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Aging Fish – Length Frequency Analysis
OBJECTIVE: Age fish using the Petersen Method, length frequency analysis.
BACKGROUND:
Length frequency distributions can help in understanding the dynamics of
populations, but for them to be truly meaningful an underlying assumption must
be met: that is that fish length within each age group is normally distributed
around a peak value.
Petersen method is the simplest to use.
A large sample of fish is collected by some method that does not yield biased
distributions of fish age or size (e.g., rotenone).
The frequency of individuals is plotted as a function of fish length, and distinct
peaks are identified in the resulting length-frequency distribution.
Each peak is assigned an age beginning with the youngest. Intervals around
each peak are determined based on the maximum length of a fish. Typically for
fish > 30 cm, 1 cm intervals are used; between 30 and 60 cm long fish, 2 cm
intervals are used, and 5 cm intervals are used for fish greater than 150 cm.
Because older fish typically grow in length more slowly than younger fish,
variation among individuals within age groups tends to increase relative to
difference between age groups.
Variation in hatching time and growth rates among individuals within an age
group can lead to size-groups that do not necessarily correspond to different
age-groups.
Proportional stock density is a numerical descriptor of length-frequency data.
Values of PSD range from 0 to 100. Stock length is defined as the approximate
length of a fish at maturity, the minimum length effectively sampled by traditional
fisheries gear, and the minimum length that provides recreational value. Quality
length is defined as the minimum size of fish most anglers like to catch.
PSD’s provide another biological interpretation of the quality of a particular
fishery.
METHODS:
For today’s lab we will be evaluating fish that were collected during a 2003 lock
and dam survey on the Monongahela River. Fish lengths were recorded and
entered into a spreadsheet so that you can produce a frequency histogram.
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Once the frequency histogram has been produced, you will determine which
ages to assign to each length class using the distinct peaks that form as a guide.
Use 20 cm intervals to determine your frequencies.
Using your length frequency data, calculate the proportional stock density for this
population, using the following formula:
PSD(%) = number ≥ quality size X 100
number ≥ stock size
Quality size is the minimum size most anglers like to catch
Stock size is the minimum size that will provide recreational value, or the
minimum size effectively sampled with traditional fisheries gears (rotenone or
nets or electrofishing).
For Drum assume that quality size = 30 cm and stock size = 5 cm
OTHER LAB QUESTIONS TO INCORPORATE:
What are some of the limitations of the Petersen technique?
Why do fisheries scientists need this sort of information?
What does the age distribution look like for the drum (provide graphs here)?
Should other capture techniques have been used?
If you were a fisheries manager, what would you conclude from your results?
What do PSD’s tell us about the fishery? What does a large and a small PSD
mean biologically?
What can PSD’s tell us about community dynamics of a fishery?
What do the drum PSD’s tell us about each pool?