Is weight an important parameter when measuring copepod growth?

We studied in situ weight increment of Calanus helgolandicus developing under field conditions. Weight increment between successive stages was rather similar for early nauplii (NII-NV ∼2.00 μgC μgC ^-1) and for early copepodites (NVI-CIII mean: ∼2.40 μgC μgC ^-1) but lower for late copepodites (CIV ∼0.80 and CV ∼0.20 μgC μgC ^-1). Weight increment of CIV and particularly CV was found to be closely linked to the method used to estimate the weight: either (i) directly measured weights, or (ii) weights estimated from a length-weight regression. The weight of CV, which has a relatively long stage duration, shows wide variations within the stage while length variations within stage are very low. For example, on the 17th of August, some CV had a carbon weight three times higher than other ones. The main cause of variation is the age of the individuals within the stage. We recommend direct weight measurement rather than the use of length-weight regression to estimate weight increment. However, and especially for stages with large variations in weight, whether the individuals are at the mid-point of their stage should be checked.