FISH POPULATION DYNAMICS

Fish Population Dynamics includes temporal (seasonal or year-to-year) variation in:

•population numbers

•age structure

•biomass

FISH POPULATION DYNAMICS

Information on Fish Population Dynamics is used to:

•Determine the current status of a fishery

•Develop fisheries management plans

•Evaluate management success and failure

FISH POPULATION DYNAMICS

2 Major Subcategories• Population Assessment

• Modeling Population Trends (dynamics of a population under different management scenarios)

Key Demographic Processes that Cause Populations to

Change over Time

Births

Immigrants

Deaths

Emigrants

Population Change = B + I – D – E

Factors that Cause a Population to Change can be the result of either

• Density-Dependent (D-D)

• Density-Independent (D-I)

Processes

Density-Dependent Processes• Demographic rates (b,d,i,e) are related to population

density.

• Forces: Food availability, availability of spawning habitats, predation, cannibalism, disease, parasites, exploitative vs. interference competition

• Example: With increasing fish density, there is a reduction in the amount of food per fish. This results in reduced fish growth and condition. With reduced condition, there is an increase in fish mortality rates and a reduction in fish reproductive rates. This causes the rate of population increase to decrease with increasing population density.

Density Dependent Mortality and Recruitment (simple linear)

Density Dependence

0.000.200.400.600.801.001.20

Population Size

Rat

e Mortality Rate

Recruitment Rate

Density Independent Processes

• Demographic rates are variable from year-to-year but not in relation to density.

• Forces: water temperature, flow extremes, water chemistry variability, demographic stochasticity

• Example: year-to-year variation in the severity of spring time flows causes scour of stream bottoms. This results in high mortality rates of trout eggs and larvae, and mortality rates are independent of the number of eggs present to start with.

Density Independent Mortality and Recruitment

Density Independence

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D-D and D-I Interaction

Non-Linear D-D

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Relative Importance of D-D and D-I

In General:

• Ponds, Lakes, Oceans are dominated by D-D processes with D-I processes important but secondary.

• Rivers and Streams are dominated by D-I processes with D-D processes important but secondary.

• Most populations regulated in part by both D-D and D-I processes.

Fish Population Modeling

Births

Immigrants

Deaths

Emigrants

Population Change = B + I – D – E

EXPONENTIAL MODEL OF POPULATION GROWTH

Nt+1 = Nt x (1+R)

Nt = N0 x (1+R)t

Where R = b – d

= “Finite Rate of Population Increase”

EXPONENTIAL MODEL

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Time (years)

Po

pu

lati

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Siz

e (

N a

t ti

me

t)

R=0.15

R=0.13

R=0.10

Logistic Model of Population Growth

Nt+1 = Nt + Nt x R x (1- Nt / K)

Where:

K = Carrying Capacity

Maximum population size that can be supported in a particular environment.

Encompasses many potential limiting factors: food, space, shelter, mates

LOGISTIC MODEL OF POPULATION GROWTH

Nt = K; N

Nt < K; N

Nt > K; N

LOGISTIC MODEL OF POPULATION GROWTH

LOGISTIC

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R=0.21

R=0.15R=0.18

Characteristic Dynamics of Fish Populations

• Equilibrium Concept – Populations tend to stay at or near a certain level

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Carrying Capacity

Complementary vs Supplementary Habitats

Complementary Habitat: necessary for the completion of an individual’s life cycle and maintenance of the population

Supplementary Habitat: unnecessary but results in increased population productivity (density and/or biomass)

Complementary vs Supplementary Habitats: Steelhead Example

Small StreamOcean

Reproduce

Refuge

Forage

Forage

Small Streams COMPLEMENT Ocean

Ocean SUPPLEMENTS Small Streams

Scale of Spatial Links is Determined by Movement Rates

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Movement Rate (m / 45 days)

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uen

cy

Mottled Sculpin

Max Distance = 225 m

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Max Distance = 6.5km

Brook Trout

Scale of Spatial Links

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2m

Sculpin

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2km

Brook Trout

Good for reproduction• groundwater• stable temp• stable flow• bed-moving flows rare

Good for eating

•high light

•high productivity

•lots of small fishes

Headwaters Larger Tributaries

Mainstem

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ay A

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C) Shavers 1999

Shavers 2000Shavers 2001Rocky Run 2001

Despite higher summer temperatures in the mainstem

Cumulative Daily Growth

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/day

)High productivity leads to high growth rates

Mainstem

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Brook Trout

Brown Trout

Brook trout survive summer by finding coldwater “pockets” in the mainstem