Harvesting of Populations Avoid overexploitation Avoid underexploitation (economic, conservation) How much? How often?

Harvesting of Populations AJ Nicholson ( ) High intraspecific competition Low intraspecific competition Nicholson’s Blowflies BHT, table 16.6 harvest reduce competition… … thereby increasing fecundity, survivorship and lifespan harvest reduce population size Carrying Capacity high yields if populations are below carrying capacity!

Harvesting of Populations Maximum Sustainable Yield (MSY): fixed quota BHT, fig Low recruitment rate (few individuals) Low recruitment rate (high competition) Fixed-quota harvesting High quota Low quota Medium quota MSY: the highest harvesting rate that the population can match with its own recruitment Stable Unstable MSY (risky business!)

Harvesting of Populations Maximum Sustainable Yield (MSY): fixed quota model The logistic equation Population size (N) Intrinsic rate of increase Carrying capacity K = 1000 r = 0.6

Harvesting of Populations Maximum Sustainable Yield (MSY): fixed quota model The logistic equation with harvest Population size (N) K = 1000 r = 0.6 – h Harvest quota Sustainable yield: dN/dt = 0

Harvesting of Populations Maximum Sustainable Yield (MSY): fixed quota model The logistic equation with harvest Population size (N) K = 1000 r = 0.6 – h Harvest quota Sustainable yield: dN/dt = 0 h N K Max sustainable yield: dh/dN = 0 = 0 N m = NmNm MSY harvested from a population size of K/2 … … to yield (rK/4) animals per year h m =150 =500

Harvesting of Populations Maximum harvesting frequency: at what N? BHT, fig harvest... Highest frequency The logistic equation: N = K/2

Harvesting of Populations Maximum Sustainable Yield (MSY): fixed quota BHT, fig 6.12 extinction! – h(…) NmNm Fixed-quota: not a very safe management strategy Population size Effort (/days) Efficiency (catch/vessel)

Harvesting of Populations h(…) = q E N Maximum Sustainable Yield (MSY): fixed effort Population size Effort Efficiency Stable MSY Stable SY Fixed-effort: a much safer management strategy (requires monitoring of N which can be expensive) cost Large net yield Economy is important! BHT, fig 16.14a BHT, fig 16.14b

Harvesting of Populations h(…) = q E N Maximum Sustainable Yield (MSY): fixed effort model Population size Effort Efficiency Finding E m : The logistic equation Population size (N) K = 1000 r = 0.6 h N K NmNm h m =150 =500 E m = 0.3

Harvesting of Populations Instability of harvested populations: climate BHT, fig 16.17