell us its gender. Several species, including economically and ecologically important grouper species, are generally female when smaller and male when they’ve reached a certain length. When fish gather to spawn, we typically observe a balanced ratio between males and females, so estimating lengths helps us get a better idea of the composition of fish groups.

Bigger fish will also produce more eggs and offspring. For example, a red grouper that’s 60cm long will produce 10 times more eggs than a 40cm specimen of the same species. This is why it’s crucial to prevent excessive fishing and let fish grow large enough to birth future generations and ensure the sustainability of fisheries.

So, there are a great many reasons why fish length estimation is a valued skill out here in the Raja Ampat Islands. On this trip, we have five people assigned to record fish: Devy Pada and Edy Setyawan of Conservation International; Aser Burdam and Elvis Mambraku from the Raja Ampat community; and me. Through consistent practice, we’ve become more than 75% accurate in recording the length of fish, so we can be confident about the accuracy of the data we capture in the field.

Learn more about the Conservancy’s involvement in the game-changing Coral Triangle Initiative on Coral Reefs, Fisheries and Food Security.

(First image: The team diving with fish. Second image credit: Sangeeta Mangubhai/TNC. Second image: The correlation between fish size and offspring. Second image credit: PISCO.)

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