Methods—Biological and environmental data: Jellyfish
data used in this study were collected by the CPR survey.
This is an upper-layer plankton-monitoring program that
has been undertaken in the North Atlantic and North Sea
since 1946. Sampling is carried out by a high-speed
plankton recorder that is towed at monthly intervals
behind merchant ships (at about 20 km h21) on regular
routes at a standard depth of approximately 6.5 m. Use of
such vessels means that the sampling frequency in the
North Sea has been consistent over both months and years.
One CPR sample corresponds to about 3 m3 of seawater
filtered (Warner and Hays 1994).