cod fish

Shrinking Atlantic Cod Forced North By Collapse Of Ocean Ecosystem

A new paper reports on the collapse of the Atlantic Ocean pastures over the past 40 years.

As the ocean fish pastures collapse mother fish lose their maternal health and bear very few and sickly eggs.

Freshly hatched cod fish fry simply starve to death.

According to a study just published (13 Feb. 2014) in the journal Plos One, the Atlantic Ocean has been changing dramatically over past 40 years.  As a result collapse of the main food of juvenile cod specimens is shown to be more critical than the density of breeding adults in the recruitment of this species (read overfishing), reported the IEO.

The scientists determined that the stock of cod in the North Sea experienced a dramatic reduction over the past four decades. And the two main factors that are considered responsible for this decline are overfishing and reduction in productivity of ocean fish pastures. Ocean pastures like those on land have a carrying capacity for “livestock/fish” proportional to their primary productivity. When a pasture has no grass it can sustain no livestock. Ocean pasture grass is plankton.

The stock of cod now has a ‘truncated population structure,’ which is dramatically changed reflecting both smaller populations of fish and also dramatic reduction in the size of adult fish. It is well established that larger fish produce more young as well as more healthy young. The main obstacle that the fish face for proper recovery is the mortality rate in the early stages of life due to lack of planktonic and mesopelagic food supply.  Atlantic Cod are continuing to starve on decimated ocean pastures.

The key factors are the ambient temperature, and the availability and quality of food:

  • The temperature determines the spawning time and the number of eggs produced during the winter;
  • Food affects the survival of larvae in the spring.

During the study, Spanish, French and English researchers evaluated the changes that occurred between 1971 and 2011 from a spatial-temporal viewpoint in order to detect those periods which have a similar spatial distribution.

Cod PLOS_1

Click to enlarge

In the first place, the changes observed major distribution and evaluated their speed. Then, they investigated whether these changes were directly related (changes in temperature) or indirectly (changes in the abundance of breeding specimens and quality and quantity of plankton) related to effects common to climate change.  (Ed note: The most dramatic impact on North Atlantic ocean pasture productivity is declining amounts of mineral micronutrients which have resulted from dramatic declines in Aeolian dust deposition. As mineral dust has declined so has ocean pasture productivity.)

The IEO explained that, they used information sources “with more extensive spatial resolution existing in the area, combining the estimates of abundance of adults and recruits performed by international demersal surveys gathered by the International Council for the Exploration of the Sea (ICES),” and sea surface temperature data generated by the British Atmospheric data Centre and estimates of plankton abundance provided by the Continuous Plankton Recorder.

“The results of this study suggest that the direct (temperature) and indirect effects (abundance and occurrence period of their favourite food) of climate change are responsible for the continuous recruitment failures recorded in the last decades despite the significant reduction of fishing pressure on this species,” summarizes the IEO.

Scientists estimate that if global warming and related climate change effects continues to change the North Sea, mainly in its southern and shallow areas, the direct and indirect effects will result in further and progressive reductions in cod stocks that historically occupied these habitats.

For more about the collapse of Atlantic Cod as a result of the loss of their plankton food read this item about the copepods of the Atlantic.

Read about how Atlantic Ocean Pastures can be replenished and restored.