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

Abstract

Measurements of hydrography, water transparency, chlorophyll (Chl)<it>a</it> and egg production rates (EPRs) by females of<it>Calanus marshallae</it> Frost, <it>Calanus pacificus</it> Brodsky, <it>Eucalanus californicus</it> Johnson, <it>Epilabidocera longipedata</it> Sato,<it>Pseudocalanus mimus</it> Frost, <it>Centropages abdominalis</it> Sato, <it>Acartia longiremis</it> Lilljerborg and <it>Paracalanus parvus</it> (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 Ni&ntilde;o, which allowed us to determine how El Ni&ntilde;o events affect EPRs of coastal copepods. The larger copepods <it>Calanus marshallae, Calanus pacificus</it> and <it>Centropages abdominalis</it> showed the highest egg production and specific growth rates during the period of active upwelling (18 July-13 August, water temperatures 8-13<IMG SRC="/math/deg.gif">C, Chl <it>a</it> concentration 4.7-16.2 <IMG SRC="/math/mu.gif"> l^-1 and water transparency 3-5 m). After 27 August, the 1997-98 El Ni&ntilde;o arrived off Oregon, creating a downwelling situation. Upwelling winds ceased, the thermocline intensified, temperature and transparency increased (to &gt;18&deg;C and 16 m), and Chl<it>a</it> declined to &lt;2 &mgr;g l^-1. Densities of the common coastal species declined greatly as well. <it>Paracalanus parvus</it> became the dominant species, and <it>Eucalanus californicus, Epilabidocera longipedata</it> and <it>Corycaeus anglicus</it> became common in our samples. EPRs for the larger boreal copepods (<it>Calanus</it> and <it>Centropages</it>) declined greatly during El Ni&ntilde;o;; the smaller copepods,<it>Pseudocalanus mimus, A.longiremis</it> and<it>Paracalanus parvus</it>, 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 its maximum rate (24-28 eggs female^-1 day^-1),<it>Pseudocalanus mimus</it> which averaged 4 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.