Lecture 14 Ecology and Evolution of Harvested Populations

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11 Apr 2012
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Lecture 14 BIO220 Ecology and Evolution of Harvested Populations
Density-dependent growth rate can limit population size
Tern carrying capacity on islands is limited by nesting space
As time increases, N increases until it reaches plateau that reflects capacity that
depends on number of available nesting sites
Maximizing population growth
Per capita rate of increase x population size = overall rate of population increase
Ideal harvesting strategy: harvest ½ K individuals/year so population is at
maximum growth rate
Ideal scenario
Harvest k/2 per year
Population always in region of maximum growth
oHighest rate of replenishment
Allowing harvesting and self-sustaining populations
Best case scenario does not account for random chance, weather, season/year to
year flux, predation, etc
Harvesting costs increase with increased carrying capacity
MEY: Maximum economic yield, where benefit-cost maximized
MSY: maximum population growth
Harvesting costs increase with increased K
[B-C]MEY > [B-C]MSY
Harvest @ MEY >> Harvest @ MSY
Consequences of MEY > MSY
Constant financial pressure to harvest more
oNot just for gross returns, but for net returns
Populations harvested to point where they have low population growth rates
oSame absolute amount removed each year
oBut: population experiencing slower population growth
oAnnual stock flux hard to track optimally
oWill lead to extinction
Improvements in technology make cost curve “flatter”, but do not alter the
population dynamics
Until the fishery collapses,
Net profits more pressure to fish, and get more fish
Tight regulations, enforcement, quotas below estimated MST negative effects on
population, negative effects on society (no jobs)
Catch quotas < MSY “natural insurance” via ecological stability
There can be economic incentive to fish more even when industry is failing
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