Sustainable use of wildlife

Question 1

Describe the precautionary objective

Answer 1

• 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

Question 2

Define maximum sustainable yield

Answer 2

• 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

Question 3

Define sustainable level

Answer 3

a level of that stock above biomass levels capable of producing maximum sustainable yield

Question 4

Give the equation for logistic population growth

Answer 4

• dN/dt = rN(1-N/K)
• r = intrinsic rate of increase

Question 5

Describe the parabolic population curve as number of new individuals against population size (% of K)

Answer 5

• 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

Question 6

Describe the relationship between our capita growth rate against population size (% of K)

Answer 6

• linearly decreasing growth rate with population size
• 1-(N/K)

Question 7

Describe the relationship between population size (% of N) against time

Answer 7

• sigmoid
• Nt = K / 1+(K/N-1)e^-n

Question 8

Give some organisms for which logistic growth has been observed

Answer 8

• yeast
• Yellowstone elk

Question 9

Describe the economic fundamentals of harvesting

Answer 9

• populations are capital
• we can harvest the surplus

Question 10

Modify the logistic equation for harvesting

Answer 10

dN/dt = rN(1-N/K) - H

Question 11

H

Answer 11

can be constant number oc constant proportion (aN)

Question 12

When are harvested populations stable

Answer 12

• when they are not growing
• dN/dt = 0
• when the population growth rate = harvest rate

Question 13

Give the equation for stably harvested populations

Answer 13

rN(1-N/K) = H

Question 14

Populations can be sustainably harvested because

Answer 14

• they exhibit a density dependent response
• assumptions about the shape of the density dependence are crucial

Question 15

Describe what happens when a constant number of individuals are harvested

Answer 15

• 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

Question 16

Describe what happens when the number harvested individuals is proportional to population size

Answer 16

• 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

Question 17

Give the equation for harvesting size, harvesting effort and population size

Answer 17

H = qEN
- E = harvesting effort
- q = catchability coefficient

Question 18

Give an expression for yield (H) as a function of effort:

Answer 18

H = qKE(1-qE/R)

Question 19

What are the advantages of harvesting a constant number?

Answer 19

• guarantees the same yield each year
• strategy is very simple to administer

Question 20

Give the disadvantages of harvesting at a constant number

Answer 20

• 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

Question 21

Describe harvesting with a constant effort

Answer 21

leads to an offtake that is proportional to population size

Question 22

Give the advantages to harvesting with a constant effort

Answer 22

• 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

Question 23

Describe the disadvantages to harvesting with constant effort

Answer 23

• 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

Question 24

Give the advantages of harvesting a constant proportion of the population

Answer 24

• 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

Question 25

Describe the disadvantages of harvesting at a constant proportion

Answer 25

• 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

Question 26

List some common priorities for managers

Answer 26

• 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

Question 27

Profits =

Answer 27

revenues - opportunity costs
- pi = profit from harvesting
- p = price per unit of offtake
- c = cost per unit of effort expended

Question 28

Describe open access equilibrium

Answer 28

• Ninfinity = c/pq
• Einfinity = r/q(1-c/pqK)

Question 29

What is Epi

Answer 29

• the sole owner static equilibrium
• lower effort level (and so a higher population size) than Einfinity, and than MSY

Question 30

Describe MSY

Answer 30

• 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

Question 31

Why is the open access equilibrium unsatisfactory

Answer 31

• 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

Question 32

When is Einfinity hypothetically sustainable

Answer 32

IF the environment is deterministic and the stock-production relationship is smooth

Question 33

Describe monopoly

Answer 33

• 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

Question 34

Describe open access

Answer 34

• 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.