| Summary: | Sockeye salmon (Oncorhynchus nerka) can return from the Gulf of Alaska to the Fraser
River by migrating around the north or south end of Vancouver Island and the proportion of fish
using each route varies considerably among years. This thesis consists of three separate studies
that contribute to the overall objective of understanding the migration route variation. The first
two components are concerned with the estimation of migration routes from fisheries data, and
potentially have additional practical implications for fisheries management. The third
component examines some explicit interactions between individual behaviour and oceanographic
variability that could potentially affect migration route selection.
The first study investigated how standard methods of estimating salmon fishery harvest
rates introduce substantial errors because of violations to migration dynamics assumptions. This
involved: 1) defining plausible stochastic salmon migration rate variability scenarios from
observations of salmon migratory timing distributions and tagging studies, and 2) using Monte
Carlo simulations of fisheries to examine how this variability affects harvest rate estimation
when uniform migration rates are assumed during the process of run-reconstruction. A unique
migration dynamics scenario could not be defined. Within the migration constraints that were
identified, the simulations suggested that, in general, harvest rate estimates can be expected to be
beta-distributed, and there is a potential underestimation bias for high harvest rates. The
magnitude of the error variance and bias are dependent upon the migration dynamics, fishery
temporal and spatial structure, and run-reconstruction method.
The second study presents methods for estimating harvest rates and migration routes in a
multiple approach salmon fishery. The Bayesian approach explicitly admits uncertainty from 1)
the confounded relationship between harvest rates and migration routes, 2) escapement
estimation error that arises from run-reconstruction, and 3) the unknown relationship between
harvest rates and effort. The resulting parameter uncertainty is reflected in posterior probability
distributions. Potential advantages and disadvantages of adopting this method for the Fraser
River sockeye fishery are examined.
The third study involved simulating adult sockeye salmon migration routes from the Gulf
of Alaska to the coastal approaches of the Fraser River. A spatially-explicit individual-based
model was used to explore potential mechanisms that could explain the observed interannual
variation in migration routes. Assuming that sockeye are initially distributed throughout the
central Gulf of Alaska and orient on a compass bearing, the following mechanisms were
simulated to produce migration route variations among years in a physical environment described
by dynamic surface temperatures and currents: 1) the distribution prior to homing was
constrained by southern thermal limits, 2) sockeye were advected by currents during open ocean
migration, and 3) sockeye tended to avoid high water temperatures. Optimization of the
behavioral component of the model for a least squares fit between hindcast and observed coastal
migration routes suggested that thermal limits and offshore-currents could not explain very much
coastal migration route variability. Avoidance of high water temperature explained about 33% of
the interannual variability and suggested that coastal processes (although poorly resolved in the
model) could be more important than the offshore processes examined.
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