| Summary: | Measuring growth rates of zooplankton is essential to studies of secondary production of pelagic food webs. Ribonucleic acid (RNA) content offers a simple way to estimate growth rates of natural populations of zooplankton without incubating animals or repeatedly sampling cohorts. I investigated the relationship of RNA concentration of the copepod Calanus finmarchicus to food availability, temperature, and growth rates and developed several models for estimating growth rates from RNA. Growth rates were measured in terms of carbon, nitrogen, protein, and DNA content. RNA:DNA ratios of stages N6-adult were affected by food concentration and were inversely related to temperature. RNA:DNA ratios increased with stage in all food and temperature treatments. To account for the effects of stage and temperature on RNA:DNA and its relation to growth, I developed the Relative RNA:DNA Index (RRD), which was strongly related to relative protein and carbon growth rates (r 2 = 0.70). Growth rates were a direct function of RNA:protein and the interaction Temperature × RNA:protein (r2 = 0.63–0.83). Both the RRD and the RNA:protein ratio are useful indices of growth for C. finmarchicus. The RRD method requires less laboratory work but more extensive data manipulation. I used the Relative RNA:DNA Index (RRD) to evaluate the nutritional status of natural populations of C. finmarchicus in relation to growth rates, egg production rates, and phytoplankton biomass (chl a). In the Georges Bank region of the NW Atlantic Ocean, during winter–spring 1995 and spring 1997, RRD was related to rates of growth and egg production. For stages C1–C5, high RRD was associated with higher relative growth rates (carbon and nitrogen, r2 = 0.13–0.19, P < 0.05). RRD of adult females was strongly related to egg production rates (eggs female−1 day−1, r2 = 0.70). RRD was hyperbolically related to chl a during stages N6–C4, but not related during C5 and C6. Phytoplankton biomass may limit the growth of the younger stages, which experience their highest growth rates during the spring diatom bloom, whereas the late juvenile and adult stages are less dependent on phytoplankton concentration and can maintain high growth rates in some areas until as late as mid-June.
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