17.8. Discussion
387
(see Giske and Gro Yea Salvanes 1999). A carrying capacity for a particular
species can result from either endogenous or exogenous controls. In the
current model , we have not invoked any exogenous control , but have assumed density dependence at three different times in the fishes' life history.
An increasing rate of natural mortality that is a function of the sum of the
mature fish in the population has been imposed on age 1, age 2, and mature fish. Such a control may be a cannibalistic predator/prey situation
whereby larger fish prey on their smaller conspecifics . On the other hand,
one might interpret the limit to juvenile population growth as reflecting
habitat limitations (Auster and Langton 1999; Langton and Auster 1999) as
an alternative to, or component of, a predatory control mechanism. Density
dependence, from whatever cause , has not been imposed on the survival
of eggs or larval fish. These two life history stages have been assumed to
have a constant rate of mortality, albeit one that can be altered in consecutive simulations. If there is a control on these life history stages, it is most
likely an exogenous factor and may be an environmental extreme that occurs during this stage of the fishes' development, or the result of prey limitation that is not included in the current model design .
The key to success for enhancement is for the exponential growth of the
population to outstrip the growing interest costs on the debt. Changing
mortality rate input in the model can slow, or speed, the rate of population
growth, but unless population growth is checked by a low carrying capacity for the system, the results are surprisingly consistent. The model is relatively insensitive to the amount of the initial loan, the annual operating
costs, the cost per fish, and the interest rate on the debt. Changing anyone
of these parameters may shift the break-even point along the time axis, but
will not usually negate the economic reward of exponential growth that is
reflected in value of the total catch. What is important, however, is to recognize that there is a substantial time lag before the loan can be repaid and,
in the real world, that opens the process to variation not currently accounted for in the model. The lag-time is the result of high mortality rates
for all life history stages, followed ultimately by the dramatic and almost immediate impact of exponential population growth. Enhancement cannot,
therefore, be considered a rapid cure for mismanagement of a productive
fishery, but it does hold out hope for a more rapid recovery than the random chance that fish will migrate to an area and repopulate it. Enhancement is developing into its own applied science (Leber 1999) and , as the
parameters for the survival of hatchery-reared fish upon release in the wild
are understood, a proper experimental protocol will be established to empirically determine how enhancement can be utilized as a management and
conservation tool.
Stocking the ocean with fish is stocking the public domain . It is therefore
unlikely that a private hatchery would invest in such a venture unless the
owners were granted exclusive rights to the fishery. Even then, the lag-time
prior to generating income makes the investment very risky. What is more
387
(see Giske and Gro Yea Salvanes 1999). A carrying capacity for a particular
species can result from either endogenous or exogenous controls. In the
current model , we have not invoked any exogenous control , but have assumed density dependence at three different times in the fishes' life history.
An increasing rate of natural mortality that is a function of the sum of the
mature fish in the population has been imposed on age 1, age 2, and mature fish. Such a control may be a cannibalistic predator/prey situation
whereby larger fish prey on their smaller conspecifics . On the other hand,
one might interpret the limit to juvenile population growth as reflecting
habitat limitations (Auster and Langton 1999; Langton and Auster 1999) as
an alternative to, or component of, a predatory control mechanism. Density
dependence, from whatever cause , has not been imposed on the survival
of eggs or larval fish. These two life history stages have been assumed to
have a constant rate of mortality, albeit one that can be altered in consecutive simulations. If there is a control on these life history stages, it is most
likely an exogenous factor and may be an environmental extreme that occurs during this stage of the fishes' development, or the result of prey limitation that is not included in the current model design .
The key to success for enhancement is for the exponential growth of the
population to outstrip the growing interest costs on the debt. Changing
mortality rate input in the model can slow, or speed, the rate of population
growth, but unless population growth is checked by a low carrying capacity for the system, the results are surprisingly consistent. The model is relatively insensitive to the amount of the initial loan, the annual operating
costs, the cost per fish, and the interest rate on the debt. Changing anyone
of these parameters may shift the break-even point along the time axis, but
will not usually negate the economic reward of exponential growth that is
reflected in value of the total catch. What is important, however, is to recognize that there is a substantial time lag before the loan can be repaid and,
in the real world, that opens the process to variation not currently accounted for in the model. The lag-time is the result of high mortality rates
for all life history stages, followed ultimately by the dramatic and almost immediate impact of exponential population growth. Enhancement cannot,
therefore, be considered a rapid cure for mismanagement of a productive
fishery, but it does hold out hope for a more rapid recovery than the random chance that fish will migrate to an area and repopulate it. Enhancement is developing into its own applied science (Leber 1999) and , as the
parameters for the survival of hatchery-reared fish upon release in the wild
are understood, a proper experimental protocol will be established to empirically determine how enhancement can be utilized as a management and
conservation tool.
Stocking the ocean with fish is stocking the public domain . It is therefore
unlikely that a private hatchery would invest in such a venture unless the
owners were granted exclusive rights to the fishery. Even then, the lag-time
prior to generating income makes the investment very risky. What is more