Determining Age, Growth, and Sexin Fish and Wildlife

NR 250Justin VanDeHey

M. FincelD. Isermann

Outline• Estimating and using age information.

• Estimating and using growth information.

• Determining and using sex.

Important to know and remember:

• It is often difficult to accurately determine ages for many species of fish and wildlife.

Fish aging = calcified structuresScales

Otoliths

Spines

Cleithra

Nucleus or focus

Annuli

Otolith from an age-6 white crappie, Kentucky Lake, TN

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

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

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

Winter

Summer

Scales

• Very common- Easy removal, non-lethal

• Poor accuracy for older fish

• Annuli “crowd” as growth slows

Otoliths• Inner ear bones

• Lethal (duh!)

• Higher accuracy than scales

• Can be sectionedSagittal otolith Cross-section

D. Isermann

D. Isermann

D. Isermann

D. Isermann

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

Age Estimation Techniques for Wildlife

• Wide variety of techniques

• Very difficult for many species

• Some techniques are relatively “coarse”

Cementum annuli - mammals

• Annual deposits in teeth

• Seasonal growth causes “annuli” formation

• Section teethMatson Labs photo

National Geographic

http://www.backpackingintherubymountains.info/animals/bighornsheepaging.html

Tooth eruption and wear- mammals

http://www.tpwd.state.tx.us/publications/pwdpubs/media/pwd_bk_w7000_0755.pdf

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.

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)

Birds - Feathers

Turkeys

Waterfowl- Feathers

• Which feathers are present?

• Feather appearance.

Other Age Estimation Methods

• Anatomical measurements

• Gonad development

• Morphology

• Coloration

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?

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

Estimate Mortality

0 2 4 6 8 10Age (years)

0

5000

10000

15000

20000N

um

be

r

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

)

Age Ratios in Birds

• Adult : juvenile, HY : AHY, etc…

• Recruitment

• Survival

• Cost-benefit

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)

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

Measuring growth:Recapture tagged individuals.

Often difficult – low recapture rates.

D. Isermann

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

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

5 mm

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.

Using Growth Information

• Population indicator

• What does poor growth indicate?

• Fisheries – length limits?

D. Isermann

Determining Sex - External

• Nonlethal

• Sexual dimorphism

• Sexual dichromatism

• Urogenital morphology

• Gamete extrusion

• Ultrasound

Determining Sex - Internal

Not just eggs and sperm!External ID can be difficult

Maturity, seasonal gonad development

D. Isermann

M. Fincel

Using Sex Information

• Sex ratios (M:F)

• Sex-selective harvest

• Estimate mortality

• Estimate spawning stock size

Summary