2 Today’s goalsUnderstand why and how population dynamics are important in fisheries ecologyGain 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
5 Why study population dynamics? Often most relevant response to ecosystem manipulation/perturbationEndangered species (population viability analysis, PVA)Fisheries management (sustainable yield)Understand ecosystem dynamics and ecological processes
6 Why study population dynamics? Atlantic salmon PVAFrom Legault 2004Often most relevant response to ecosystem manipulation/perturbationEndangered species (population viability analysis, PVA)Fisheries management (sustainable yield)Understand ecosystem dynamics and ecological processesPVA: 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/perturbationEndangered species (population viability analysis, PVA)Fisheries management (sustainable yield)Understand ecosystem dynamics and ecological processesfrom Hilborn and Walters 1992
8 Why study population dynamics? Often most relevant response to ecosystem manipulation/perturbationEndangered species (population viability analysis, PVA)Fisheries management (sustainable yield)Understand ecosystem dynamics and ecological processesWhen do ecological shifts occur?Are they stable?
10 Density DependenceRate of Change (per capita)Population Density
11 Rate of population increase per capita annual increaseNDensity independentDensity 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-dependentDensity-independentTimePopulation density?TimePopulation density?
13 Logistic population growth dN/dt=r0N(1-N/K)K= carrying capacityr0 = maximum rate of increaseper capita annual increaseNKr0
17 Survival Eggs and larvae suffer the largest losses Recruit! HATCH 2 cohorts each produce 10,000,000 eggs90.5% survivorship/day yields 24,787 survivors at 60 days95.1% 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 yearNumber of fish achieving age/size at which they are vulnerable to fishing gearSomewhat arbitrary, varies among populationsMajor 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-independentRickerWhat determines recruitment?RecruitmentBeverton-Holtspawning stock biomass (SSB)From: Wootton (1998). Ecology of teleost fishes.
22 From: Cushing (1996). Towards a science of recruitment in fishpopulations
23 Highly variable recruitment results in naturally very variable catches From: Jennings, Kaiser and Reynolds (2001). Marine Fisheries Ecology
24 Population AbundanceOn rare occasions, abundance can be measured directlySmall enclosed systemsMigration
25 Catch per unit effort (CPUE) Very coarse and very common index of abundance1Catch= 4 fishCPUE=4/48=0.083Effort= 4 nets for 12 hours each= 48 net hours2Catch=8 fishCPUE=8/48=0.167Effort= 4 nets for 12 hours each= 48 net hoursWe conclude population 2 is 2X larger than population 1
26 Population abundance Density estimates (#/area) Eggs estimated with quadratsPelagic larvae sampled with modified plankton netsJuvenile and adult fish with nets, traps, hook and line, or electrofishingDensity 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 passCollection efficiency constantOften not simple linear regression**N**Time (or pass)
30 Modified Petersen method Assumptions:Closed populationEqual catchability in first sampleMarking does NOT influence catchabilityMarked and unmarked fish mix randomlyMortality rates are equalMarks are not lost
31 How to avoid violation of assumptions? Two sampling gearsDistribute marked individuals widely; allow time for mixingCan be separated into different groupsLengthSexGeographic regions
32 How many to mark/recapture? Requires some knowledge of population size!Trade-off between precision and sample sizePopulation 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 sizePopulation 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 catchabilityMultiple recapturesEasier to pick up on violation of assumptions
34 Jolly Seber method Open populations Allows estimation of births and deathsThree or more sampling periods neededEqual catchability of all individuals in all samplesEqual probability of survivalMarks are not lostSampling time is negligible compared to intervals between samples
35 Importance of uncertainty Confidence intervalsLong-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” valuePrecision vs. accuracyxxxxxxxxxxAccurate, not preciseNot accurate, preciseAccurate, precise