1. Population dynamics
Zoo 511 Ecology of Fishes

2. Today’s goals
Understand why and how population dynamics are important in fisheries ecology
Gain experience in a variety of mark-recapture methods

3. What are population dynamics?
“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

4. Major role of ecology: understand change

5. 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

6. Why study population dynamics?
Atlantic salmon PVA
From Legault 2004
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.

7. 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

8. 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
When do ecological shifts occur?
Are they stable?

9. How do populations change?

10. Density Dependence
Rate of Change (per capita)
Population Density

11. Rate of population increase
per capita annual increase N
Density independent
Density dependent

12. ? ? Small group exercise 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?
Density-dependent
Density-independent
Time
Population density ?
Time
Population density ?

13. Logistic population growth
dN/dt=r0N(1-N/K)
K= carrying capacity
r0 = maximum rate of increase
per capita annual increase N K r0

14. R-selected vs. K-selected
Environment
variable and/or unpredictable
constant and/or predictable
Lifespan
short
long
Growth rate
fast
slow
Fecundity
high
low
Natural mortality
Population dynamics
unstable
stable

15. How do populations change?
Nt+1 = Nt + B – D + I – E
B = births
D = deaths
I = immigration
E = emigration
Immigration
Stocking
Population
Births
Deaths
Angling
Emigration

16. 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

17. Survival Eggs and larvae suffer the largest losses Recruit! HATCH
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

18. 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

19. What determines recruitment? -Stock size (number and size of females)

20. What determines recruitment?
Density-independent
Ricker
What determines recruitment?
Recruitment
Beverton-Holt
spawning stock biomass (SSB)
From: Wootton (1998). Ecology of teleost fishes.

21. The problem? Stochasticity!

22. From: Cushing (1996). Towards
a science of recruitment in fish
populations

23. Highly variable recruitment results in naturally very variable catches
From: Jennings, Kaiser and Reynolds (2001). Marine Fisheries Ecology

24. Population Abundance
On rare occasions, abundance can be measured directly
Small enclosed systems
Migration

25. Catch per unit effort (CPUE)
Very coarse and very common index of abundance 1
Catch= 4 fish
CPUE=4/48=0.083
Effort= 4 nets for 12 hours each= 48 net hours 2
Catch=8 fish
CPUE=8/48=0.167
Effort= 4 nets for 12 hours each= 48 net hours
We conclude population 2 is 2X larger than population 1

26. 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

27. Depletion methods Closed population
Vulnerability constant for each pass
Collection efficiency constant
Often not simple linear regression * * N * *
Time (or pass)

28. Mark recapture
M=5
C=4
R=2
N=population size=????

30. 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

31. 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

32. 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

33. Schnabel method Closed population Equal catchabilty in first sample
Marking does NOT influence catchability
Multiple recaptures
Easier to pick up on violation of assumptions

34. 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

35. 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 x x x x x x x x x x
Accurate, not precise
Not accurate, precise
Accurate, precise

36. Lets count some beans!