274
13. Nile Perch Population Dynamics in Lake Victoria:
realism. Before our model can be applied to real-world management scenarios, explicit mass-balance models must be develop ed for the other fishes
of Lake Victoria. But there is still much to be learned from what we have
develo ped so far.
13.3. Model Behavior
The gene ral flow of energy in the model is from prey popul ations, to Nile
perch, to the fishe ry. Some energy is transferred , through cannibalism, between Nile perch cohorts. If the fishery keeps the Nile pe rch popul ations in
check, the haplochromine populations will remain large, and the growth
rate of Nile perch will be higher. If the fishery allows the Nile pe rch stock
to explode, the haplochromines will be depleted and growth rates will be
slower, but the Nile perch standing stocks will be larger.
The model will crash if eithe r the haplochromin e prey, or the Nile perch
popul ations, are redu ced to zero . We consider these two extremes (complete overfishing of Nile perch or complete collapse of the prey base) to be
both the mathematical and theoretical bounds of the model. These conditions are experienced in nature, but we cannot pred ict the species ' true responses to them. When Nile pe rch prey is dep leted , will some members of
the po pulation emigrate? Perhaps they will fast or simply starve. Undo ubted ly, these dynamics are at the core of metapopulation and community
dynamics in Lake Victoria, but we do not pretend to understand them at
this time.
Within these bounds, however, the model behaves in a familiar and interesting way . Traditional fishery theory predicts that at some intermediate
effort maximum yield can be achieved (Ricker 1975), though the mechanisms of this relationship are rarely considered. It has been sugges ted that
these dynamics occur because the target species is freed from interspecific
com pe tition at intermediate fishing effort, but more generally it could be
stated that the flow of energy to the fished popul ation is increased as its absolute density is decreased. Ricker's equation (975) subsequently demand s
that the produ ctivity of the stock as a whole will increase to greate r-thancompe nsatory levels when its density is decreased by fishing, so that the
accumulation of biomass in the stock is highest when its inputs and outputs
are intermediate.
We ran the model one hundred times using the same gill net and seine
fishery curves , and holding all othe r variables at their default values (Table
13.1), but at various levels of fishing effort. For the Nile perch po pulation,
the specific rate of biomass accumulation in the stock (in gig/d) is highest
when haplochromine populations are highest, but the absolute rate (in g/d)
appears to be highest when fishing effort is intermediate (Figure 13.9). This
coincides with an intermed iate density of haplochromines, although this relationship is not linear.
13. Nile Perch Population Dynamics in Lake Victoria:
realism. Before our model can be applied to real-world management scenarios, explicit mass-balance models must be develop ed for the other fishes
of Lake Victoria. But there is still much to be learned from what we have
develo ped so far.
13.3. Model Behavior
The gene ral flow of energy in the model is from prey popul ations, to Nile
perch, to the fishe ry. Some energy is transferred , through cannibalism, between Nile perch cohorts. If the fishery keeps the Nile pe rch popul ations in
check, the haplochromine populations will remain large, and the growth
rate of Nile perch will be higher. If the fishery allows the Nile pe rch stock
to explode, the haplochromines will be depleted and growth rates will be
slower, but the Nile perch standing stocks will be larger.
The model will crash if eithe r the haplochromin e prey, or the Nile perch
popul ations, are redu ced to zero . We consider these two extremes (complete overfishing of Nile perch or complete collapse of the prey base) to be
both the mathematical and theoretical bounds of the model. These conditions are experienced in nature, but we cannot pred ict the species ' true responses to them. When Nile pe rch prey is dep leted , will some members of
the po pulation emigrate? Perhaps they will fast or simply starve. Undo ubted ly, these dynamics are at the core of metapopulation and community
dynamics in Lake Victoria, but we do not pretend to understand them at
this time.
Within these bounds, however, the model behaves in a familiar and interesting way . Traditional fishery theory predicts that at some intermediate
effort maximum yield can be achieved (Ricker 1975), though the mechanisms of this relationship are rarely considered. It has been sugges ted that
these dynamics occur because the target species is freed from interspecific
com pe tition at intermediate fishing effort, but more generally it could be
stated that the flow of energy to the fished popul ation is increased as its absolute density is decreased. Ricker's equation (975) subsequently demand s
that the produ ctivity of the stock as a whole will increase to greate r-thancompe nsatory levels when its density is decreased by fishing, so that the
accumulation of biomass in the stock is highest when its inputs and outputs
are intermediate.
We ran the model one hundred times using the same gill net and seine
fishery curves , and holding all othe r variables at their default values (Table
13.1), but at various levels of fishing effort. For the Nile perch po pulation,
the specific rate of biomass accumulation in the stock (in gig/d) is highest
when haplochromine populations are highest, but the absolute rate (in g/d)
appears to be highest when fishing effort is intermediate (Figure 13.9). This
coincides with an intermed iate density of haplochromines, although this relationship is not linear.