Environmental Engineering Reference
In-Depth Information
constitute a somewhat arbitrary control of all fish present in the system. All
processes in this model were described by utilizing a matrix of interlinked
differential equations (Chap. 6).
Through variation of the state variables 'fishing experiments' were performed in
order to assess the effects of fishing intensity and catch selection on fisheries yield,
community structure and ecological processes. McClanahan performed a total of
five simulation runs to specifically determine (1) the model's prediction of succes-
sional dynamics, (2) the effect of removing all bony fish, (3) the effect of removing
only piscivores, (4) the effect of removing all fish except invertivores, and (5)
model predictions of a scenario where sea urchins (or fishing) do not have a
detrimental effect on live coral. Model simulations of different scenarios revealed
that the modification of a single variable in this web of highly interrelated compo-
nents has important ramifications for reef development. One of the major findings
of the model (and later verified by field studies) was that coral reefs are prone to
have more than one equilibrium state for realistic parameter ranges (see Chap. 6),
influenced by the extent of fishing or the abundance of piscivores (Fig. 17.2 ). The
simulation results showed that if all fish groups were harvested, two equilibrium
states could occur, one governed by herbivorous fish and the other by sea urchins.
A third ecological state was manifest by high algal and low coral cover associated
with the low abundance of either herbivorous fish or sea urchins. The fisheries
management strategy that is predicted to produce the highest yields whilst main-
taining high primary productivity and calcium carbonate deposition was to harvest
piscivores and herbivorous fish and to leave invertivores unharvested. For this
case, the model predicted the amount of piscivores to quickly decline, taking
predation pressure off the herbivorous fish and channelling the majority of algal
production into herbivorous fish, while invertivores kept the sea urchin abundance
low. When fishing levels were highest, algal biomass was predicted to increase and
Fig. 17.2 Transition processes between different equilibrium states of the coral reef ecosystem
indicated by model simulations and field studies. Adapted from McClanahan (1995)
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