4. determining age, growth and sex of fish and wildlife
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Determining Age, Growth, and Sexin Fish and Wildlife
NR 250Justin VanDeHey
M. FincelD. Isermann
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Outline• Estimating and using age information.
• Estimating and using growth information.
• Determining and using sex.
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Important to know and remember:
• It is often difficult to accurately determine ages for many species of fish and wildlife.
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Fish aging = calcified structuresScales
Otoliths
Spines
Cleithra
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Nucleus or focus
Annuli
Otolith from an age-6 white crappie, Kentucky Lake, TN
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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
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Age-0 walleye otolith from Leech Lake, MN (magnified)
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Annuli form during winter, but they are not visible until growth resumes.
Winter
Summer
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Scales
• Very common- Easy removal, non-lethal
• Poor accuracy for older fish
• Annuli “crowd” as growth slows
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Otoliths• Inner ear bones
• Lethal (duh!)
• Higher accuracy than scales
• Can be sectionedSagittal otolith Cross-section
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D. Isermann
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D. Isermann
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D. Isermann
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D. Isermann
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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
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Age Estimation Techniques for Wildlife
• Wide variety of techniques
• Very difficult for many species
• Some techniques are relatively “coarse”
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Cementum annuli - mammals
• Annual deposits in teeth
• Seasonal growth causes “annuli” formation
• Section teethMatson Labs photo
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National Geographic
http://www.backpackingintherubymountains.info/animals/bighornsheepaging.html
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Tooth eruption and wear- mammals
http://www.tpwd.state.tx.us/publications/pwdpubs/media/pwd_bk_w7000_0755.pdf
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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.
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In many cases specific ages cannot beestimated for all individuals…
• Juvenile or adult
• Young-of-year (YOY) or adult
• Hatching year (HY) or after hatching year (AHY)
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Birds - Feathers
Turkeys
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Waterfowl- Feathers
• Which feathers are present?
• Feather appearance.
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Other Age Estimation Methods
• Anatomical measurements
• Gonad development
• Morphology
• Coloration
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Age StructureBull Elk Harvested in Nevada 2001
1 2 3 4 5 6 7 8 9 10 11 12 1305
10152025303540
Num
ber
Age (years)
How do we use age information?
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Female Yellow Perch Age Structure
1986 1988 1990 1992 1994 1996 19980
1
2
3
4
5
6
7
8
Year
Mea
n ag
e of
fem
ales
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Estimate Mortality
0 2 4 6 8 10Age (years)
0
5000
10000
15000
20000N
um
be
r
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Age-Class Diversity
1 2 3 4 5 6 7 8 90
20
40
60
1 2 3 4 5 6 7 8 90
20
40
60
Age (years)
Fre
qu
ency
(n
um
ber
)
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Age Ratios in Birds
• Adult : juvenile, HY : AHY, etc…
• Recruitment
• Survival
• Cost-benefit
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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)
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Measuring growth:Mean Length or Weight at Age
• Collect a sample
• Weigh and measure
• Estimate ages
• Calculate mean length / weight at each age
Age (years)0 1 2 3 4
Mean
tota
l len
gth
(mm
)
150
175
200
225
250
275
300
325
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Measuring growth:Recapture tagged individuals.
Often difficult – low recapture rates.
D. Isermann
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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
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White crappie = 300 mm total length.Otolith radius = 5 mm.1 mm of otolith growth = 60 mm of fish growth.
5 mm
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First-year otolith growth1.5 mm
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.
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Using Growth Information
• Population indicator
• What does poor growth indicate?
• Fisheries – length limits?
D. Isermann
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Determining Sex - External
• Nonlethal
• Sexual dimorphism
• Sexual dichromatism
• Urogenital morphology
• Gamete extrusion
• Ultrasound
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Determining Sex - Internal
Not just eggs and sperm!External ID can be difficult
Maturity, seasonal gonad development
D. Isermann
M. Fincel
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Using Sex Information
• Sex ratios (M:F)
• Sex-selective harvest
• Estimate mortality
• Estimate spawning stock size
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Summary