POPULATION DYNAMICSZoo 511 Ecology of Fishes 2009

Today’s goals Understand why and how population

dynamics are important in fisheries ecology

Gain experience in a variety of mark-recapture methods

“A population is a group of fish of the same species that are alive in a defined area at a given time” (Wootton 1990)

Population dynamics: changes in the number of individuals in a population or the vital rates of a population over time

What are population dynamics?

Major role of ecology: understand change

Major role of ecology: understand change

Myers et al. 2007

Why study population dynamics?

Often most relevant response to ecosystem manipulation/perturbation

Endangered species (population viability analysis, PVA)

Fisheries management (sustainable yield)

Understand ecosystem dynamics and ecological processes

Why study population dynamics?

Often most relevant response to ecosystem manipulation/perturbation

Endangered species (population viability analysis, PVA)

Fisheries management (sustainable yield)

Understand ecosystem dynamics and ecological processes

PVA: Modeling the probability that a population will go extinct or drop below the minimum viable population size within a given number of years.

Atlantic salmon PVAFrom Legault 2004

Why study population dynamics?

Often most relevant response to ecosystem manipulation/perturbation

Endangered species (population viability analysis, PVA)

Fisheries management (sustainable yield)

Understand ecosystem dynamics and ecological processes

from Hilborn and Walters 1992

Why study population dynamics?

Often most relevant response to ecosystem manipulation/perturbation

Endangered species (population viability analysis, PVA)

Fisheries management (sustainable yield)

Understand ecosystem dynamics and ecological processesWhen do ecological shifts occur?Are they stable?

Nt+1 = Nt + B – D + I – E

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

How do populations change?

DeathsPopulationBirths

Emigration

Immigration

Stocking

Angling

Density Dependence

Population Density

Rate of Change (per capita)

Rate of population increase

Density independent

Density dependent

per c

apita

ann

ual i

ncre

ase

N

Small group exercise

Time

Pop

ulat

ion

dens

ity

Time

Pop

ulat

ion

dens

ity

Density-dependent Density-independent

Population starts at low density.What happens to density over time

under density-dependent rate of increase?

What happens if rate of increase is density-independent?

Logistic population growth

K= carrying capacityr0 = maximum rate of increase

dN/dt=r0N(1-N/K)

per c

apita

ann

ual

incr

ease

N K

r0

R-selected vs. K-selectedr-selected K-selected

Environment variable and/or unpredictable

constant and/or predictable

Lifespan short long

Growth rate fast slow

Fecundity high low

Natural mortality high low

Population dynamics unstable stable

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 Predation Disease Prey availability Competition for food Harvest

“Natural Mortality”

Age 1

Age 2

Age 3

Year 1 N1,1 N1,2 N1,3

Year 2 N2,1 N2,2 N2,3

Year 3 N3,1 N3,2 N3,3

S

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

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

*

*

*

*

N

Time (or pass)

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

Mark recaptureM=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

How to avoid violation of assumptions? Two sampling gears Distribute marked individuals widely;

allow time for mixing Can be separated into different groups

Length Sex Geographic regions

How many to mark/recapture? Requires some knowledge of population size! Trade-off between precision and sample size

Population of 10,000: Mark 400 and examine 600 for +/- 50% OR mark 1,000 and examine 1,500 for +/- 10%

Trade-off between marked and recapture sample size Population of 10,000: Mark 1,000 and

examine1,500 OR Mark 4,500 and examine 500

Schnabel method

Closed population Equal catchabilty in first sample Marking does NOT influence catchability Multiple recaptures

Easier to pick up on violation of assumptions

Jolly Seber method Open populations

Allows estimation of births and deaths Three or more sampling periods needed Equal catchability of all individuals in all

samples Equal probability of survival Marks are not lost Sampling time is negligible compared to

intervals between samples

Importance of variability Confidence intervals

Long-term frequency, not probablity! 95% confidence intervals if you repeated

procedure an infinite number of times, 95% of the time the interval you create would contain the “true” value

Precision vs. accuracy

xx

x xx

x

x

x

x

xxxx x

xxxx

Accurate, not precise Not accurate, precise Accurate, precise

Lets count some beans!