65 research outputs found
Frequency responses of age-structured populations: Pacific salmon as an example
Increasing evidence of the effects of changing climate on physical ocean
conditions and long-term changes in fish populations adds to the need to
understand the effects of stochastic forcing on marine populations. Cohort
resonance is of particular interest because it involves selective sensitivity
to specific time scales of environmental variability, including that of mean
age of reproduction, and, more importantly, very low frequencies (i.e.,
trends). We present an age-structured model for two Pacific salmon species with
environmental variability in survival rate and in individual growth rate, hence
spawning age distribution. We use computed frequency response curves and
analysis of the linearized dynamics to obtain two main results. First, the
frequency response of the population is affected by the life history stage at
which variability affects the population; varying growth rate tends to excite
periodic resonance in age structure, while varying survival tends to excite
low-frequency fluctuation with more effect on total population size. Second,
decreasing adult survival strengthens the cohort resonance effect at all
frequencies, a finding that addresses the question of how fishing and climate
change will interact.Comment: much revised: the version accepted by Theoretical Population Biolog
Optimal wind patterns for biological production in shelf ecosystems driven by coastal upwelling
Major upwelling systems around the world provide marine productivity and fishery yield out of proportion to their area. Upwelling winds have the counteracting effects that stronger winds upwell more nutrients to the surface for higher production, but they also transport that production off continental shelves where it may not be consumed by shelf-dwelling species. Because the patterns of wind fluctuations vary in these systems, we determined the conditions for maximal biological production using a simple conveyor belt model. The conditions are that: (1) the average cross-shelf velocity produced by the winds be the value that provides the maximum production with constant winds and (2) that the wind pattern be periodic with period equal to the cross-shelf transport time that results from maximizing production with constant winds. Examination of an example using winds in central California indicated wind patterns optimal for phytoplankton occurred more frequently than those for zooplankton
Human impacts on marine ecosystems
Marine Ecosystems and Global Change provides a detailed synthesis of the work conducted under the auspices of the Global Ocean Ecosystems Dynamics (GLOBEC) programme. This research spans two decades, and represents the largest, multi-disciplinary, international effort focused on understanding the impacts of external forcing on the structure and dynamics of global marine ecosystems
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