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Population dynamics Zoo 511 Ecology of Fishes

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

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**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

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**Major role of ecology: understand change**

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**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

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**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.

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**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

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**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?

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**How do populations change?**

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Density Dependence Rate of Change (per capita) Population Density

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**Rate of population increase**

per capita annual increase N Density independent Density dependent

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**? ? 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 ?

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**Logistic population growth**

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

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**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

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**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

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**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

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**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

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**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

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

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**What determines recruitment?**

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

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**The problem? Stochasticity!**

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**From: Cushing (1996). Towards**

a science of recruitment in fish populations

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**Highly variable recruitment results in naturally very variable catches**

From: Jennings, Kaiser and Reynolds (2001). Marine Fisheries Ecology

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

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**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

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**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

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**Depletion methods Closed population**

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

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

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**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

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**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

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**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

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**Schnabel method Closed population Equal catchabilty in first sample**

Marking does NOT influence catchability Multiple recaptures Easier to pick up on violation of assumptions

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**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

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**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

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Lets count some beans!

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