How many fish are there in the oceans?

How many fish are there in the oceans?

This question is not only challenging but of great importance for the assessment of fish stocks. Counting fish in the oceans is a tedious and difficult task. A research group at the Danish Institute for Fisheries Research (DIFRES) might have found a way to make the counting easier, as they have discovered a relation between the number of fish in the oceans and their size.

"Master, I marvel how the fishes live in the sea. Why, as men do a-land; the great ones eat up the little ones."

Inspired by this Shakespeare quote the Danish scientist started investigating the relation between the number of fish in the oceans and their size.

Imagine that the sea is being emptied of all fish and then sorted after weight, such that those with a weight of 1 g to 10 g go in one basket, those from 10 g to 100 g in another and so forth. After sorting begins the counting, to find out how many fish there are in the basket with small fish compared to the one with large fish? You will then find out that there are approximately 10 times more of the smaller fish compared to the larger, and that the weight of fish in the two baskets is almost the same.

Simple calculation
You could also calculate it, and this is what the Danish team has done. The rule of large fish eating small fish combined with a mathematical description of how large a volume of water fish of different size patrol was sufficient to make the calculation.

"The real novelty is not the result in itself, but the fact that we were able to derive the result using very simple arguments", say physicist Ken Andersen from The Danish Institute for Fisheries Research (DIFRES).

Can show the effect of fishing
The result is valid for a pristine ecosystem, with no fishing, and can therefore be used to examine the effect of fishing, by comparing the relation between the number of fish of different sizes in a pristine and a “fished” ecosystem”.

For practical reasons fish stocks are often managed in isolation, without considering the interaction, e.g. cod eats herring, herring eats cod larvae etc.

“Our approach takes the interaction of different stocks into account, and therefore provides a starting point for managing the ecosystem as a whole" ads senior biologist Jan Beyer, DIFRES.

Further reading:
The American Naturalist, July 2006, K. H. Andersen and J. E. Beyer, DIFRES: Asymptomatic Size Determines Species Abundance in the Marine Size Spectrum.