Changes in Fisheries Halflogo2

Fish stocks are a measure of the biomass and abundance of fish in 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. 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.

SAHFOS has examined whether variations in plankton abundance and timing is 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 is reducing the survival of young cod.

The research found that 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. ยท There is now a mis match in predator/prey relationships due to the modification of plankton community composition.

Calanus Finmarchicus  Fish Larva Photo _2  Cod Swimming

Further information:  Beaugrand et al (2003) Plankton effect on cod recruitment in the North Sea. Nature 426: 661 -664