# Age-Length Key

Lake Erie Freshwater Drum

Construct and apply an age-length key for Lake Erie Freshwater Drum.
Exercise
Age-Length Key
Drum
Author

Derek H. Ogle

Published

Mar 8, 2019

Modified

Feb 13, 2023

# Background

Bur (1984) examined the population dynamics of Freshwater Drum (Aplodinotus grunniens) in Lake Erie in the late 1970s. In one part of his study, he measured the total length (TL) of all 1577 drum sampled and extracted scales for age estimation from a proportionate sample from each 10 mm length interval. The length and age data are recorded in FWDrumLE2.1

# Construct an ALK

1. Add a variable to the data frame that contains the 10 mm TL categories and then separate the observed data into age- and length-samples. How many fish are in each sample?
2. Construct a table of the number (not proportion) of fish in each age and 10 mm TL category in the age-sample. From these results, compute each of the following by hand (i.e., not using R, but you can use a calculator).
1. How many Freshwater Drum are in the 230 mm TL category?
2. How many Freshwater Drum are age 5?
3. What proportion of Freshwater Drum in the 300 mm TL category are age 5?
4. What proportion of Freshwater Drum in the 200 mm TL category are age 4?
3. Construct an observed age-length key from the table above (using R). From these results answer the following questions.
1. What proportion of Freshwater Drum in the 210 mm TL category should be assigned age 5?
2. How many of thirty Rock Bass in the 250 mm TL category should be assigned age 4?
3. Construct a plot of the observed age-length key. Are there any potential anomalies in the plot that would suggest that a smoothed age-length key could be appropriate?
4. Construct a smoothed age-length key. From these results answer the following questions.
1. What proportion of Freshwater Drum in the 210 mm TL category should be assigned age 5?
2. How many of thirty Rock Bass in the 250 mm TL category should be assigned age 4?

# Apply an ALK I

Continue with the age- and length-sample data frames and the observed age-length key from the previous section.

1. Use the semi-random age assignment technique from Isermann and Knight (2005) and the observed age-length key to assign ages to the unaged fish in the length-sample. Combine the age-sample and the age-assigned length-sample into a single data frame to answer the following questions.
1. How many fish are estimated to be age 3?
2. How many fish are estimated to be age 8?
3. Plot the age distribution for all fish.
4. How many fish are in the 150 mm TL interval?
5. What is the mean TL of age-4 fish?
6. Plot the length-at-age with the mean length-at-age superimposed for all fish.
2. Compare your results from the previous question to someone else’s results (or repeat the previous question). Did you both get the exact same results? Why or why not? If not, how different were they?

# Apply an ALK II

Continue with the age- and length-sample data frames and the observed age-length key from the first section.

1. Use the “classical” method to estimate the age distribution (with standard errors) for all sampled fish.
1. How many fish are estimated to be age 3?
2. How many fish are estimated to be age 8?
3. Plot the age distribution for all fish.
2. Use the “classical” method to estimate the mean length-at-age (with standard deviations) for all sampled fish.
1. What is the mean TL of age-4 fish?
2. Plot the length-at-age with the mean length-at-age superimposed for all fish.
3. Compare your results to someone else’s results (or repeat the steps above). Did you both get the exact same results? Why or why not? If not, how different were they?
4. Compare your results using the “classical” method here to your results from using the Isermann and Knight (2005) method in the previous section.

Solution Code:

Available upon request to students not in a class. Contact fishR maintainers.

## References

Bur, M. T. 1984. Growth, reproduction, mortality, distribution, and biomass of Freshwater Drum in Lake Erie. Journal of Great Lakes Research 10:48–58.
Isermann, D., and C. Knight. 2005. A computer program for age–length keys incorporating age assignment to individual fish. North American Journal of Fisheries Management 25:1153–1160.