BIO220 Lecture 16 Flashcards
Ecology & evolution of harvested populations
General harvesting stragety
harvest out the recruitment and leave the stock alone
recruitment
new additions to the population
stock
current population
logistic equation of population growth
dN/dt = rN[(K-N)/K]
= Change in abundance / change in time
= rate of population growth
r = intrinsic rate of population growth of the species K = carrying capacity N = number of individuals in population
What is the logistic equation of population growth saying?
As number of individuals in population nears the carrying capacity of that population (N = K), rate of population growth is slow (logistic growth)
What happens when N -> 0 (logistic population growth equation)
dN/dt = rN
= exponential growth
____ growth rate can limit population size
density-dependent
example of density-dependent population
Tern (type of seagull) on islands is limited by nesting space
- Bird island
- Ram island
- Penikese island
logistic curves can be used to predict…
harvesting & its consequences
- what would give us the best yield without harvesting the stock?
Ideal harvesting strategy would be to harvest ____ individuals per year. This ensures ____
K/2;
Max growth rate
calculation: overall rate of population increase
per capita rate of increase x population size = overall rate of population increase
How do we know that we should harvest K/2 individuals per year?
inflection point on “overall rate of population increase”
= max rate of population increase is at K/2
Recruitment axis = ___ axis
harvest amount
A “best case scenario” for harvesting does not account for…
- random chance
- weather
- seasonal year-to-year fluctuations
- predation
MEY
max economic yield
= max profit (benefit - cost)
MEY is good for business // species populations?
business & fishermen
MSY
max sustained yield
= point of max population growth
= K/2
MSY is good for business // species populations?
species population
The fisheries tend to fish to the ___ point. Why?
break even point
- incentive to catch more fish until benefit doesn’t > cost
- fish as long as there is positive profit
Cons of the break even point
- not max profit
- populations have low growth rates
- high effort
- more harvest short-term, but screwed long-term
why do fisheries harvest to the break even point, not MSY or MEY?
constant financial pressure to harvest more
Effect of new technologies on fishing curve
- decrease slope of cost curve
- no effect on population dynamics curve
what can be found on the fishing curve?
- population dynamics (“n” parabola)
2. cost of harvesting (“/” slope)
3 methods to improve the overfishing situtation
- fixed quota (set at MSY)
- fixed effort (time, size of net…)
- fixed escapement (allow some fish to escape fishery before fishing begins)
Problems with fixed quota harvesting strategy
In a bad year, population won’t grow as fast. Next year, stock will be harvested. Eventually deplete population.
- Do not account for the random factors influencing population growth
requirements for using fixed effort harvesting strategy
define “effort” strictly
Problem with fixed effort harvesting strategy
- political pressure to change “effort” leads to rachet effect
- Good years: increased pressure to harvest more
- Bad years: political & economic pressure will always increase harvest, never decrease
Rachet effect
Political pressure to harvest more when there is a good year, but no pressure to harvest less when there are bad/normal years in the future
What is the ideal harvesting curve?
Set so that break-even point (B=C) is @ MSY
- fisheries will harvest to MSY
What does the fishing curve look like when there is over-harvesting?
When cost curve is very steep
What does the fishing curve look like when there is under-harvesting?
Cost curve is very flat
What further complicates fixed effort harvesting strategy?
multiple fish caught in the same net
- different fishing curves for different fish, so cost curve of 1 fish can’t be applied to the other without some kind of consequence
What is the best solution to overfishing (currently)?
Fixed escapement
fixed escapement works best with…
salmon & migratory fisheries
Benefit of fixed escapement
ensures that a fraction of the population escapes into the wild to spawn every generation
Cons of fixed escapement
- expensive & complicated
- not useful for non-migratory fish
