Background
Less than 1% of the ocean floor is covered by coral. Yet, 25% of the ocean’s biodiversity
is supported in these areas. Thus, conservationists are concerned when coral
disappears, since the biodiversity of the region disappears shortly thereafter.
Consider an area in the Philippines located in a narrow channel between Luzon Island
and Santiago Island in Bolinao, Pangasinan, that used to be filled with coral reef and
supported a wide range of species (Figure 1). The once plentiful biodiversity of the area
has been dramatically reduced with the introduction of commercial milkfish (Chanos
chanos) farming in the mid 1990’s. It's now mostly muddy bottom, the once living corals
are long since buried, and there are few wild fish remaining due to over fishing and loss
of habitat. While it is important to provide enough food for the human inhabitants of the
area, it is equally important to find innovative ways of doing so that allow the natural
ecosystem to continue thriving; that is, establishing a desirable polyculture system that
could replace the current milkfish monoculture. The ultimate goal is to develop a set of
aquaculture practices that would not only support the human inhabitants financially and
nutritionally, but simultaneously improve the local water quality to a point where reef-
building corals could recolonize the ocean floor and co-exist with the farms.
A desirable polyculture is a scenario where multiple economically valuable species are
farmed together and the waste of one species is the food for another. For example, the
waste of a fin-fish can be eaten by filter feeders and excess nutrients from both fish and
filter feeders can be absorbed by algae which can also be sold, either as food or
commercially useful by-products. Not only does this reduce the amount of nutrient input
from the fish farming into the surrounding waters, it also increases the amount of profit a
farmer can make by using the fish waste to generate a greater quantity of usable
products (mussels, seaweed, etc.)
For modeling purposes, the primary animal organisms involved in these biodiverse
environments can be partitioned into predatory fish (phylum Chordata, subphylum
Vertebrata); herbivorous fish (phylum Chordata, subphylum Vertebrata); molluscs (such
as mussels, oysters, clams, snails, etc., phylum Mollusca); crustaceans (such as crabs,
lobsters, barnacles, shrimp, etc., phylum Arthropoda, subphylum Crustacea);
echinoderms (such as star fish, sea cucumbers, sea urchins, etc.; phylum
Echinodermata); and algae. By feeding types, there are primary producers
(photosynthesizers—these can be single cell phytoplankton, cyanobacteria, or
multicellular algae); filter feeders (strain plankton, organic particles, and sometimes
bacteria out of the water); deposit feeders (that eat mud and digest the organic
molecules and nutrients out of it); herbivores (eat primary producers); and predators
(carnivores). Just as on land, most of the carnivores eat herbivores or smaller
carnivores, but in the ocean they can also eat many of the filter feeders and deposit
feeders. Most animals have growth efficiencies of 10–20%, so 80–90% of what they
ingest ends up as waste in one form or another (some dissipated heat, some physical
waste, etc.). The role of coral in this biodiverse environment is largely to partition the
space and allow species to condense and coexist by giving a large number of species
each its own chance at a livable environment in a relatively small space—the aquatic
analogue of high-rise urbanization. Coral also provides some amount of filter feeding,
which helps clean the water. The ability of an area to support coral depends on many
factors, the most important of which is water quality. For example, corals in Bolinao are
able to live and reproduce in waters that contain half a million to a million bacteria per
milliliter and 0.25ug chlorophyll per liter (a proxy for phytoplankton biomass). The fish
pen channel currently sees levels upwards of ten million bacteria per milliliter and 15ug
chlorophyll per liter. Excess nutrients from the milkfish farms encourage fast-growing
algae to choke out coral growth, and particulate influx from the milkfish farms reduces
corals ability to photosynthesize. Therefore, before coral larvae can begin to grow,
acceptable water quality must be established. Other threats to coral include degradation
from increasing ocean acidity due to increased atmospheric CO2, and degradation from
increasing ocean temperature due to global warming. These can be considered second
order threats which we will not specifically address in this problem.
Problem Statement
The challenge for this problem is to come up with viable polyculture systems to replace
the current monoculture farming of milkfish that would improve water quality sufficiently
that coral larvae could begin settling and recolonizing the area. Your polyculture
scenario should be economically interesting and environmentally friendly both in the
short and long term.
1. MODEL THE ORIGINAL BOLINAO CORAL REEF ECOSYSTEM BEFORE
FISHFARM INTRODUCTION: Develop a model of an intact coral reef foodweb
containing the milkfish as the only predatory fish species, one particular herbivorous fish
(of your choice), one mollusc species, one crustacean species, one echinoderm species,
and one algae species. Specify the numbers of each species present in a way you find
reasonable; cite the sources you use or show the estimates you make in arriving at
these population numbers. In articulating your model, specify how each species interacts
with the others Show how your model predicts a steady state level of water quality
sufficient for the continued healthy growth of your coral species. If your model does not
yield a high enough level of water quality, then adjust your number of each species in a
way you find most reasonable until you do achieve a satisfactory quality level, and
describe clearly which species numbers you adjusted and why your changes were
reasonable.
2. MODEL THE CURRENT BOLINAO MONOCULTURE MILKFISH:
a. First examine the impact if milkfish farming were to suppress other animal species.
Do this by removing (setting the population to zero of) all herbivorous fish, all molluscs,
all crustaceans, and all echinoderms. Set all other populations to be the same as in your
full model above. Since you have removed the milkfish’s natural food supply, you will
need to introduce a constant term that models farmer feeding of the penned milkfish;
choose this term to keep your model in equilibrium. What steady state level of water
quality does your model now predict? Is water quality sufficient for the continued healthy
growth of your coral species? Compare and describe how your result compares to
observations.
b. Milkfish farming does not totally suppress all other animal species and water quality
is probably not as bad as your results from part 2a suggest, so use your model to
simulate the current Bolinao situation by reintroducing all deleted species and adjust
only those populations until water quality matches that currently observed in Bolinao.
Compare your populations with those currently observed in Bolinao and discuss what
changes to your model could bring your population predictions into closer agreement
with observations.
3. MODEL THE REMEDIATION OF BOLINAO VIA POLYCULTURE: You now strive to
replace the current monoculture with a polyculture industry, seeking to make the water
clear enough that the original reef ecosystem that you modeled in part 1 can re-establish
itself without any help from humans. The idea is to introduce an interdependent set of
species such that, whatever feed the milkfish farmer puts in, the combination of all of
his/her “livestock” will use it entirely so that there are no (or only minimal) leftover
nutrients and particles (feed and feces) falling onto the newly growing reef habitat below.
Additionally, you seek to commercially harvest edible biomass from this polyculture in
order to feed humans and increase value.
a. Develop a commercial polyculture to remediate Bolinao. Do this by starting with your
“current” penned model from part 2b, and introduce into it additional species that both
help clean the water and yield valuable, harvestable biomass. For example, you could
line the pens with mussels, oysters, clams or other economically valuable filter feeder to
remove some of the waste from the milkfish. Economically valuable algae could be
grown on the sides of the pens near the surface (where they get enough light), and
some of these could feed the small herbivorous fish that feed the milkfish. Clearly
present your model and its steady state populations.
b. Report on the outputs of your model. What did you optimize, what constraints did
you enforce, and why? What water quality does your model yield? How much harvest
does your model yield, and what is its economic value? How much does it cost you to
further improve water quality? In other words, from your optimal scenario, how many
dollars of harvest does it cost you to improve water quality by one unit?
4. SCIENCE: Discuss the harvesting of each species for human consumption. How do
we use your model for predicting or understanding harvesting for human consumption?
Does a harvested pound of carnivorous fish count the same as a harvested pound of
seaweed so that we seek to maximize total weight harvested, or do we differentiate by
value (as measured by price of each harvested species) so that we seek to maximize
the value of the harvest? Or do we seek to maximize the total value of harvest minus
cost of milkfish feed? Should we define the value of edible biomass as the sum of the
values of each species harvested, minus the cost of milkfish feed?
5. MAXIMIZE THE VALUE OF THE TOTAL HARVEST: We now wish to maintain an
acceptable (maximal) level of water quality while harvesting a high (maximal) value of
marketable (because edible and sell-able for byproducts are equally legitimate ways to
maximize value) biomass from all living species in the model for human consumption.
Change your model to harvest a constant amount from each species. What is the total
value of biomass (as defined above) you can harvest and the corresponding water
quality? Try different harvesting strategies and different levels of milkfish feeding (always
choosing values that will keep your model in equilibrium), and graph water quality as a
function harvest value. What strategy is optimal and what is the optimal harvest?
6. CALL TO ACTION: Write an information paper to the director of the Pacific Marine
Fisheries Council summarizing your findings on the relationship between biodiversity
and water quality for coral growth. Include a strategy for remediating an area like Bolinao
and how long it will take to remediate. Present your optimal harvesting/feeding strategy
from part 5 above along with persuasive justification, and present suggested
fishing/harvest quotas that will implement your plan. Show the leverage of your strategy
by presenting the ratio of the harvest value under your plan to the harvest value under
the current Bolinao scenario. Discuss the pros and cons from an ecological perspective
of implementing your polyculture system.
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发表于 2009-2-6 09:34
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