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

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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