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WF4133‐Fisheries Science
Module 3: Harvesting biomassClass 13
Housekeeping
• Now is the time to be looking for summer jobs!– State websites– USA jobs– Fisheries.org– Texas A&M
• No lab today!
HARVESTING BIOMASS CONTINUEDHow do we deal with the issues assuming equilibrium?
Fmsy
0.0 0.2 0.4 0.6 0.8 1.0
0
50
100
150
hrate
Yie
ld
Fishing mortality rate
Fmsy
Fishing mortality rate that maximizes sustained yield
Biological overfishing!
0.0 0.2 0.4 0.6 0.8 1.0
0
50
100
150
hrate
Yie
ld
Fishing mortality rate
Fmsy
Yield DecreasesWith More Effort
Epitaph for MSY
Solution for equilibrium = F0.1The use of F0.1 has emerged as a useful “rule of thumb” for managing fisheries, but according to Hilborn and Walters (1992) this is an arbitrary, ad hoc strategy with no theoretical basis.
OK, so what is it exactly?
Shifting MSY to the left…
0.0 0.2 0.4 0.6 0.8 1.0
0
50
100
150
hrate
Yie
ld
Fishing mortality rate
Shift Fmsy to the left… reduce it
Fmsy
How do we figure out F0.11. Find slope at origin2. Plot line with 10%
of this slope3. Find tangent of
curve at this slope
Slope at origin
0.0 0.2 0.4 0.6 0.8 1.0
0
50
100
150
Fishing mortality
Yie
ld
10% of slope at origin
0.0 0.2 0.4 0.6 0.8 1.0
0
50
100
150
Fishing mortality
Yie
ld
10% of slope at origin
0.0 0.2 0.4 0.6 0.8 1.0
0
50
100
150
Fishing mortality
Yie
ld
F0.1
0.0 0.2 0.4 0.6 0.8 1.0
0
50
100
150
Fishing mortality
Yie
ld F0.1 = 0.28
FMSY = 0.3
F0.1
Continuous harvest assumption
Suppose harvest does not occur continuously…Is this realistic?Examples?
Solution for equilibrium = F0.1The use of F0.1 has emerged as a useful “rule of thumb” for managing fisheries, but according to Hilborn and Walters (1992) this is an arbitrary, ad hoc strategy with no theoretical basis.
Continuous harvest
http://www.mdwfp.com/media/218652/creel_limits_pt._3_chaper_1.pdf
Continuous harvest?
Colvin, M.E., Pierce, C.L., Stewart, T.W., 2012. Semidiscretebiomass dynamic modeling: an improved approach for assessing
fish stock responses to pulsed harvest events. Canadian Journal of Fisheries and Aquatic Sciences 69, 1710‐1721.
Traditional biomass models
• Assumes harvest occurs continuously
• Biomass models guide stock management
• Pulsed harvest?
0 2 4 6 8 10
0
50
100
150
200
250
300
Years
Biom
ass
Continuous harvest
Does assuming continuous harvest make a difference?
Pulsed harvest & biomass dynamics
0 2 4 6 8 10
0
50
100
150
200
250
300
Years
Biom
ass
A Pulsed Harvest System
Pulsed biomass removal
Development of Semi‐Discrete Biomass Dynamics Models (BDMs)
Semi‐discrete models• Hybrid class of models that allow pulsed events in continuous time
• Continuous processes‐ intrinsic growth rate
• Pulsed harvest
0 2 4 6 8 10
0
50
100
150
200
250
300
Years
Biom
ass
Pulsed harvest
0 2 4 6 8 10
0
50
100
150
200
250
300
Years
Biom
ass
Can biomass dynamics models be improved by accounting for pulsed harvests? YES
0 2 4 6 8 10
Years
Pulsed harvestContinuous harvest
Better!
0.00 0.05 0.10 0.15 0.20 0.25 0.30
0
5
10
15
20
25 A: Schaefer
Continuous FMSYSemi-discrete FMSY
ContinousSemi-discrete
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
0
5
10
15
20
25
30 B: Fox
Continuous FMSYSemi-discrete FMSY
0.00 0.05 0.10 0.15 0.20 0.25 0.30
0
5
10
15
20 C: Pella-Tomlinson
Continuous FMSYSemi-discrete FMSY
Equ
ilbriu
m y
ield
(kg
ha1
)
Fishing mortality (kg ha1yr1)
Equilibrium yields
Assuming continuous harvest over estimates MSY!
0
50
100
150
200
250
Biom
ass
(kg/
ha) A: Biomass
468
10121416
Cat
ch p
er u
nit
effo
rt (k
g/tra
wl)
B: CPUE
2007 2008 2009 2010 2011
0
5
10
1520
25
Biom
ass
(kg/
ha)
Year
C: Weekly harvest April-JuneOctober-November
Applying this to carp
Carp biomass dynamics model
Fit to data by maximum likelihood to estimate r & q, given B, I, and C
Estimating standing biomass
Mark Weigh
Recapture
Trawling to index biomass (CPUE) A: 2007
B: 2008
C: 2009
D: 2010
N
Seine locationTraw l location
Tracking carp biomass harvested
When & how much biomass is harvested
0
50
100
150
200
250Bi
omas
s (k
g/ha
) A: Biomass
468
10121416
Cat
ch p
er u
nit
effo
rt (k
g/tra
wl)
B: CPUE
2007 2008 2009 2010 2011
0
5
10
1520
25
Biom
ass
(kg/
ha)
Year
C: Weekly harvest April-JuneOctober-November
100
150
200
250
Bio
mas
s (k
g/ha
)
2007 2008 2009 2010 2011
4
6
8
10
12
14
16
Cat
ch p
er u
nit
effo
rt (C
PU
E: k
g/tra
wl)
Model predicted biomass
Model predicted CPUE
Fittedmodel
The big picture
• Global fisheries estimated to be worth $91.2 billion according to United Nations– 82 billion for saltwater– 10 billion for freshwater– In the round…
• Managing fisheries is serious economic business
• Lots of stakeholders…
Not just about yield
• MSY is tough to attain• Really not interested in catching fish for the sake of catching fish
• Interested in $$$• Linking yield to money• Lets look at Spring Chinook salmon
Spring Chinook life historyRedds & eggs Parr (0+)Hatch Fry (0+)
Smolt (1)
Mini jack (2)
Jack (3)
Adult (4)
Adult (5)
Adult (6)
Willamette Basin Spring Chinook
Fish return ~ February
Pacific Ocean
Commercial fisheries & profits
Spring Chinook Salmon
• Average 25 pounds• 16 to 25 USD per pound in the round• 400 to 625 USD per fish!
– 1000 fish= 40 to 62.5K USD– 10,000 fish = 400 to 625K USD
Why so expensive?
• Life history• Return to freshwater early!
Biomass & harvest dynamics
tt
biomass
economic
K BiomassdBiomass r F Biomassdt K
dYield F Biomassdt
dYield F Biomass Landing pricedt
MSY & economic yield
Link yields to profits• Graham Schaefer
r=0.3K=10,000B0=9,000Landing price=4 usd/kg
Sustained yield
0 100 200 300 400
0
100
200
300
400
500
600
Effort (hours)
Yie
ld (t
ons)
Economic yield
0 100 200 300 400
0.0
0.5
1.0
1.5
2.0
Effort
Pro
fit (x
1000
)
Same as MSY but for profit
No free lunch
• Harvest of fish requires effort!• Effort cost something… money, person hours
Fisheries & effort
Incorporating effort
Effort
Gear Effort (days)
Totalcatch
Catch per effort
Gill nets 55 6600 120
Need to link effort and yieldExample if I fish for 4 hours how much fish will I catch?
Catchability
6600 55catchability Biomass
catchCatch ef
abilityfo
Biomassrt
Just need to know biomass….Or catchability
Estimating biomass & catchability
• Mark recapture• Calibrated fishery independent surveys• Catchability is much more difficult
– Function of biomass–Need catch, biomass, and effort
Biomass & harvest dynamics
tt
biomasst
economict
K BiomassdBiomass r catchability Biomassdt K
dYield ca
eff
tchability Biomassdt
dYield catchability Biomass Landing price
ort
effort
effortdt
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