Variability of mortality rates for Calanus finmarchicus early life stages in the Labrador Sea and the significance of egg viability

Mortality rates of eggs and nauplii are essential for understanding and modelling dynamics of copepod populations. Abundances of Calanus finmarchicus females, eggs and nauplii were determined at 88 stations in the Labrador Sea. Egg production rates (EPRs) and egg and naupliar stage durations were calculated using published relationships with in situ chlorophyll concentration and temperature. The data were used to estimate mortality rates for eggs (ME), eggs and early naupliar stages (ME-NIII) and naupliar stages (MNI – NVI). Estimated mortality rates in the central basin were higher than those on the shelves and within regions generally ME . ME-NIII . MNI – NVI. The “Basic Method” for eggs, arguably the most reliable method, gave a high proportion of seemingly erroneous (negative) values. These became positive when a modified estimation formula was used, which assumes that some eggs being laid could not hatch. Egg hatching success is often ,100%, but this is rarely considered when calculating mortality rates, although it affects all estimates that include egg abundances and/or EPRs as variables, mostly at low mortality rates. Egg and early life stage mortality rates were correlated with female abundance, but cannibalism may not be the appropriate interpretation. The issues of egg viability and cannibalism require more careful consideration.