Instant expert

How to count the fish in the sea

It turns out that finding out how many fish are in the sea is not an easy task

School of fish

Credit: iStockphoto

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There are two ways to estimate the number of fish in the sea: direct observations and indirect methods.

Direct observation

As the name suggests, means scientists go out to sea and use special equipment to count the number of fish in each school. In shallower waters, say for abalone, divers get in the water to count the fish directly.

Although it doesn't sound like a 'direct observation', scientists also tag some fish in the stock and release them back into the ocean.

They then recapture fish from the stock, and count how many are tagged and how many aren't. The ratio of tagged to untagged fish allows scientists to estimate the size of the fish population.

Direct observations are very costly, though, and the results are complex, so they're only used to estimate the size of expensive fish stocks.

Indirect methods

Scientists typically use indirect methods because they're cheaper.

However, according to Keith Sainsbury, board member for the Australian Fisheries Management Authority, indirect methods are "also the most commonly misleading".

For example, if you want to know how many fish are in a small pond, you would use a small scoop net to take samples. At the beginning, your 'catch rate' (the number of fish you catch in each scoop) might be quite high, but with each scoop you catch fewer fish.

Your catch rate reflects the number of fish that are still left in the pond.

Now comes the important bit. To work out the number of fish that were in the pond before you came along with your net, you need to keep track of the total number of fish you catch in the pond, or your 'catches'.

If you catch half the original number in each scoop (your catch rate), it means you are down to half the original number of fish in the pond.

So, to calculate the original number of fish in the pond, you simply double the total number of fish you have taken out of the pond.

Unfortunately for fisheries scientists, our little pond is not quite like the big blue sea.

In real life, fish might dodge your little scoop net (or big trawl net), they might breed and add extra little fish to the pond, or they might die before you can count them. They might also live in the ocean (instead of a convenient little pond), which does not have obvious boundaries.

All of these factors play havoc with interpreting how your 'catch rates' relate to the number of fish in the pond. The numbers are also somewhat retrospective, reflecting what was there before you detected a decline in the catch rate, not what is now left in the pond.

Consequently, it's no mean feat for fisheries scientists to calculate the exact number of fish in the sea.