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POPULATION DYNAMICS Zoo 511 Ecology of Fishes. Today’s goals  Understand why and how population dynamics are important in fisheries ecology  Gain experience.

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Presentation on theme: "POPULATION DYNAMICS Zoo 511 Ecology of Fishes. Today’s goals  Understand why and how population dynamics are important in fisheries ecology  Gain experience."— Presentation transcript:

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 “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?

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?  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 PVA From Legault 2004

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? Population

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

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

12 Small group exercise Time Population density Time Population density Density-dependentDensity-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? 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?

13 Logistic population growth K= carrying capacity r 0 = maximum rate of increase dN/dt=r 0 N(1-N/K ) per capita annual increase N K r0r0

14 R-selected vs. K-selected r-selectedK-selected Environmentvariable and/or unpredictable constant and/or predictable Lifespanshortlong Growth ratefastslow Fecundityhighlow Natural mortalityhighlow Population dynamicsunstablestable

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

16 Survival  Predation  Disease  Prey availability  Competition for food  Harvest “Natural Mortality” Age 1Age 2Age 3 Year 1N 1,1 N 1,2 N 1,3 Year 2N 2,1 N 2,2 N 2,3 Year 3N 3,1 N 3,2 N 3,3 S

17 Survival  Eggs and larvae suffer the largest losses Egg Not Fertile Inviable Eaten Other Larva Viable & Competent Starvation Eaten 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

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

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 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 1 2

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

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

29

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 xx xx x x x x x Accurate, not precise Not accurate, preciseAccurate, precise

36 Lets count some beans!


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