This document discusses methods for determining age, growth, and sex in fish and wildlife. It describes how structures like scales, otoliths, and teeth can be used to estimate age by examining annual growth rings. Growth can be estimated by measuring structures at each age, tracking recaptured individuals over time, or back-calculating based on ring distances. Sex can be identified externally through morphology or coloration differences, or internally by examining gonads. This information is useful for understanding population dynamics, growth patterns, and managing harvest.

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