Age and Growth

Growth & age patternsMeasurement techniques

Growth patterns• Determinate Growth

– Mammals & birds

• Indeterminate Growth– Fishes

Determinate

Indeterminate

Age

Siz

e

Indeterminate growth & fecundity

Fish growth – von Bertalanffy equationLt=Lmax(1-e-kt)

Fish growth – von Bertalanffy equation

Length – Weight relation (power function)

W=a Lb

Growth patterns

• Great Plasticity in growth• Size at age: High variability

– Between species– Between populations– Between individuals

Environmental factors influencing growth

• Temperature• Food and Nutrient Availability• Light Regime• Oxygen Concentration• Salinity• Pollutants• Predator Densities• Intraspecific Social Interactions• Genetics

Example: Species polymorphism

SalmonidaeArtic CharrSalvelinus alpinus

Large benthic feeder

Small benthic feeder

Piscivorous feeder

Planktivorous feeder

Annual growth variation

Population Age-Size structure

Population Size-Age relationship

Age measurement methods

• Scales• Otoliths• Vertebrae• Rays/Spines

Age measurement through scales

Age measurement through otoliths

Otolith uses

• Age determination– Daily ring counts– Annual ring counts– Radioactive isotopes

• Species identification• Paleoclimate studies (018)• Life history studies (elemental

tracers)

(Oncorhynchus clarkii)

Weakfish (Cynoscion regalis)

Otolith age validation

Otolith age validation

Otolith age validation – nuclear fallout

AnoplomatidaeSablefishAnoplopoma fimbria

Age calculation error case

Scales: 3-8 years

Otoliths: 4-40 & up to 80

Species identification

Dolphin stomach contents

Climate studies (isotope 018)

Climate studies (isotope 018)(6000 year old fossil)

Elemental tracers – Life history(Zn, Sr, Ba, Mn, Fe and Pb)

Thorrold et al. 2001

Elemental tracersof weakfish

Proof of Natal Homing!

Thorrold et al. 2001

How many fish are there?

Nt+1 = Nt + B – D + I – E

B = births D = deaths I = immigration E = emigration

How do populations change?

DeathsPopulationBirths

Emigration

Immigration

Stocking

Angling

Survival• Eggs and larvae suffer the largest

losses

Egg

Not Fertile

Inviable

Eaten

Other

Larva Viable & Competent

Starvation

Eaten

HATCHRecruit!

2 cohorts each produce 10,000,000 eggs

90.5% survivorship/day yields 24,787 survivors at 60 days

95.1% survivorship/day yields 497,871 survivors at 60 days

Recruitment• Can mean many things!

– Number of young-of-year (YOY) fish entering population in a year

– Number of fish achieving age/size at which they are vulnerable to fishing gear

• Somewhat arbitrary, varies among populations

• Major goal of fish population dynamics: understanding the relationship between stock size and recruitment

What determines recruitment?-Stock size (number and size of females)

What determines recruitment?

spawning stock biomass (SSB)

Ricker

Beverton-Holt

Density-independent

From: Wootton (1998). Ecology of teleost fishes.

What determines recruitment?

spawning stock biomass (SSB)

Ricker

Beverton-Holt

Density-independent

From: Wootton (1998). Ecology of teleost fishes.

What determines recruitment?

spawning stock biomass (SSB)

Ricker

Beverton-Holt

Density-independent

From: Wootton (1998). Ecology of teleost fishes.

The problem?

• Stochasticity = variable recruitment!

From: Cushing (1996). Towards a science of recruitment in fishpopulations

Highly variable recruitment results in naturally very variable catches

From: Jennings, Kaiser and Reynolds (2001). Marine Fisheries Ecology

Population Abundance • On rare occasions, abundance can be measured

directly– Small enclosed systems– Migration

Catch per unit effort (CPUE)• Very coarse and very common

index of abundance

Effort= 4 nets for 12 hours each= 48 net hours

Catch= 4 fish

CPUE=4/48=0.083

Effort= 4 nets for 12 hours each= 48 net hours

Catch=8 fish

CPUE=8/48=0.167

We conclude population 2 is 2X larger than population 1

1

2

Estimates of Population Size

• Proportional sampling• Rp = size of the range of the population

(Rp), (uniform distribution) • Rs = size of sampling a region• Ns/Np = Rs/Rp. • Np = (Ns Rp)/Rs = Population

Abundance

No Accuracy Estimate

Population abundance

• Density estimates (#/area)– Eggs estimated with quadrats– Pelagic larvae sampled with modified

plankton nets– Juvenile and adult fish with nets, traps,

hook and line, or electrofishing

• Density is then used as index of abundance, or multiplied by habitat area to get abundance estimate

Depletion methods

*

*

*

*

Num

ber

Cau

ght

Number previously caught

Closed populationVulnerability constant for each passCollection efficiency constantOften not simple linear regression

Estimates of Population Size

• Mark & Recapturecapture – mark – release - recapture

• Np = population abundance• M = number of individuals that are marked • n = size of the second sample of organisms • R: number of marked organisms in second

sample

Np = (M*n)/R

Accuracy Estimates Available

Mark recapture

M=5 C=4 R=2

N=population size=????

Modified Petersen method

• Assumptions:– Closed population– Equal catchability in first sample– Marking does NOT influence catchability

• Marked and unmarked fish mix randomly• Mortality rates are equal

– Marks are not lost

Schnabel method

• Closed population• Equal catchabilty in first sample• Marking does NOT influence

catchability• Multiple recaptures

– Easier to pick up on violation of assumptions