Graduate Thesis Or Dissertation
 

Comparative study of the population dynamics, secondary productivity, and reproductive ecology of the euphausiids Euphausia pacifica and Thysanoessa spinifera in the Oregon upwelling region

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  • I compare the seasonal abundance variation, population dynamics, fecundity, egg hatching mechanism and success, and apostome ciliate parasites of the euphausiids Euphausia pac?fica and Thysanoessa spinfera from the Oregon coast, USA. Community structure and nearshore distributions were examined from bi-weekly oceanographic surveys (1970-1972). This region has a strong cross-shelf change in euphausiid assemblages located about 45 km from shore. Euphausia pacflca and T. spinfera have life stage-segregated distributions, suggesting active location-maintenance strategies. Morphology and biometry of all the post-spawning embryonic stages and the hatching mechanisms of three broadcast-spawning (E. pacfica, T. spinfera and Thysanoe;sa inspinata) and one sac-spawning (Nematoscelis djfficilis) euphausiids are described. The average embryo and chorion diameters were significantly larger for E. pacca (0.378,0.407 mm) than for T. spinfera (0.35 3, 0.363 mm) and T. inspinata (0.312, 0.333 mm). There are four hatching mechanisms. Some broadcast-spawning species have delayed hatching schedules, hatching as nauplius 2, metanauplius or calyptopis 1, rather than as the usual nauplius 1. Sac-spawning species sometimes hatch early as nauplius 2, rather than as the normal pseudometanauplius or metanauplius. Late and early hatching schedules were associated with low hatching success and small brood size. The brood sizes (BS) of E. pacca and T. spin(fera were estimated during incubations on 23 cruises. Thysanoessa spinifera had higher BS and extended its spawning areas farther offshore during 2002 than in 1999-2001. Euphausia pacfica had a stable inshore-offshore broodsize pattern from 2001 to 2003, and had greater BS than during 1999-2000. Elevated egg production during summer 2002 was associated with cooler, high chl-a conditions resulting from anomalously high equatorward transport.
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