Download Article cod in the North Sea

Cod in the North Sea:
Certainly food for thought!
Food for thought…
ood chains show us the feeding relationships between organisms. The position of an organism in
a food chain is called its trophic level, defined by its feeding strategy. The organisms at the base
of a food chain are described as primary producers, using light energy from the sun to make
their own energy via the process of photosynthesis. The organisms which feed on primary
producers are known as primary consumers; these in turn are eaten by secondary consumers and so
on until the top predator in the community is reached with no natural predators. An example of a North
Sea food chain in presented in figure 1.
Light
energy
Sun
Primary
producers
Photosythesisers
Phytoplankton
Primary
consumers
Herbivores
Secondary
consumers
Carnivores
Top
predators
Carnivores
Zooplankton
Herring
Cod
Figure 1 – An example of a North Sea food chain.
Plankton in plentiful supply?
One of the most important components of all North Sea food chains are
plankton; free-floating plants (phytoplankton) and animals (zooplankton). In
fact, they are the basis for nearly every food web in the ocean, a habitat which
covers over two thirds of our planet. Not only that, phytoplankton provide us
with just over 50% of the oxygen we breathe on a day-to-day basis through the
process of photosynthesis.
Much of the plankton production each year is transferred to the ocean floor,
either by dead plankton falling directly downwards or via food webs as top
predators die and are decomposed by organisms on the sea floor. As a result,
A phytoplankton
carbon dioxide, which is in balance between surface sea water and the
atmosphere, is lost to the deep ocean. This is an important part of the
global carbon cycle, which in turn is linked with the maintenance of
steady average global temperatures.
A zooplankton
Given the importance of plankton in regulating global processes such
as climate change, we need to understand exactly how marine
organisms are affected by changes in the environment. Plankton are
monitored on a routine basis in parts of the world’s oceans by using
Continuous Plankton Recorders (CPRs). This has been going on since
1931 and there exists a large database comprising results from 190
000 samples taken over 9 million km of ocean transects.
What have we learned?
Primary production in the sea is largely based on the growth of algae. CPR runs in the North Sea show that until 1987
algal growth was seasonal — taking place in spring and autumn. Since 1987 phytoplankton growth has continued
throughout much of the year. Growth is still much slower in January and February, but this is explained by the low
levels of light at this time of year. However, although there appears to be more primary production, this does not
necessarily mean there is more food for the primary consumers — the herbivorous zooplankton — because the timing
of their development and life cycles may not match the increase in primary production.
Copepods in crisis!
Copepods and larval fish are both types of zooplankton, even though larval fish, such as larval cod, eat copepods.
Young cod like eating a particular type of copepod called Calanus finmarchicus. This is a spring-breeding cold water
copepod. The North Sea has warmed up in recent years, perhaps due to climate change, and as a result C.
finmarchicus has moved further north to cooler waters, replaced by another copepod called Calanus helgolandicus
which breeds in the autumn and is happy in warmer water. However, these new arrivals are no substitute for a tasty
meal of C. finmarchicus as far as larval cod are concerned!
What will become of our cod?
There has been a significant drop in numbers of North Sea cod since 1987. Although overfishing is partly to blame,
the correlation between decreasing cod numbers and a drop in the abundance of C. finmarchicus provides striking
evidence that the numbers of cod present in the North Sea may also be affected by warming sea surface
temperatures caused by climate change.
The take home message?
The cod that you might eat with chips is finding its food is in short supply out in the North Sea. Food for thought
indeed!
What can be done about it?
That’s for you to decide!
Discuss in your group the implications of these findings. How do you think we can sustain our cod stocks for the
future?
Is the farming of cod the answer?
Do we have the technology, money and expertise to do that?
Can we somehow stop global climate change?
What is the most realistic option?