382
17. An Age-structured Mod el of Fish Population Enhancement
which are introdu ced from other sectors, are single-value numbers that can
be changed prior to a simulation to evaluate their impact on net profits.
There are six state variables in this sector, which reflect debt and income
generated from the fishery. Four of these are exp enses, while the remaining
two reflect receipts. The stock (DEBT) has an initial value equal to the (INITIAL LOAN AMOUNT), but as the model runs the cost of opera ting expen ses, interest on the rema ining loan amount and a hatchery tax are
added to this total. Since there is no income to cancel the debt before the
fish are caught, the debt will increase until there is adequate catch to offset
the debt. The accumulated interest payment (ACCUMULATED INTEREST
PAY) reflects the summation of the annual interest costs on the debt , or the
current debt value times the annual interest rate. The annual operating
costs (ANNUAL OP COSTS) of the hatchery are calculated by multiplying
the annual (COST PER FISH) times the number of (FISH RAISED) and the
total is summed in the (ACCUM OP COSTS) stock. Revenu e is generated
from fishery landings and is the product of the price per unit weight
(PRICE) times the total annu al catch in weight of all age classes of fish
(SUM CATCH). In a similar fashion to the costs and interest values, the total
revenues are accumulated in the (ACCUM REVENUE) stock. A component
of the (ANNUAL REVENUE) is dedicated to (HATCHERY TAXES) which are
calculated using a variable interest rate that initially increases to a maximum
of 5% with catch, but then declines as the catch approac hes its maximum
levels.
NET PROFITS are the ANNUAL REVENUE minus the ANNUAL INTEREST
COST minus the ANNUAL OP COSTS minus HATCHERY TAXES. Net profits can be either negative or positive, and because of a lack of revenue from
a fishery to initially offset deb t and expe nses the net profits are initially always negative in any model run . Net profits are used to make ANNUAL
PAYMENTS, and either add to or redu ce the debt , dep ending on the ANNUAL REVENUE being gene rated by the fishe ry. The stock (ACCUM PROFITS) is similar to the other stocks that accumulate total values over the
course of a simulation.
17.6. Model Input Values
The enhancement model allows for the exploration of a complex, dynamic
system whose behavior might otherwise be difficult to predict , given the
number of different input variables. The model should reflect reality to the
extent that there are empirical values that can be used as inputs for any
simulation. In this model , these empirical values are based on Atlantic cod.
On the other hand , variable inputs and sensitivity analyses allow for the exploration of different input values on model beh avior. Nevertheless , these
values should represent a range that wo uld bracket what are expected to
be realistic, but currently unk nown or poorly und erstood , variables. The
17. An Age-structured Mod el of Fish Population Enhancement
which are introdu ced from other sectors, are single-value numbers that can
be changed prior to a simulation to evaluate their impact on net profits.
There are six state variables in this sector, which reflect debt and income
generated from the fishery. Four of these are exp enses, while the remaining
two reflect receipts. The stock (DEBT) has an initial value equal to the (INITIAL LOAN AMOUNT), but as the model runs the cost of opera ting expen ses, interest on the rema ining loan amount and a hatchery tax are
added to this total. Since there is no income to cancel the debt before the
fish are caught, the debt will increase until there is adequate catch to offset
the debt. The accumulated interest payment (ACCUMULATED INTEREST
PAY) reflects the summation of the annual interest costs on the debt , or the
current debt value times the annual interest rate. The annual operating
costs (ANNUAL OP COSTS) of the hatchery are calculated by multiplying
the annual (COST PER FISH) times the number of (FISH RAISED) and the
total is summed in the (ACCUM OP COSTS) stock. Revenu e is generated
from fishery landings and is the product of the price per unit weight
(PRICE) times the total annu al catch in weight of all age classes of fish
(SUM CATCH). In a similar fashion to the costs and interest values, the total
revenues are accumulated in the (ACCUM REVENUE) stock. A component
of the (ANNUAL REVENUE) is dedicated to (HATCHERY TAXES) which are
calculated using a variable interest rate that initially increases to a maximum
of 5% with catch, but then declines as the catch approac hes its maximum
levels.
NET PROFITS are the ANNUAL REVENUE minus the ANNUAL INTEREST
COST minus the ANNUAL OP COSTS minus HATCHERY TAXES. Net profits can be either negative or positive, and because of a lack of revenue from
a fishery to initially offset deb t and expe nses the net profits are initially always negative in any model run . Net profits are used to make ANNUAL
PAYMENTS, and either add to or redu ce the debt , dep ending on the ANNUAL REVENUE being gene rated by the fishe ry. The stock (ACCUM PROFITS) is similar to the other stocks that accumulate total values over the
course of a simulation.
17.6. Model Input Values
The enhancement model allows for the exploration of a complex, dynamic
system whose behavior might otherwise be difficult to predict , given the
number of different input variables. The model should reflect reality to the
extent that there are empirical values that can be used as inputs for any
simulation. In this model , these empirical values are based on Atlantic cod.
On the other hand , variable inputs and sensitivity analyses allow for the exploration of different input values on model beh avior. Nevertheless , these
values should represent a range that wo uld bracket what are expected to
be realistic, but currently unk nown or poorly und erstood , variables. The