BIO220 Lecture 16

Question 1

General harvesting stragety

Answer 1

harvest out the recruitment and leave the stock alone

Question 2

recruitment

Answer 2

new additions to the population

Question 3

stock

Answer 3

current population

Question 4

logistic equation of population growth

Answer 4

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

Question 5

What is the logistic equation of population growth saying?

Answer 5

As number of individuals in population nears the carrying capacity of that population (N = K), rate of population growth is slow (logistic growth)

Question 6

What happens when N -> 0 (logistic population growth equation)

Answer 6

dN/dt = rN

= exponential growth

Question 7

____ growth rate can limit population size

Answer 7

density-dependent

Question 8

example of density-dependent population

Answer 8

Tern (type of seagull) on islands is limited by nesting space

• Bird island
• Ram island
• Penikese island

Question 9

logistic curves can be used to predict…

Answer 9

harvesting & its consequences

- what would give us the best yield without harvesting the stock?

Question 10

Ideal harvesting strategy would be to harvest ____ individuals per year. This ensures ____

Answer 10

K/2;

Max growth rate

Question 11

calculation: overall rate of population increase

Answer 11

per capita rate of increase x population size = overall rate of population increase

Question 12

How do we know that we should harvest K/2 individuals per year?

Answer 12

inflection point on “overall rate of population increase”

= max rate of population increase is at K/2

Question 13

Recruitment axis = ___ axis

Answer 13

harvest amount

Question 14

A “best case scenario” for harvesting does not account for…

Answer 14

• random chance
• weather
• seasonal year-to-year fluctuations
• predation

Question 15

MEY

Answer 15

max economic yield

= max profit (benefit - cost)

Question 16

MEY is good for business // species populations?

Answer 16

business & fishermen

Question 17

MSY

Answer 17

max sustained yield
= point of max population growth
= K/2

Question 18

MSY is good for business // species populations?

Answer 18

species population

Question 19

The fisheries tend to fish to the ___ point. Why?

Answer 19

break even point

• incentive to catch more fish until benefit doesn’t > cost
• fish as long as there is positive profit

Question 20

Cons of the break even point

Answer 20

• not max profit
• populations have low growth rates
• high effort
• more harvest short-term, but screwed long-term

Question 21

why do fisheries harvest to the break even point, not MSY or MEY?

Answer 21

constant financial pressure to harvest more

Question 22

Effect of new technologies on fishing curve

Answer 22

• decrease slope of cost curve

- no effect on population dynamics curve

Question 23

what can be found on the fishing curve?

Answer 23

1. population dynamics (“n” parabola)

2. cost of harvesting (“/” slope)

Question 24

3 methods to improve the overfishing situtation

Answer 24

1. fixed quota (set at MSY)
2. fixed effort (time, size of net…)
3. fixed escapement (allow some fish to escape fishery before fishing begins)

Question 25

Problems with fixed quota harvesting strategy

Answer 25

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

Question 26

requirements for using fixed effort harvesting strategy

Answer 26

define “effort” strictly

Question 27

Problem with fixed effort harvesting strategy

Answer 27

• 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

Question 28

Rachet effect

Answer 28

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

Question 29

What is the ideal harvesting curve?

Answer 29

Set so that break-even point (B=C) is @ MSY

- fisheries will harvest to MSY

Question 30

What does the fishing curve look like when there is over-harvesting?

Answer 30

When cost curve is very steep

Question 31

What does the fishing curve look like when there is under-harvesting?

Answer 31

Cost curve is very flat

Question 32

What further complicates fixed effort harvesting strategy?

Answer 32

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

Question 33

What is the best solution to overfishing (currently)?

Answer 33

Fixed escapement

Question 34

fixed escapement works best with…

Answer 34

salmon & migratory fisheries

Question 35

Benefit of fixed escapement

Answer 35

ensures that a fraction of the population escapes into the wild to spawn every generation

Question 36

Cons of fixed escapement

Answer 36

• expensive & complicated

- not useful for non-migratory fish