Seminar Abstract
John Field - June 3, 2003
Modeling Northeast Pacific Ecosystems
We [John Field, Kerim Aydin, Robert Francis and Sarah Gaichas] have
been involved in modeling the major trophic interactions in Northern
California
Current (NCC) in close collaboration with workers
at the Alaska Fisheries Science Center modeling the Gulf of Alaska, the
Eastern Bering Sea and the Aleutian Islands. The key objectives are to
develop of conceptual models of food webs, estimate how total removals
relate to standing biomass, production, optimum yield, natural mortality
and trophic structure. In addition to the static snapshots, we have also
been exploring dynamic ecosystem simulations; which can be best described
as a multi-species biomass-driven predator/prey surplus production model.
Although the modeling framework is limited in its ability to account
for some important ecosystem processes, in particular age structure,
the simplicity of its assumptions suggest that it is a useful tool for
integrating available data and knowledge in a dynamic way. The approach
is particularly relevant to evaluating whether observed trends and results
from single species assessments are consistent with commonly held notions
of ecosystem abundance, productivity, interactions and behavior.
A key
challenge in doing so has been determining how best to incorporate climate
impacts and forcing processes into model dynamics, an effort that requires
numerous simplifying assumptions in order to accommodate the modeling
framework.
Climate is known to affect the Northern California Current
both from the bottom up (through short and long term variability in primary
and secondary productivity) as well as from the top-down (through variability
in the spatial distribution of key middle and top trophic level predators
such as Pacific hake, arrowtooth flounder, albacore, sardine and mackerel).
Consequently, it is clear that neither a single index, nor a suite of
indices, offers a clear forcing mechanism to explain the dynamics of
the system over time. We are hopeful that this approach may be useful
in evaluating the role of both fisheries and climate under alternative
assumptions of stability, and consequently lead to insights regarding
the potential impacts of fisheries at a systems level (albeit that of
a simplified system), rather than solely a single species perspective.