Database Reference
In-Depth Information
We have created a model that explores factors influencing the growth, removal,
and size structure of bluegill populations. These factors include density dependent
growth, natural mortality (a variable that accounts for losses due to predation, dis-
ease, winterkill, and starvation), and finally, losses due to removal via recreational
angling (the variable of interest in this model). Often these processes are size and sex
specific. To account for this, we divided the bluegill population into seven 30-mm
size categories from a 0- to 30-mm cohort up to a 180- to 210-plus-mm cohort. To
account for difference between sexes, we split the population into a male compo-
nent and a female component to give our model further resolution (Figure 11.8). We
added a section to account for egg production and the resulting fry production of
the population. Following the fry stage of the model, we included a component to
determine the sex and life history strategy of the fry. The sex ratio of the population
was assumed to be 50:50. The life history component was determined through a sub-
model, which bases life history of male fry on the proportion of adult males in the
population (the greater the density of adult males, the greater the proportion of male
fry that become “sneakers”). Sneakers are male bluegills that adopt a cuckoldry life
history strategy. The numbers used throughout the model were averages from data
collected from numerous fish populations around the Midwest. Numbers for para-
meters that were not available from empirical data were estimated from discussions
with fishery biologists in the Illinois Natural History Survey in 1997.
To address the questions listed above we ran the model three ways:
1. With no fishing mortality and with just growth and natural mortality occurring,
to establish a baseline with which to test the effects of management regulations
on the population.
2. With growth and natural mortality plus mortality due to fishing, where fishing is
unregulated and set by the number of anglers, hours fished, and the catchability
of fish in the cohort.
3. With growth and natural mortality plus mortality due to fishing, but where fishing
is regulated such that a creel is set for selected fishable cohorts, establishing an
effective size limit and maximum number of fish that can be removed from the
population per year.
In the case where there is no fishing mortality, similar patterns emerge for similar
size classes for both males and females (Figures 11.9 and 11.10). There are a greater
number of individuals (150,000 to 200,000) in the smaller size classes (0 to 30 mm
and 30 to 60 mm), and a smaller number of fish (70,000 to 90,000) in the larger size
classes (
90 mm). This represents a typical size distribution found in many lakes.
After some initial fluctuation, the selected cohorts stabilize around the ranges stated
earlier, but increase slightly through time.
Note that in these graphs and in all the following ones, a period of approximate
6 years is required for the population to come to equilibrium. This is due to the
initially stocked cohort of fry moving through all of the size classes before reaching
equilibrium. Also note that the youngest size class (0 to 30 mm) is increasing most
rapidly.
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