Sustainable use of wildlife Flashcards
Describe the precautionary objective
- precautionary approach to fisheries management is applied and
- exploitation of marine stocks restores and maintains populations of harvested species above biomass levels capable of producing maximum sustainable yield
Define maximum sustainable yield
- the highest theoretical equilibrium yield that can be continuously taken on average from a marine stock under existing environmental conditions without significantly affecting the reproduction process
Define sustainable level
a level of that stock above biomass levels capable of producing maximum sustainable yield
Give the equation for logistic population growth
- dN/dt = rN(1-N/K)
- r = intrinsic rate of increase
Describe the parabolic population curve as number of new individuals against population size (% of K)
- when N is low there is very little density dependence
- however, population of breeding females is small: limited growth
- centrally, both growth rate and population size are middle; maximum new individuals added
- when N is high there is very strong density dependence, so growth is limited despite the large population of breeding females due to competition for resources
Describe the relationship between our capita growth rate against population size (% of K)
- linearly decreasing growth rate with population size
- 1-(N/K)
Describe the relationship between population size (% of N) against time
- sigmoid
- Nt = K / 1+(K/N-1)e^-n
Give some organisms for which logistic growth has been observed
- yeast
- Yellowstone elk
Describe the economic fundamentals of harvesting
- populations are capital
- we can harvest the surplus
Modify the logistic equation for harvesting
dN/dt = rN(1-N/K) - H
H
can be constant number oc constant proportion (aN)
When are harvested populations stable
- when they are not growing
- dN/dt = 0
- when the population growth rate = harvest rate
Give the equation for stably harvested populations
rN(1-N/K) = H
Populations can be sustainably harvested because
- they exhibit a density dependent response
- assumptions about the shape of the density dependence are crucial
Describe what happens when a constant number of individuals are harvested
- harvesting > MSY level leads to a rapid population decline to extinction
- harvesting < MSY level leads to a stable equilibrium population, if the starting population is above the unstable equilibrium population size.
- the further below the MSY level the harvest is the safer it is
Describe what happens when the number harvested individuals is proportional to population size
- a single stable equilibrium population size results
- so long as the harvesting rate is low enough to cross the population growth curve at some point
Give the equation for harvesting size, harvesting effort and population size
H = qEN
- E = harvesting effort
- q = catchability coefficient
Give an expression for yield (H) as a function of effort:
H = qKE(1-qE/R)
What are the advantages of harvesting a constant number?
- guarantees the same yield each year
- strategy is very simple to administer
Give the disadvantages of harvesting at a constant number
- assumption that stock size is deterministic, with no natural variation or measurement errors
- if harvests are high, rapid population decline (reinforced by positive feedback) could ensue
- if population is at an unstable equilibrium, any decline in population size will accelerate unless the level of harvest is reduced.
- to avoid population collapse, a large margin of error is needed
- never harvest more than 75% of MSY
- leads to a significantly lower yield than would be possible under other management strategies
Describe harvesting with a constant effort
leads to an offtake that is proportional to population size
Give the advantages to harvesting with a constant effort
- easier to monitor and control
- safer and more efficient
- allows higher yields because no large margins of error are needed
- can be administered without monitoring population size or determining the relationship between population size and the population growth rate
- yield should automatically track population size, with the catch per unit effort decreasing as population size declines
Describe the disadvantages to harvesting with constant effort
- incentive for the hunters to use technological innovation to circumvent regulations.
- undermines the policy by leading to constant pressure to increase effort.
- yields vary from year to year depending on the population size.
- indirect relationship between policy and biology.
- any major violations of the assumption of logistic growth could lead to unexpected population collapse
Give the advantages of harvesting a constant proportion of the population
- clearer relationship between harvesting a constant proportion of the population and the biology of the system than for constant effort: more transparent policy.
- harvesters do not make constant attempts to circumvent the regulations, if they are free to use any technology they please
- economically more efficient than constant effort
Describe the disadvantages of harvesting at a constant proportion
- manager needs to know the population size in order to set the number that can be killed that year
- yields vary from year to year
- any major violations of the assumption of logistic growth could lead to unexpected population collapse
List some common priorities for managers
- stability of yield -> constant numbers
- conservation of the harvested stock -> constant proportion
- size of yield -> constant proportion
- ease of enforcement -> constant effort
- profitability of enterprise -> constant proportion
Profits =
revenues - opportunity costs
- pi = profit from harvesting
- p = price per unit of offtake
- c = cost per unit of effort expended
Describe open access equilibrium
- Ninfinity = c/pq
- Einfinity = r/q(1-c/pqK)
What is Epi
- the sole owner static equilibrium
- lower effort level (and so a higher population size) than Einfinity, and than MSY
Describe MSY
- unsafe biologically because it is a semi-stable equilibrium
- unlikely to be the most profitable rate to harvest at, because of the costs of harvesting
Why is the open access equilibrium unsatisfactory
- biologically over-harvested (if on the descending portion of the revenues curve)
- population might be in danger of sudden collapse and the productivity of the resource is low
- economically overharvested
- profits are dissipated rather than maximised
When is Einfinity hypothetically sustainable
IF the environment is deterministic and the stock-production relationship is smooth
Describe monopoly
- a sole owner has full control of the resource and can set the harvesting level
- likely aim is to maximise profits, and thus to harvest at Epi
Describe open access
- anyone can harvest the resource
- aka perfect competition.
- harvesting is at Einfinity, because as long as there are profits to be made, new harvesters enter the industry
- effort levels stabilise when profits are zero.
