Seasonal abundance and egg production rates of Oithona similis and Pseudocalanus elongatus in the northern North Sea: a first comparison of egg-ratio and incubation methods

In this study, we examined egg production rates (EPR), clutch sizes, numbers of spawning females and prosome lengths of 2 common marine egg carrying copepods, Oithona similis and Pseudocalanus elongatus, over an annual cycle in the northern North Sea. For both species, we determined EPR using the incubation method, with rates being assessed after 24 (EPR0-24), 48 (EPR0-48), and between 24 and 48 h (EPR24-48) of incubation. For Oithona similis, EPR were also determined by applying the egg-ratio method EPRp, allowing a first ever comparison between these 2 approaches. In both species, mean rates of egg production from EPR24-48 incubations were lower on average than from EPR0-24 and EPR0-48 incubations; however, paired t-tests showed these differences to be insignificant (p > 0.05). In Oithona similis, the egg-ratio method EPR, gave higher rates (mean +/- 95% CI: 1.06 +/- 0.476 eggs female(-1) d(-1)) than the incubation method, but only the comparison with EPR24-48 (mean 95% CI: 0.51 +/- 0.337 eggs female(-1) d(-1)) was significantly different (p = 0.031). Possible reasons for the differences are explored and recommendations made. EPR were significantly correlated with temperature in P. elongatus (p < 0.05), but the correlation was significant in O. similis only when using the egg-ratio method (p < 0.05). Applying a modified Michaelis-Menten relationship showed EPR to be significantly related to chlorophyll a in both species (p < 0.001). Changes in prosome lengths explained little of the variability in clutch size in the 2 species over the course of the year. Finally, seasonality in the abundance and biomass of nauplii, copepodites and adults was examined, together with adult female production and weight-specific fecundity rates, and these were compared across a variety of regions.