13.5. Questions and Tasks
279
We view the modeling process as a form of communication in which researchers attempt to parameterize their understanding of system processes.
But we also see the process as a source for novel data. This first version of
the Nile perch model suggests that the road to recovery in this system will
be long and hard, but ultimately exhilarating and rewarding. The sensitivity
required on the part of the fishery consortium to effectively control Nile
perch populations in our model is daunting. We hope this will not discourage, but rather will inspire, scientists, managers, and fishermen to bring energy and resources to this question for the sake of the fishes, the fishery,
and the people of East Africa.
13.5. Questions and Tasks
1) a) Decrease the larval mortality to 95%. The model will now enter deep
cycles in the fishing intensity and haplochromine standing stock. Is
this realistic? Why or why not?
b) Now increase the haplochromine Rand K values . Does this change
the cycles? Why or why not?
2) Now increase the management regime delay to 100 days . How does this
affect the variance in haplochromine standing stock?
3) Leaving the larval mortality at 95%, change the uncertainty variables to include 10% random flux in the model. At what "Ideal haplochromine" values can you keep the model from crashing? (You will need to run the
model several times, making adjustments until the model does not crash.)
4) Reset the model (you may need to open and close it). Run the model
several times, altering both the management delay and the fishing intensity. Try to find the bounds of the model where either the haplochromines or the Nile perch are extirpated. Are these bounds realistic?
Can we work within them?
5) Set the cohort structure of the model to include eight living cohorts of
decreasing fish size. Be sure to set the cohort number and weight tables ,
and set the starting date to 2920. Now decrease the larval mortality rate
to 90% (0.90). Set the "Ideal haplochromine community" to 75%. You
will now have to adjust the two fishery curves until the fishery can keep
the Nile perch populations in check. Why is this so difficult?
NILE PERCH MODEL
Environment
Hap_availability
hap_standin9_stock /(hap_standin9_stock+other_standin9_s
tock)
279
We view the modeling process as a form of communication in which researchers attempt to parameterize their understanding of system processes.
But we also see the process as a source for novel data. This first version of
the Nile perch model suggests that the road to recovery in this system will
be long and hard, but ultimately exhilarating and rewarding. The sensitivity
required on the part of the fishery consortium to effectively control Nile
perch populations in our model is daunting. We hope this will not discourage, but rather will inspire, scientists, managers, and fishermen to bring energy and resources to this question for the sake of the fishes, the fishery,
and the people of East Africa.
13.5. Questions and Tasks
1) a) Decrease the larval mortality to 95%. The model will now enter deep
cycles in the fishing intensity and haplochromine standing stock. Is
this realistic? Why or why not?
b) Now increase the haplochromine Rand K values . Does this change
the cycles? Why or why not?
2) Now increase the management regime delay to 100 days . How does this
affect the variance in haplochromine standing stock?
3) Leaving the larval mortality at 95%, change the uncertainty variables to include 10% random flux in the model. At what "Ideal haplochromine" values can you keep the model from crashing? (You will need to run the
model several times, making adjustments until the model does not crash.)
4) Reset the model (you may need to open and close it). Run the model
several times, altering both the management delay and the fishing intensity. Try to find the bounds of the model where either the haplochromines or the Nile perch are extirpated. Are these bounds realistic?
Can we work within them?
5) Set the cohort structure of the model to include eight living cohorts of
decreasing fish size. Be sure to set the cohort number and weight tables ,
and set the starting date to 2920. Now decrease the larval mortality rate
to 90% (0.90). Set the "Ideal haplochromine community" to 75%. You
will now have to adjust the two fishery curves until the fishery can keep
the Nile perch populations in check. Why is this so difficult?
NILE PERCH MODEL
Environment
Hap_availability
hap_standin9_stock /(hap_standin9_stock+other_standin9_s
tock)