1. FISH POPULATION DYNAMICS
Fish Population Dynamics includes temporal (seasonal or year-to-year) variation in:
population numbers
age structure
biomass

2. 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

3. FISH POPULATION DYNAMICS
2 Major Subcategories
Population Assessment
Modeling Population Trends (dynamics of a population under different management scenarios)

4. Key Demographic Processes that Cause Populations to Change over Time
Births
Immigrants
Deaths
Emigrants
Population Change = B + I – D – E

5. Factors that Cause a Population to Change can be the result of either
Density-Dependent (D-D)
Density-Independent (D-I)
Processes

6. 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.

7. Density Dependent Mortality and Recruitment (simple linear)

8. 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.

9. Density Independent Mortality and Recruitment

10. D-D and D-I Interaction

11. 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.

14. Fish Population Modeling
Births
Immigrants
Deaths
Emigrants
Population Change = B + I – D – E

15. 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”

16. EXPONENTIAL MODEL
R=0.15
R=0.13
R=0.10

17. 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

18. LOGISTIC MODEL OF POPULATION GROWTH
Nt = K; N
Nt < K; N
Nt > K; N

19. LOGISTIC MODEL OF POPULATION GROWTH

20. Characteristic Dynamics of Fish Populations
Equilibrium Concept – Populations tend to stay at or near a certain level

25. 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)

26. Complementary vs Supplementary Habitats: Steelhead Example
Small Stream
Ocean
Reproduce
Forage
Refuge
Forage
Small Streams COMPLEMENT Ocean
Ocean SUPPLEMENTS Small Streams

27. Scale of Spatial Links is Determined by Movement Rates
Max Distance = 225 m
Mottled Sculpin
Brook Trout
Max Distance = 6.5km

28. Scale of Spatial Links 2m 2km R R F Sculpin F Re Re R Brook Trout R Re

29. Good for reproduction
groundwater
stable temp
stable flow
bed-moving flows rare
Good for eating
high light
high productivity
lots of small fishes

30. Headwaters
Larger Tributaries
Mainstem

31. Despite higher summer temperatures in the mainstem

32. High productivity leads to high growth rates
Mainstem

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