This transfer of nutrients enriches the river and makes for a massive fish population.

Scientists blame a 'triple whammy' of polluting fish farming techniques, depleted fish stocks, which used to control the jelly fish population, and warmer seas.

There are also significant changes in the fish population, sometimes because of the subtle problems caused by global warming, sometimes from overfishing by people.

In the past year or so scientists have been attempting to quantify just what effect fish farming has on the wild fish population.

So catching them helps protect the rest of the fish population.