MOLTING RATES OF CALANUS-HELGOLANDICUS - AN INTERCOMPARISON OF EXPERIMENTAL METHODS

An inter-comparison of three commonly reported methods for estimating the stage duration of copepods was made on Calanus helgolandicus from both wild and laboratory-reared populations. These three methods have not been compared previously in contemporaneous experiments. The metho ds were: (1) following the stage frequency of a laboratory-reared popu lation over time, (2) sorting of individual species stages from the la boratory-reared population and from wild plankton samples, followed by incubation, and (3) sieve fractionating the wild plankton sample foll owed by incubation. We establish that estimates by these three methods made from the wild and laboratory-reared populations did not differ s ignificantly. Handling of the animals did not affect estimates of moul ting rate significantly, but there was a wide range in stage durations in identical copepodite stages which had experienced similar environm ental conditions. We therefore suggest that the 'Heinle' method would be most applicable to use on populations with a clear cohort structure in areas not subjected to high rates of advection, and which experien ce saturating food conditions. The sorted cohort method would be appro priate to areas which were subject to high rates of advection and to a population which had prolonged recruitment, but because this is a lab our intensive method the sieved cohort method may be more appropriate when the plankton hauls are dominated by a particular species of copep od. (C) 1998 Elsevier Science B.V.