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

Measurements of hydrography, water transparency, chlorophyll (Chl) a and eg g production rates (EPRs) by females of Calanus marshallae Frost, Calanus p acificus Brodsky, Eucalanus californicus Johnson, Epilabidocera longipedata Sate, Pseudocalanus mimus Frost, Centropages abdominalis Sate, Acartia lon giremis Lilljerborg and Paracalanus pan us (Claus) were estimated at weekly intervals between 17 July and 2 September 1997. Production of eggs was det ermined in 24 h incubations to examine the effects of environmental variabi lity on EPR, to detect the possibility of food limitation of EPR, and to ev aluate the hypothesis that growth rates of females are size dependent. Duri ng the study, an anomalous downwelling event occurred, possibly in response to the 1997 El Nino, which allowed us to determine how El Nino events affe ct EPRs of coastal copepods. The larger copepods Calanus marshallae, Calanu s pacificus and Centropages abdominalis showed the highest egg production a nd 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 ceas ed, 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 be came the dominant species, and Eucalanus californicus, Epilabidocera longip edata and Corycaeus anglicus became common in our samples. EPRs for the lar ger boreal copepods (Calanus and Ccntropages) declined greatly during El Ni no; the smaller copepods, Pseudocalanus mimus, A. longiremis and Paracalanu s parvus, showed low but relatively constant egg production and specific gr owth rates during both upwelling and downwelling events. Over the entire st udy 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 av eraged similar to 4 eggs day(-1) and Calanus pacificus which averaged simil ar to 40 eggs day(-1). All other species had EPRs that were two or five tim es below their maximum EPR. Thus, EPRs were not related to body size, contr ary to our expectations. Hatching success was variable among species. Value s as low as 20-40% were found for all species at least once during the stud y period, suggesting that occasionally a substantial portion of egg product ion may not be viable.