POPULATION DYNAMICS

Zoo 511 Ecology of Fishes

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

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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?

How do populations change?

Population

Density Dependence

Population Density

Rate of Change (per capita)

Rate of population increase

Density independent

Density dependent

per

cap

ita

an

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al in

crease

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Small group exercise

Time

Pop

ulat

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dens

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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

cap

ita a

nn

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incr

ease

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r0

R-selected vs. K-selected

r-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

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Survival

Eggs and larvae suffer the largest losses

HATCH Recruit!

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.

Rec

ruit

men

t

The problem? Stochasticity!

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

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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

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Time (or pass)

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

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

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 uncertainty

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

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Accurate, not precise Not accurate, precise Accurate, precise

Lets count some beans!