5
Impact of Fishing Pressure
on Mean Length of Fish
Matthias Ruth
Clearly, it is not possible to go back and redirect history. But now-not
for long-there is a chance, a brief window of opportunity to restore
and protectthe remaining healthy ecosystems that supportus. Most important , most urgent, we must protect the principal substance of the
biosphere: the sea.
Silvia Earle, from SeaChange
In this chapter you will:
• Learn how to disaggregate a population by age cohort and length;
• Specify recruitment into individual length classes as a random process;
• Introduce different fishing-induced mortality rates for different age and
length classes of fish and investigate the long-term impact of selective
fishing on the make-up of the population;
• Make use of STELLA's array features to maintain transparency of the
model diagram.
5.1. Three-Age-Cohort Model and Fishing
In Chapters 2 and 3 we dealt with each population of a species in a fairly
aggregate way. There was one state variable representing the stock of
whales , and one for the sand lance . That level of disaggregation was sufficient to capture the main features of our system-the interaction between
predators and prey . In Chapter 4 we treated juvenile and adult members of
the population alike and did not distinguish differences in their propensities to contract a disease. In contrast , in this chapter we wish to distinguish
within each population among those individuals that have different features, such as different ages , different lengths, and different rates of reproduction. With such a model at hand we can investigate, for example,
how different environmental or anthropogenic pressures may influence the
fate of an individual subgroup of the population. Specifically, we shall
model here how the age and length distribution within a fish population
changes as fishing selectively targets older and larger individuals .
There are many factors, environmental and anthropogenic, that influence
fish length. Primary among environmental factors are variations in water tern74
M. Ruth et al. (Eds.), Dynamic Modeling for Marine Conservation
© Springer-Verlag New York, Inc. 2002
Impact of Fishing Pressure
on Mean Length of Fish
Matthias Ruth
Clearly, it is not possible to go back and redirect history. But now-not
for long-there is a chance, a brief window of opportunity to restore
and protectthe remaining healthy ecosystems that supportus. Most important , most urgent, we must protect the principal substance of the
biosphere: the sea.
Silvia Earle, from SeaChange
In this chapter you will:
• Learn how to disaggregate a population by age cohort and length;
• Specify recruitment into individual length classes as a random process;
• Introduce different fishing-induced mortality rates for different age and
length classes of fish and investigate the long-term impact of selective
fishing on the make-up of the population;
• Make use of STELLA's array features to maintain transparency of the
model diagram.
5.1. Three-Age-Cohort Model and Fishing
In Chapters 2 and 3 we dealt with each population of a species in a fairly
aggregate way. There was one state variable representing the stock of
whales , and one for the sand lance . That level of disaggregation was sufficient to capture the main features of our system-the interaction between
predators and prey . In Chapter 4 we treated juvenile and adult members of
the population alike and did not distinguish differences in their propensities to contract a disease. In contrast , in this chapter we wish to distinguish
within each population among those individuals that have different features, such as different ages , different lengths, and different rates of reproduction. With such a model at hand we can investigate, for example,
how different environmental or anthropogenic pressures may influence the
fate of an individual subgroup of the population. Specifically, we shall
model here how the age and length distribution within a fish population
changes as fishing selectively targets older and larger individuals .
There are many factors, environmental and anthropogenic, that influence
fish length. Primary among environmental factors are variations in water tern74
M. Ruth et al. (Eds.), Dynamic Modeling for Marine Conservation
© Springer-Verlag New York, Inc. 2002