(^) Next, concern for sustaining both fisheries production (to generate food, money, and
jobs) and ecosystem health creates the need to go beyond static models, like Ecopath,
to evaluate the effects of changes in catches, in Ci values, often for stocks of many
species at once. Effects of changing habitat conditions must also be evaluated. Several
steps toward this are possible. Ecopath models have been converted by Carl Walters
and colleagues to dynamic equations (dBi/dt = ...) that accept various and varying
values of Qj, DCij, Ci, (P/Bi) etc. and can be implicitly integrated in a programming
system called Ecosim (Christensen & Walters 2004). Heymans et al. (2009) used the
parameter-fitting routines in Ecosim to generate a trophic-web history based on
fisheries data for 1956 to 2003. They started from their Ecopath model for the system
in 1956. The Ecosim model was fitted to time-series for the Benguela of (i) estimated
fish-stock biomass (based on catch per unit effort and virtual population
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