User:Mike Young/Sandbox1

The Relationship between Overfishing and Global Warming

It is recognised that the oceans play a crucial part in the Carbon Cycle and is both a large absorber and a large emitter of Carbon Dioxide.

The absorbtion of Carbon Dioxide is linked to the levels of Phytoplankton and their rate of metablolic activity.

One of the possible cause of Global Warming is in inbalance in plankton levels caused by overfishing. This imbalance makes the sea a net producer of carbon dioxide rather than a net absorber.

Theory

When large fish are removed from the sea this has several detrimental effects

• As the fish are no longer laying eggs, and as the eggs themselves form an important food for maturing zooplankton this means the zooplankton numbers are reduced.
• Lack of zooplankton causes starvation of the remaining fish along with the destruction of filter feeders such as barnacles and coral.
• The large fish help to move nutrients upwards from the bottom of the sea towards the surface. Therefore removing them means that fish that feed near the surface suffer from starvation. Bottom feeders suffer proportionally less.
• The remaining phytoplankton are not eaten by zooplankton, but rather die and sink to the bottom of the sea to and are food for bacteria. This effectively removes protien from the sea

Evidence

Carbon Dioxide levels: Notice they start to rise before industrialisation and the small decrease from 1939 to 1945. This fits better with the hypophesis that rising CO2 levels are correlated to overfishing than that they are correlated to burning fossil fuels

Evidence supporting this idea is:

• Zooplankton numbers decrease when fish are lost
• Carbon Dioxide levels are better correlated to pressure on marine life than to burning of fossil fuels, in particular:
  • Carbon dioxide levels started to rise about 1800, before the start of significant industiralisation.
  • During the Second World War Carbon Dioxide levels fell. During this time there was a radical reduction in fishing, but no change in the burning of fossil fuels (see diagram to right)
• When under pressure from predators, the surviving prey are usually very fit and healthy. Fish caught today do not appear healthy, but rather are smaller and more emaciated than used to be the case. This suggests that the main pressure they are under may be starvation, rather than predation.
• There is little historical evidence about the abundance of phytoplankton. However a good surrogate is whale balean where growth (over the last 50 years) has shown that phytoplankton used to be more common, than has a brief spike in numbers as the preditors are removed. This is followed by a decrese as it is unable to find adequate amounts of protien to reproduce, as dead phytoplankton is not reproduced.