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)
1.20
1.00
0.80
0.60
0.40
0.20
0.00
Mortality Rate
11
0
13
0
15
0
17
0
19
0
90
70
50
Recruitment Rate
30
10
Rate
Density Dependence
Population Size
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
0.5
0.3
0.2
Mortality Rate
0.1
Recruitment Rate
Population Size
19
0
17
0
15
0
13
0
11
0
90
70
50
30
0
10
Rate
0.4
D-D and D-I Interaction
Non-Linear D-D
1.00
0.90
0.80
0.60
0.50
Mortality Rate
Recruitment Rate
0.40
0.30
Population Size
200
190
180
170
160
150
140
130
120
110
100
90
80
70
60
50
40
30
20
0.20
0.10
0.00
10
Rate
0.70
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
=
Where
Nt x (1+R)
N0 x (1+R)t
R =b–d
= “Finite Rate of Population Increase”
Population Size (N at time t)
EXPONENTIAL MODEL
35
30
R=0.15
25
20
R=0.13
15
10
R=0.10
5
0
0
5
10
Time (years)
15
20
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
Nt
Nt
=
<
>
K;
K;
K;
N
N
N
LOGISTIC MODEL OF
POPULATION GROWTH
LOGISTIC
POPULATION SIZE
120
100
R=0.21
80
60
R=0.15
R=0.18
40
20
0
0
5
10
15
20
25
YEAR
30
35
40
45
50
Characteristic Dynamics of
Fish Populations
• Equilibrium Concept – Populations tend to
stay at or near a certain level
120
Population Size
100
80
60
Logistic
40
Carrying Capacity
20
0
0
5
10
Time (years)
15
20
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 Stream
Reproduce
Ocean
Forage
Refuge
Forage
Small Streams COMPLEMENT Ocean
Ocean SUPPLEMENTS Small Streams
Scale of Spatial Links is Determined
by Movement Rates
0.8
0.6
Max Distance = 225 m
0.4
0.2
Mottled Sculpin
0.0
0
4
8
12
16
20
24
28
32
36
40
44
Movement Rate (m / 45 days)
Cumulative %
Cumulative Frequency
1.0
100
90
80
70
60
50
40
30
20
10
0
Brook Trout
Max Distance = 6.5km
0
20
40
60
Movement Rate (m/d)
80
100
Scale of Spatial Links
R
R
F
Sculpin
F
Re
Re
R
Brook Trout
R
Re
F
2m
F
2km
Good for reproduction
• groundwater
• stable temp
• stable flow
• bed-moving flows rare
Good for eating
•high light
•high productivity
•lots of small fishes
Headwaters
Mainstem
Larger Tributaries
Despite higher summer temperatures in
the mainstem
Shavers 1999
Shavers 2000
Shavers 2001
Rocky Run 2001
26
25
24
23
22
21
20
19
18
17
16
15
14
13
8/13/01
8/6/01
7/30/01
7/23/01
7/16/01
7/9/01
7/2/01
6/25/01
6/18/01
12
6/11/01
7-Day Average Maximum Temperature (C)
27
Cumulative daily growth (g/day)
High productivity leads to high growth rates
1.2
1.0
Cumulative Daily Growth
Mainstem
0.8
0.6
0.4
0.2
0.0
SU02
FA02
SP03
SU03
FA03
Brook trout survive summer by finding
coldwater “pockets” in the mainstem
Temperature Difference (focal - ambient)
4
Brook Trout
Brown Trout
3
2
1
0
-1
-2
-3
-4
10
12
14
16
18
Ambient Temperature (C)
20
22
24