Bioenergetics and behavior of the krill Euphausia pacifica in the California Current System off the Oregon coast Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/wp988n08t

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  • Euphausia pacifica, the North Pacific krill, is a key grazer in the California Current System and an important prey item for consumers such as salmon, seabirds, and whales. As a crucial link between phytoplankton and higher trophic levels, it is essential to understand both the behavior and bioenergetics of this species in the context of its physical environment in order to anticipate distributions in time and space. The second chapter of this dissertation explores an hypothesis of differential transport of larval stages at the Oregon coast with an analysis of previously-collected and staged samples of E. pacifica. A metric was proposed to quantify the fraction of migrating late larval stages in net tows and how it was anticipated to vary with upwelling on an event-length time scale, due to differential cross-shelf transport. While the data from one year were consistent with the hypothesized relationship, the other years in the analysis did not conform to expectations. Results suggest there may be other confounding factors operating in the Oregon upwelling zone that prevent the direct observation of differential transport of larvae with field samples. The third chapter discusses egg transport between the Oregon coast shelf break and the inner shelf region. Modeling of egg trajectories demonstrated that females spawning at or beyond the shelf break are unlikely to be the primary source of eggs found at an inner shelf station. Years with strongly positive (warm) Pacific Decadal Oscillation index values are not likely to be years with large fluxes of E. pacifica eggs to the nearshore region if they originate at the shelf break or seaward, as a result of increased egg development rate. Finally, the fourth chapter introduces an individual based model (IBM) of E. pacifica bioenergetics and explores the potential for the species to maintain its position in a region off Oregon, as well as the assumption that repeated sampling in the region is resampling the same E. pacifica population over a period of months. Optimistic (i.e. no mortality) trajectories of individuals modeled off Oregon suggest that cohort analyses of juvenile and adult E. pacifica in the region may not always violate the assumption of resampling a single population, especially if samples are taken during the fall or krill are caught in mesoscale features such as eddies. In these special cases, cohort analysis may be useful for estimating in situ growth rates, which might not be expected for a region as dynamic as the Northern California Current. There are still significant gaps in our understanding of fundamental biological processes for E. pacifica, such as how it responds to an ocean environment with generally insufficient food, whether overwintering metabolism is greatly reduced, how consumption rates by larval stages change when multiple food types are available, what mechanisms cause the variability in larval stage diel-vertical migration, and what the impact of different qualities of food might have on egg characteristics such as buoyancy. Addressing these unknowns and how well E. pacifica copes under non-ideal conditions will facilitate a fuller understanding of the dynamics of this species.
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