Are there far fewer fish on the planet than we think?

The number of fish in the sea could be far less than we think according to a new report.  It comes as dozens of nations gather in Paris to set quotas for fishing.

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The report says the yardstick used for estimating ocean fish stocks, is badly flawed.  As a result, global stocks of some predators including tuna, sharks and halibut, may be closer to collapse than thought.

For over ten years scientists have used catch data to measure changes in the balance of species across so-called trophic levels – that’s the species' rank in the food chain.

For example microscopic sea algae have a trophic level of one, while large predators such as sharks or tuna are at the highest level of four.

Proportional changes within the ranking have been used as the indicator of how well a particular species is faring.

So if a species of trophic four fish was in disproportionate decline compared with trophic three fish on which they feed, this would likely indicate overfishing.

According to today’s study, published in the journal Nature, the problem is that the method presumes that humans fish down the food web by over-harvesting fish at the highest levels and then going after fish further down the chain.

The new study says this technique is not smart enough, and sometimes gives you totally the wrong result.

The method's shortcomings are illustrated by the case of the Gulf of Thailand. There the average trophic level of what is being caught is rising, which should indicate improving ecosystem health. But it turns out that fish at all levels have declined roughly tenfold since the 1950s because of overharvesting.

The report’s authors say the disastrous drop is masked because the trophic level system is based on looking at the top predators first