1. Determining Age, Growth, and Sex
in Fish and Wildlife
D. Isermann
NR 250
Justin VanDeHey
M. Fincel

2. Outline
• Estimating and using age information.
• Estimating and using growth information.
• Determining and using sex.

3. Important to know and remember:
• It is often difficult to accurately
determine ages for many species of
fish and wildlife.

4. Fish aging = calcified structures
Scales
Otoliths
Spines
Cleithra

5. Nucleus or focus
Annuli
Otolith from an age-6 white crappie, Kentucky Lake, TN

6. How do annuli form?
• Deposition of daily rings or circuli
• Circadian rhythm (circular, 24 hrs)
• Temperate regions - growth slows in winter
• Daily rings closer together in winter
• Faster growth- daily rings farther apart
• Different in marine fish, tropics, and other cases

7. Age-0 walleye otolith from Leech Lake, MN (magnified)

8. Annuli form during winter, but they are not visible
until growth resumes.
Winter
Summer

9. Scales
• Very common
-
Easy removal, non-lethal
• Poor accuracy for older fish
• Annuli “crowd” as growth slows

10. Otoliths
• Inner ear bones
• Lethal (duh!)
• Higher accuracy than scales
• Can be sectioned
Sagittal otolith
Cross-section

11. D. Isermann

12. D. Isermann

13. D. Isermann

14. D. Isermann

15. Age of Lake Trout in Lake Superior
• Scales indicated trout were ages 5 to 13
• Otoliths – ages 8 to 28
• Disagreements of 2 to 20 years!
• Scale ages resulted in biased estimates of
growth (theoretical maximum TL of 49 feet!).
• Trout had a more diverse age structure and
lower mortality than previously thought
Burnham-Curtis and Bronte 1996

16. Age Estimation Techniques for Wildlife
• Wide variety of techniques
• Very difficult for many species
• Some techniques are relatively “coarse”

17. Cementum annuli - mammals
• Annual deposits in teeth
• Seasonal growth causes “annuli” formation
• Section teeth
Matson Labs photo

18. National Geographic
http://www.backpackingintherubymountains.info/animals/bighornsheepaging.html

19. Tooth eruption and wear- mammals
http://www.tpwd.state.tx.us/publications/pwdpubs/media/pwd_bk_w7000_0755.pdf

20. When we can estimate an age…
• Age group or age class = all individuals of a
designated age.
• Year class or cohort = all individuals born in a
specific year.

21. In many cases specific ages cannot be
estimated for all individuals…
• Juvenile or adult
• Young-of-year (YOY) or adult
• Hatching year (HY) or after hatching year (AHY)

22. Birds - Feathers
Turkeys

23. Waterfowl- Feathers
• Which feathers are present?
• Feather appearance.

24. Other Age Estimation Methods
• Anatomical measurements
• Gonad development
• Morphology
• Coloration

25. How do we use age information?
Age Structure
Number
Bull Elk Harvested in Nevada 2001
40
35
30
25
20
15
10
5
0
1 2 3 4 5 6 7 8 9 10 11 12 13
Age (years)

26. Female Yellow Perch Age Structure
Mean age of females
8
7
6
5
4
3
2
1
0
1986
1988
1990
1992
Year
1994
1996
1998

27. Estimate Mortality
20000
Number
15000
10000
5000
0
0
2
4
6
Age (years)
8
10

28. Age-Class Diversity
Frequency (number)
60
40
20
0
60
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
9
40
20
0
Age (years)

29. Age Ratios in Birds
• Adult : juvenile, HY : AHY, etc…
• Recruitment
• Survival
• Cost-benefit

30. Estimating Growth
• Typically accompanies age estimation
• Define growth per unit of time (e.g., annual)
• Typically use length for fish, weight for wildlife
(e.g., hard to get an elk on a ruler)

31. • Collect a sample
• Weigh and measure
• Estimate ages
Mean total length (mm)
Measuring growth:
Mean Length or Weight at Age
325
300
275
250
225
200
175
150
0
1
2
3
4
Age (years)
• Calculate mean length / weight at each age

32. Measuring growth:
Recapture tagged individuals.
D. Isermann
Often difficult – low recapture rates.

33. Back-calculation or Biochronology
• Growth history
• Assume growth of fish is related to growth
of selected aging structure
• Proportional in some cases
• Distance between annuli = annual growth
• Useful when sampling of a population is
sporadic

34. White crappie = 300 mm total length.
Otolith radius = 5 mm.
1 mm of otolith growth = 60 mm of fish growth.
5 mm

35. 1 mm of otolith growth = 60 mm of fish growth.
1.5 mm of otolith growth = 1.5 x 60 = 90 mm of fish growth.
This crappie grew 90 mm in year 1.
First-year otolith growth
1.5 mm

36. Using Growth Information
• Population indicator
• What does poor growth indicate?
• Fisheries – length limits?
D. Isermann

37. Determining Sex - External
• Nonlethal
• Sexual dimorphism
• Sexual dichromatism
• Urogenital morphology
• Gamete extrusion
• Ultrasound

38. Determining Sex - Internal
M. Fincel
D. Isermann
Not just eggs and sperm!
External ID can be difficult
Maturity, seasonal gonad development

39. Using Sex Information
• Sex ratios (M:F)
• Sex-selective harvest
• Estimate mortality
• Estimate spawning stock size

40. Summary