**11.1 Overfishing**

Some decades ago, the decline in marine life was attributed to overfishing. International agreements were put into place to limit fishing in the oceans. Particularly well known are the bans on whaling, which were adhered to by most countries. In spite of these voluntary limits on fishing, marine life rather than rebounding, has continued it's steady decline. With the restrictions on fishing and whaling, plankton at the bottom of the food chain should actually have increased. The question arises as to why there has not been a corresponding increase in the plankton in the oceans.

Ocean plankton are at the bottom of the food chain, and are the ultimate source of food for most marine life. It is estimate that phytoplankton is responsible for about half of Earth's photosynthesis, a process that removes carbon dioxide from the atmosphere and converts it into organic carbon and oxygen that feeds nearly every ocean ecosystem.

Paradoxically, the plankton densities over the world have continued their steady decline. Clearly, there is some major factor besides global warming and overfishing which are causing the alarming decline in marine life. The increase in CO2 has clearly not been sufficient to visibly benefit the CO2 dependent marine plankton.

Nine years of NASA satellite data published in the journal Nature show that the growth rate and abundance of phytoplankton around the world decreases in warm ocean years and increases in cooler ocean years. This has been used to support that global warming is the cause of the decline of the plankton. However, this explanation is insufficient, and the variation of plankton could have some other cause besides annual temperature cycles. Contrary to their explanation, global warming is clearly not the main factor leading to the decline of plankton.
