Age-Length Key

Background

Researchers at the Center for Quantitative Fisheries Ecology at Old Dominion University in collaboration with the Virginia Marine Resources Commission annually collect Striped Bass (Morone saxatilis) from Virginia waters of the Atlantic Ocean for age assessments. The total lengths of 1201 Stiped Bass collected in 2003 and the ages estimated from otoliths for as many as 10 fish per 1 inch length interval are recorded in StripedBass3.¹

¹ See “CSV file” link in “Source” section of linked page.

 

Construct an ALK

1. Add a variable to the data frame that contains the 1 in 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 1 inch TL category in the age-sample. From these results, compute each of the following by hand (i.e., not using R, but you may use a calculator).
   1. How many fish are in the 30 in TL category?
   2. How many fish are age 10?
   3. What proportion of fish in the 35 in TL category are age 9?
   4. What proportion of fish in the 31 in TL category are age 11?
3. Construct an observed age-length key from the table above (using R). From these results answer the following questions.
   1. What proportion of fish in the 30 in TL category should be assigned age 10?
   2. How many of forty fish in the 25 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 fish in the 30 in TL category should be assigned age 10?
   2. How many of fourty fish in the 25 mm TL 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.

5. 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 and answer the following questions.
   1. How many fish are estimated to be age 8?
   2. How many fish are estimated to be age 14?
   3. Plot the age distribution for all fish.
   4. How many fish are in the 30 in TL interval?
   5. What is the mean TL of age-9 fish?
   6. Plot the length-at-age with the mean length-at-age superimposed for all fish.
6. 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

7. 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 8?
   2. How many fish are estimated to be age 14?
   3. Plot the age distribution for all fish.
8. 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-9 fish?
   2. Plot the length-at-age with the mean length-at-age superimposed for all fish.
9. 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?
10. 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.

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