Re-assessing copepod growth using the Moult Rate method

Estimating growth and production rates of mesozooplankton, and copepods in particular, is important in describing flows of material and energy though pelagic systems. Over the past 30 years, the Moult Rate (MR) method has been used to estimate juvenile copepod growth rates in ∼40 papers. Yet the MR method has been shown to have serious flaws.

Here we re-examine the results from the majority of published MR method studies and re-estimate growth rates using the modified Moult Rate (MMR) method, which ascribes changes in mass to the appropriate time period over which it was accrued. The MR method has typically over-estimated growth rates (on 80% of occasions) for life stages where the subsequent stage is actively moulting; the median and mean MR values are 138 and 164%, respectively, of the corrected MMR values.

We were unable to correct the original data for life stages that are followed by a non-moulting stage, e.g. copepodite stage 5 to adult. We performed experiments with Calanus pacificus to estimate growth of stage C5 using an alternative method. We found that the error size and sign varied between mass type (i.e.

DW, C and N). Recommendations for practical future assessments of growth in copepods are made.