Almost all epipelagic fish species in the Northeast Pacific show an increase in population size
between the late 1950s and the 1980s. The complexity of pelagic ecosystems makes speculations
on the causes of these increases easy to justify, and thus various conjectures on the chain of
events leading to increased fish survival have been put forward.
In this thesis I try to explain the variability in cohort survival, abundance and distribution of
sockeye salmon (Oncorhynchus nerka) - the fish species that has experienced the largest increase
in abundance and biomass of all epipelagic fish species in the Northeast Pacific between the late
1950s and the 1980s - by ecosystem effects. I assumed that sockeye salmon total survival rate is
largely determined in early marine life due to exposure to predators, which is set by the time at
risk of predation, itself a function of sockeye prey, i.e. mesozooplankton, abundance. I then
developed two simple food chain models with three and four trophic levels, respectively, which
include lower trophic level dynamics but not fish itself. Both population models were calibrated
and tested for two locations in the Northeast Pacific through mean field simulations driven by
abiotic environmental forcings. Using a 4-hour time step from 1950 to 1990, both calibrated
population models were then run as spatially-explicit simulations with a resolution of one degree
latitude and longitude for the whole area of the Northeast Pacific, a total of 1240 open ocean
fields. To assess the relative importance of biological processes versus physical advection both
population models were simulated with and without surface currents.
I have tried to design the best models within reason utilizing the best information on
environmental forcings and biological processes available at the time. Simulation results do not
suggest a clear linkage between prey density in the oceanic environment and sockeye salmon
cohort survival. However, there are two fundamental lessons to be learned from this modeling exercise: First, categorization of ecosystem components into trophic levels with no regard of the
many life history strategies is one of the worst aggregation errors in ecology, one that implicitly
includes errors of hierarchical organization as well as of spatio-temporal stability. And second,
the complexity of ecosystems will always make results from trophodynamic simulations
interpretable, even if these results bear no relationship to the natural system.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:BVAU.2429/8664 |
| Date | 05 1900 |
| Creators | Baumann, Michael |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Relation | UBC Retrospective Theses Digitization Project [http://www.library.ubc.ca/archives/retro_theses/] |
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