Egg production rates of eight calanoid copepod species during summer 1997 off Newport, Oregon, USA

Measurements of hydrography, water transparency, chlorophyll (Chl) a and egg production rates (EPRs) by females of Calanus marshallae Frost, Calanus pacificus Brodsky, Eucalanus californicus Johnson, Epilabidocera longipedata Sate, Pseudocalanus mimus Frost, Centropages abdominalis Sate, Acartia longiremis Lilljerborg and Paracalanus pan us (Claus) were estimated at weekly intervals between 17 July and 2 September 1997. Production of eggs was determined in 24 h incubations to examine the effects of environmental variability on EPR, to detect the possibility of food limitation of EPR, and to evaluate the hypothesis that growth rates of females are size dependent. During the study, an anomalous downwelling event occurred, possibly in response to the 1997 El Nino, which allowed us to determine how El Nino events affect EPRs of coastal copepods. The larger copepods Calanus marshallae, Calanus pacificus and Centropages abdominalis showed the highest egg production and specific growth rates during the period of active upwelling (18 July-13 August, water temperatures 8-13 degrees C, Chi a concentration 4.7-16.2 mu g l(-1) and water transparency 3-5 m). After 27 August, the 1997-98 Fl Nino arrived off Oregon, creating a downwelling situation. Upwelling winds ceased, the thermocline intensified, temperature and transparency increased (to >18 degrees C and 16 m), and Chi a declined to <2 mu g l(-1). Densities of the common coastal species declined greatly as well. Paracalanus parvus became the dominant species, and Eucalanus californicus, Epilabidocera longipedata and Corycaeus anglicus became common in our samples. EPRs for the larger boreal copepods (Calanus and Ccntropages) declined greatly during El Nino; the smaller copepods, Pseudocalanus mimus, A. longiremis and Paracalanus parvus, showed low but relatively constant egg production and specific growth rates during both upwelling and downwelling events. Over the entire study period, only three species produced eggs at or very near their maximum: Calanus marshallae, which during the upwelling period produced eggs at its maximum rate (24-28 eggs female(-1) day(-1)), Pseudocalanus mimus which averaged similar to 4 eggs day(-1) and Calanus pacificus which averaged similar to 40 eggs day(-1). All other species had EPRs that were two or five times below their maximum EPR. Thus, EPRs were not related to body size, contrary to our expectations. Hatching success was variable among species. Values as low as 20-40% were found for all species at least once during the study period, suggesting that occasionally a substantial portion of egg production may not be viable.