380
17. An Age-structured Model of Fish Population Enhancement
17.4. Life History Sector
The life history sector incorporates factors affecting fish as they grow , reproduce, suffer natural mortality, migrate, and pass through the fishery. In
the current model , fish are assumed to have a ten-year lifespan and this is
incorporated into the model array. This parameter can be changed to accommodate the actual life span of any species.
There are six state variables in the present model configuration: (1)
hatch ery REARED age 1 fish (HAT AGE 1); (2) juvenile age 2 fish (JUV AGE
2); (3) mature fish (MATURE FISH); (4) eggs that are spawned into the
water column; (5) naturally produced age 0 fish (NAT AGE 0); and (6) naturally produced age 1 fish (NAT AGE 1). Identification of these six distinct
stocks allows for the incorporation of differential rates of loss, primarily in
the form of different mortality rates, in the model. These rates of loss can be
modified to evaluate their effect on the rate of repopulation resulting from
the survival and reproductive success of the hatchery-reared animals and
their progeny.
Age 1 fish have been separated into two stocks. In the case of hatcheryreared fish, a single cohort of fish is released, via the ENHANCEMENT flow
from the hatchery , into the environment as HAT AGE 1 fish. The second
stock of age 1 fish accounts for natural reproduction (NAT AGE 1) from the
fish that survive and mature, as well as their offspring and subsequent generations that resulted from the initial releasers ) of hatch ery raised fish. Like
the young fish from the hatche ry, fish resulting from natural reproduction
experience a variable rate of natural mortality (MORT AGE 1) but , unlike
their hatchery-re ared counterparts, their initial survival has not been confounded by any additional behaviorally related mortality.
Regardless of their source, once fish are introduced into the environment,
they experience natural mortality (DIE HAT AGE 1 and DIE NAT AGE 1) at
a mortality rate (MORT AGE 1) that is a function of the numbers of mature
fish in the system. This variable rate of mortality is set to range from 80% to
99.9%, with 99.9% mortality of all age 1 fish occurring when the numbers of
mature animals (SUM MATURES) exceeds 5,000,000. Those that survive
grow and advance to the juvenile stage at age 2 (JUV AGE 2) via the TO
]UV 2 or the NAT TO]UV 2 flows. At this juvenile stage , they continue to
suffer natural mortality (DIE AGE 2), being subject to a variable rate of natural mortality (MORT AGE 2) , which ranges from 40% to 100% as the sum
of mature animals reaches 5,000,000.
Fish grow , mature sexually and reproduce over the remainder of their
lives as reflected in the array labeled MATURE FISH. This reproduction is a
direct result of the enh ancement effort and needs to be accounted for in
evaluating the costs and benefi ts of re-establishing spawning populations.
The model recognizes an age- and we ight-depe ndent rep rodu ctive contribution (FECUNDITY AT AGE; WEIGHT AT AGE) from the mature fish.
Since eggs are a produ ct of the female s in the population , the SEX RATIO
17. An Age-structured Model of Fish Population Enhancement
17.4. Life History Sector
The life history sector incorporates factors affecting fish as they grow , reproduce, suffer natural mortality, migrate, and pass through the fishery. In
the current model , fish are assumed to have a ten-year lifespan and this is
incorporated into the model array. This parameter can be changed to accommodate the actual life span of any species.
There are six state variables in the present model configuration: (1)
hatch ery REARED age 1 fish (HAT AGE 1); (2) juvenile age 2 fish (JUV AGE
2); (3) mature fish (MATURE FISH); (4) eggs that are spawned into the
water column; (5) naturally produced age 0 fish (NAT AGE 0); and (6) naturally produced age 1 fish (NAT AGE 1). Identification of these six distinct
stocks allows for the incorporation of differential rates of loss, primarily in
the form of different mortality rates, in the model. These rates of loss can be
modified to evaluate their effect on the rate of repopulation resulting from
the survival and reproductive success of the hatchery-reared animals and
their progeny.
Age 1 fish have been separated into two stocks. In the case of hatcheryreared fish, a single cohort of fish is released, via the ENHANCEMENT flow
from the hatchery , into the environment as HAT AGE 1 fish. The second
stock of age 1 fish accounts for natural reproduction (NAT AGE 1) from the
fish that survive and mature, as well as their offspring and subsequent generations that resulted from the initial releasers ) of hatch ery raised fish. Like
the young fish from the hatche ry, fish resulting from natural reproduction
experience a variable rate of natural mortality (MORT AGE 1) but , unlike
their hatchery-re ared counterparts, their initial survival has not been confounded by any additional behaviorally related mortality.
Regardless of their source, once fish are introduced into the environment,
they experience natural mortality (DIE HAT AGE 1 and DIE NAT AGE 1) at
a mortality rate (MORT AGE 1) that is a function of the numbers of mature
fish in the system. This variable rate of mortality is set to range from 80% to
99.9%, with 99.9% mortality of all age 1 fish occurring when the numbers of
mature animals (SUM MATURES) exceeds 5,000,000. Those that survive
grow and advance to the juvenile stage at age 2 (JUV AGE 2) via the TO
]UV 2 or the NAT TO]UV 2 flows. At this juvenile stage , they continue to
suffer natural mortality (DIE AGE 2), being subject to a variable rate of natural mortality (MORT AGE 2) , which ranges from 40% to 100% as the sum
of mature animals reaches 5,000,000.
Fish grow , mature sexually and reproduce over the remainder of their
lives as reflected in the array labeled MATURE FISH. This reproduction is a
direct result of the enh ancement effort and needs to be accounted for in
evaluating the costs and benefi ts of re-establishing spawning populations.
The model recognizes an age- and we ight-depe ndent rep rodu ctive contribution (FECUNDITY AT AGE; WEIGHT AT AGE) from the mature fish.
Since eggs are a produ ct of the female s in the population , the SEX RATIO