Sustainable yield refers to harvesting renewable resources at a rate that allows the resource population to replenish itself naturally over time. The theory assumes populations have stable growth rates and age structures. However, sustainable yield models have problems, as populations often have many immature individuals and removing breeding stock before immature individuals mature to replace them can reduce populations over time instead of maintaining them.

1. Topic 3: Human population, carrying
capacity and resource use
3.2 : Resources Natural Capital
3.2.7: Sustainable Yield

2. Topic 3: Human population, carrying
capacity and resource use
3.2 : Resources Natural Capital
The extent to which a given interaction with the
environment exploits and utilizes the natural
income without causing long term deterioration
to the natural capital. For example, a system of
harvesting renewable resources at a rate that
will be replaced by natural growth might be
considered to demonstrate sustainability.
A Long Definition

3. Topic 3: Human population, carrying
capacity and resource use
3.2 : Resources Natural Capital
'development which meets the needs
of the present without compromising
the ability of future generations to
meet their own needs'
A Better Definition

4. Topic 3: Human population, carrying
capacity and resource use
3.2 : Resources Natural Capital
Sustainable Yield
Sustainable yield (SY) may be calculated as the rate of
increase in natural capital
Or
The idea that a “crop” can be taken from an ecosystem
in a way that leaves the ecosystem viable for the future

5. Topic 3: Human population, carrying
capacity and resource use
3.2 : Resources Natural Capital
Sustainable Yield
total biomass total biomass
SY = @ time t + 1 - @ time t
energy energy
OR
SY = (annual growth and recruitment) - (annual death and emigration)
The model works on the idea that as long as there are more new
members of a population at the end of the year than at the beging it
is possible to exploit the extra without harming the population

6. Topic 3: Human population, carrying
capacity and resource use
3.2 : Resources Natural Capital
The theory

7. Topic 3: Human population, carrying
capacity and resource use
3.2 : Resources Natural Capital
The theory
26.0
19.5
13.0
6.5
0
2007 2008

8. Topic 3: Human population, carrying
capacity and resource use
3.2 : Resources Natural Capital
The theory
26.0
19.5
13.0
Population in
6.5 the first year
0
2007 2008

9. Topic 3: Human population, carrying
capacity and resource use
3.2 : Resources Natural Capital
The theory
26.0
19.5
13.0
Population in
Population in
the second
6.5 the first year
year
0
2007 2008

10. Topic 3: Human population, carrying
capacity and resource use
3.2 : Resources Natural Capital
The theory
26.0
19.5
13.0
Population in
Population in
the second
6.5 the first year
year
0
2007 2008

11. Topic 3: Human population, carrying
capacity and resource use
3.2 : Resources Natural Capital
The theory
26.0
19.5 “surplus”
13.0
Population in
Population in
the second
6.5 the first year
year
0
2007 2008

12. Topic 3: Human population, carrying
capacity and resource use
3.2 : Resources Natural Capital
The theory
26.0
19.5 sustainable
“surplus”
yield?
13.0
Population in
Population in
the second
6.5 the first year
year
0
2007 2008

13. Topic 3: Human population, carrying
capacity and resource use
3.2 : Resources Natural Capital
Assumptions of Sustainable Yield models
There is enough data to know the populations real size and growth
rate
That the population has an even age structure
Harvesting individuals is random and does not disproportionately
affect the breeding stock
That the population is not affected by natural fluctuations e.g.
outbreaks of disease

14. Topic 3: Human population, carrying
capacity and resource use
3.2 : Resources Natural Capital
Problems with assumptions
In a stock where most individuals are breeding age and grow to
maturity is slow
Year 1 Year 2 Year 3
immature
immature
breeding stock breeding
breeding stock
stock
immature
old and diseased old and
old and diseased
diseased
Stock
harvested

15. Topic 3: Human population, carrying
capacity and resource use
3.2 : Resources Natural Capital
Problems with assumptions
The a lot of immature stock can make the population appear to be
in surplus
Year 1 Year 2 Year 3
immature
immature
breeding stock breeding
breeding stock
stock
immature
old and diseased old and
old and diseased
diseased
Stock
harvested

16. Topic 3: Human population, carrying
capacity and resource use
3.2 : Resources Natural Capital
Problems with assumptions
The old continue to die at the same rate as breeding stock become
old
Year 1 Year 2 Year 3
immature
immature
breeding stock breeding
breeding stock
stock
immature
old and diseased old and
old and diseased
diseased
Stock
harvested

17. Topic 3: Human population, carrying
capacity and resource use
3.2 : Resources Natural Capital
Problems with assumptions
So when harvesting starts it is the breeding stock that is removed
Year 1 Year 2 Year 3
immature
immature
breeding stock breeding
breeding stock
stock
immature
old and diseased old and
old and diseased
diseased
Stock
harvested

18. Topic 3: Human population, carrying
capacity and resource use
3.2 : Resources Natural Capital
Problems with assumptions
And the immature do not grow fast enough to replace the breeding
stock lost
Year 1 Year 2 Year 3
immature
immature
breeding stock breeding
breeding stock
stock
immature
old and diseased old and
old and diseased
diseased
Stock
harvested

19. Topic 3: Human population, carrying
capacity and resource use
3.2 : Resources Natural Capital
The Story of the Orange Roughy

20. Topic 3: Human population, carrying
capacity and resource use
3.2 : Resources Natural Capital
Orange Roughy: The ecology
Habitat: cool deep waters
Slow-growing, very long-lived fish, up to 150 years
Feeds on prawns, fish and squid
Do not breed until about 25 - 30 years old
Lay about 10% less eggs than most fish species

21. Topic 3: Human population, carrying
capacity and resource use
3.2 : Resources Natural Capital
Orange Roughy: Australia
Found in deep water of Southern Australia
Large scale fishing started in the 1990’s
Within a decade shoals reduced to 10% of original
size
Australian Government introduced quotas

22. Topic 3: Human population, carrying
capacity and resource use
3.2 : Resources Natural Capital
Orange Roughy: Australia
30 years ago found large numbers in deep water of
Southern Australia
Large scale fishing started in the 1990’s
Within a decade shoals reduced to 10% of original
size
Australian Government introduced quotas

23. Topic 3: Human population, carrying
capacity and resource use
3.2 : Resources Natural Capital
Orange Roughy: Australia
30 years ago found large numbers in deep
water of Southern Australia
Large scale fishing started in the 1990’s
Within a decade shoals reduced to 10% of
original size
A haul of orange roughy. Photo AFMA Australian Government introduced quotas

24. Topic 3: Human population, carrying
capacity and resource use
3.2 : Resources Natural Capital
Catch (tonnes)
Year

25. Topic 3: Human population, carrying
capacity and resource use
3.2 : Resources Natural Capital
Orange Roughy Catch: Austrailian waters 1990 - 2001
90,000
80,000
Catch (tonnes)
70,000
60,000
50,000
40,000
30,000
20,000
10,000
0
1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001
Year

26. Topic 3: Human population, carrying
capacity and resource use
3.2 : Resources Natural Capital
Orange Roughy: Declining catches
Early population counts were based on fishing records
No account taken of low reproductive rate and long
time to maturity
Management plans have not stopped the decline
Since discovered that Orange Roughy reproduction is
also spasmodic and irregular