Plankton effect on cod recruitment in the North Sea
Gregory Beaugrand, Keith Brander, Alistair Lindley, Sami Souissi and Chris Reid
Nature, Volume 426, 2003 (www.nature.com/nature)
What is this about? Fish stocks are a measure of the biomass and abundance of fish in a given area
and the fishing industry relies upon healthy stocks for a good yield of fish. Fish stocks can fluctuate
naturally over the years/ decades and in different areas but the reasons for this are not fully
understood by scientists. This paper examines whether variations in plankton abundance and timing
are having an effect on the number of larval cod surviving to adulthood. Larval cod rely upon a high
level of good quality food (copepods and euphausiids) to survive to adulthood. Larval cod emerge in
the spring in time for the high abundance of Calanus copepods. They feed predominately on C.
finmarchicus in the early spring and C. helgolandicus in the summer. Modifications to the plankton
ecosystem due to a rise in temperature since the 1980s are reducing the survival of young cod.
Why is it important? There has been growing concern about the decline in Atlantic cod biomass
since the 1960s. If the decline is an ongoing trend it could pose a serious problem to the fishing
industry and socio-economic issues surrounding it.
Why does it matter? A prolonged or semi - permanent change in the plankton community of the
North Sea is highly likely to affect the abundance of other species of fish and other organisms further
up the food web which rely on them as their prey.
What does the paper tell us?
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The abundance of Atlantic cod is affected by overfishing and fluctuations in plankton
abundance.
The survival of larval cod has been shown to be highly dependent on mean size of prey,
seasonal timing of the prey and the abundance. This is an example of bottom-up control.
During the period 1963- 1983 there was a very high population of cod recruitment (one year
olds) and this is called the ‘gadoid outburst’.
The ‘gadoid outburst’ is a consequence of a plankton ecosystem that could support a large
number of larval fish.
This occurred despite an increase in fishing mortality during 1963 – 1983.
Variability in temperature of the North Sea since the 1980s has modified the plankton
ecosystem in the area where larval cod are feeding.
Calanus finmarchicus (copepod prey for the larval cod) has declined in numbers in the North
Sea since 1980. It has been substituted in places with Calanus helgolandicus which emerges
in high numbers in the summer. Euphausiids have also suffered a long term decline in
numbers.
There is now a mis-match in predator/prey relationships due to the modification of plankton
community composition.
Unfavourable changes in the plankton ecosystem have exacerbated the impact of
overfishing on reducing the recruitment of Atlantic cod in the North Sea.