linking flow to flow directly?
Posted: Fri Apr 21, 2000 10:32 am
I appreciate all the suggestions and comments and suggested sources of
modeling ideas.
regarding:
> "Try linking the inflow
> directly to the outflow
> (bypassing the stock) ...
It seems that if we link inflow to outflow then the stock becomes
meaningless. The actual value of the stock can be anything, minus
values included, and has no effect on remainder of the model! Therefore,
it might be better to just leave out that stock, and have the "inflow"
go directly to a delayed inflow of the next stock.
ford stone asked:
> is this an actual (i.e., physical) shrimp farm you are
> analysing?
This is a natural shrimp population on the south coast of Java near
Cilacap. The ocean fishery is dominated by 5 to 7 species of about 15,
but one important species in paticular uses an endangered large coastal
lagoon as a nursery area. This species is caught both in the nursery
area as small juveniles and in the ocean as adults (much more
valuable). This is a common problem with shrimp fisheries in developing
countries. We are interested in looking at the effects on ocean catches
of limiting the lagoon fishing. We have only very limited information on
the actual mortality rates.
If there are no reproductive pulses, and "new juviniles arriving" is
constant over time then a two stock model is sufficient (shrimp in
lagoon) (Shrimp in ocean) with both natural and fishing mortality
draining both. For a general idea as to what is going on this is
sufficient.
If there are pulses in "new juveniles arriving" then if time in lagoon
is correct pulses in the ocean occur too soon in relation to the lagoon
pulse. A simple way to handle this is to have "shrimp leaving lagoon" go
to a cloud and then connect to "shimp moving into the ocean" as a fixed
delay. This allows the shrimp to be in the lagoon long enough for the
mortality rates to have the correct effect and has them arriving in the
ocean at the right time.
However, during the delay time the shrimp are essentially in
hyperspace... they have already spent three months in the lagoon. This
solution only works if there are no month-dependent changes in the
mortality rates. That is there may be specific times when mortality
rates are higher or lower (fishing or mortality patterns related to
weather for example).
Otherwise the best approach is to make a more detailed model of the
lagoon stocks using multiple stocks of shorter duration, though in this
case the ocean pulses appear too attenuated.
--
Richard G. Dudley
rdudley@indo.net.id
http://home.indo.net.id/~rdudley
http://ourworld.compuserve.com/homepages/drrdudley
modeling ideas.
regarding:
> "Try linking the inflow
> directly to the outflow
> (bypassing the stock) ...
It seems that if we link inflow to outflow then the stock becomes
meaningless. The actual value of the stock can be anything, minus
values included, and has no effect on remainder of the model! Therefore,
it might be better to just leave out that stock, and have the "inflow"
go directly to a delayed inflow of the next stock.
ford stone asked:
> is this an actual (i.e., physical) shrimp farm you are
> analysing?
This is a natural shrimp population on the south coast of Java near
Cilacap. The ocean fishery is dominated by 5 to 7 species of about 15,
but one important species in paticular uses an endangered large coastal
lagoon as a nursery area. This species is caught both in the nursery
area as small juveniles and in the ocean as adults (much more
valuable). This is a common problem with shrimp fisheries in developing
countries. We are interested in looking at the effects on ocean catches
of limiting the lagoon fishing. We have only very limited information on
the actual mortality rates.
If there are no reproductive pulses, and "new juviniles arriving" is
constant over time then a two stock model is sufficient (shrimp in
lagoon) (Shrimp in ocean) with both natural and fishing mortality
draining both. For a general idea as to what is going on this is
sufficient.
If there are pulses in "new juveniles arriving" then if time in lagoon
is correct pulses in the ocean occur too soon in relation to the lagoon
pulse. A simple way to handle this is to have "shrimp leaving lagoon" go
to a cloud and then connect to "shimp moving into the ocean" as a fixed
delay. This allows the shrimp to be in the lagoon long enough for the
mortality rates to have the correct effect and has them arriving in the
ocean at the right time.
However, during the delay time the shrimp are essentially in
hyperspace... they have already spent three months in the lagoon. This
solution only works if there are no month-dependent changes in the
mortality rates. That is there may be specific times when mortality
rates are higher or lower (fishing or mortality patterns related to
weather for example).
Otherwise the best approach is to make a more detailed model of the
lagoon stocks using multiple stocks of shorter duration, though in this
case the ocean pulses appear too attenuated.
--
Richard G. Dudley
rdudley@indo.net.id
http://home.indo.net.id/~rdudley
http://ourworld.compuserve.com/homepages/drrdudley