Seasonal dynamics and mortality rates of Calanus helgolandicus over two years at a station in the English Channel
The stage-specific abundance and egg production rates of Calanus helgolandicus were determined on a near-weekly basis over 2 yr at a 50 m deep station in the SW English Channel (Stn L4). Mortality rates were derived using a vertical life-table approach across eggs, nauplii and also the CV–adult stage pair. The results demonstrate strong seasonal patterns in the mortality rates of egg and nauplii and the CV–adult stage pair, but with different relative rates and somewhat different seasonalities. Mortality was highest in the egg and egg–NI stages, averaging 6.1 and ~1.5 d–1, respectively, with the percentage surviving through the egg–NI stage pair often <<10%. Although the instantaneous removal rate of eggs was significantly related to adult abundance (p < 0.001, r2 = 0.221), densities of adult C. helgolandicus seemed too low to account for these rates. Examination of the relationship between CV–adult female mortality and the abundance of the dominant invertebrate predators revealed statistically significant relationships (p < 0.001 r2 = 0.276 for chaetognaths; p < 0.001 r2 = 0.125 for siphonophores); however, the variability explained by temperature was much higher (p < 0.001 r2 = 0.652). The egg–NI and NI–NII stage pairs also showed a highly statistically significant positive relationship between mortality and temperature. For the first time we compared mortality rates for egg–NI using 2 vertical methods—one using measurements of egg and NI abundance (Method A) the other using egg production rates and NI abundance (Method B)—and found the two to be similar, although Method B gave higher values. Finally, as many mortality equations do not consider the bias resulting from the presence of eggs incapable of hatching in the field, we derived and applied new equations for mortality of eggs and egg–NI (Method A) that incorporated egg hatching success. At low hatching success or low mortality rates, this correction can alter estimates of mortality rates significantly.