# Age-Length Key

Lake Ontario Rock Bass

Construct and apply an age-length key for Lake Ontario Rock Bass.
Exercise
Age-Length Key
Bass
Author

Derek H. Ogle

Published

Mar 8, 2019

Modified

Feb 13, 2023

# Background

Wolfert (1980) measured the total length (TL) of 1288 Rock Bass (Ambloplites rupestris) from Eastern Lake Ontario in the late 1970s. In addition, scales were removed for age estimation from as many as 10 specimens from each 10 mm length interval. All data are recorded in RockBassLO2.1

• 1 See “CSV file” link in “Source” section of linked page. Also note that the filename contains an “oh” not a “zero.”

•

# 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 Rock Bass are in the 180 mm TL category?
2. How many Rock Bass are age 7?
3. What proportion of Rock Bass in the 140 mm TL category are age 4?
4. What proportion of Rock Bass in the 200 mm TL category are age 8?
3. Construct an observed age-length key from the table above (using R). From these results answer the following questions.
1. What proportion of Rock Bass in the 210 mm TL category should be assigned age 5?
2. How many of thirty Rock Bass in the 180 mm TL category should be assigned age 5?
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 Rock Bass in the 210 mm length category should be assigned age 5?
2. How many of thirty Rock Bass in the 180 mm length category should be assigned age 5?

# 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 5?
2. How many fish are estimated to be age 11?
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-5 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 your code and answers to the previous question). Did you both get the exact same results? Why or why not? If not, how different were they?

The data frame with ages assigned to all fish will be used in this growth exercise.

# 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 5?
2. How many fish are estimated to be age 11?
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-5 fish?
2. Plot the mean length-at-age.
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

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.
Wolfert, D. R. 1980. Age and growth of Rock Bass in eastern Lake Ontario. New York Fish and Game Journal 27:88–90.