Age Structure

1. Age Structure

2. Learning Objectives • Understand the uses of an age-length key • Calculate age structure using an age-length key • Identify uses of age structure data and common pitfalls

3. Age Structure

5. Age Structure

6. Subsampling

7. Subsampling

8. Subsampling

10. Is this correct?

11. Age-Length Key

17. Subsample only Which one do you think is most accurate? From ALK

18. Age-Length Key • Mean length-at-age Li = ΣNijlij (Ni)-1 Si2 = [ΣNij (lij-Li)2] (Ni–1)-1 where, Li= mean length of the ith age group, Si2 = variance in mean length of the ith age group, Nij = Nj(nij/nj), Nj = total number of fish in the jth length group, nij = number of fish of the ith age group subsampled from the jth length group, nj = number of fish subsampled in the jth length group, lij is the mean length of fish of the ith age group subsampled from the jth length group, and Ni = ΣNij over all j length groups.

19. Age-Length Key

20. Age-Length Key If we only use the subsample, mean length is 77 mm Ni = ΣNij This is simply a weighted mean: Li = (ΣNij × lij) / Ni = 10,447 / 141 = 74 mm

21. Age-Length Key Si2 = [ΣNij (lij– Li)2] lij) / (Ni – 1) = 41,559 / 140 = 297

22. Age-Length Key