The cod-end of the third WP-2 net w

The cod-end of the third WP-2 net was not rinsed and its content
was immediately taken after sampling, filtered onto a paper filter and
stored at −20 °C in complete darkness until laboratory analysis for
assessing gut pigment contents of copepods with the gut fluorescence
method (Mackas and Bohrer, 1976). From each sample, we took three
sub-replicates of 75 copepods. Gut pigments were extracted overnight
in 5 mL of 90% acetone at 4 °C in darkness, and fluorescence was
measured with a spectrofluorometer (Perkin Elmer LS 55) before and
after acidification with 0.15 mL of 0.1 N HCl. This method provides a
coarse estimate of the feeding rates, but represents a useful method to
observe variations in grazing behavior in relation to vertical migration
(Båmstedt et al., 2000).