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WF4133‐Fisheries Science
Module 3: Harvesting biomassClass 15
Housekeeping
• Now is the time to be looking for summer jobs!– State websites– USA jobs– Fisheries.org– Texas A&M
• Half way there
Lab today @ 1pm3/18Group 1Group 2
4/1Group 3Group 4
Lab today @ 1pm
• Bring– Rain gear– Sun screen– Bug repellent– Waders/boots
BIOLOGICAL OVERFISHING
Reduced yield with increasing fishing mortalityA combination of growth and recruit overfishing
Biological overfishing
0.0 0.2 0.4 0.6 0.8 1.0
0
50
100
150
Fishing mortality
Yie
ld
Biological overfishing
Combination of growth & recruit overfishing
GROWTH OVERFISHING
Occurs when fish are harvested at an average size that is smaller than the size that would produce the maximum yield per recruit
Growth overfishing
Harvest fish before they have time to growGrowth is a function of age, therefore we need to account for age structure
Example:
Growth process in fish
The assimilation of food as biomass (i.e., tissue). Primarily refers to somatic tissue but also includes gonad tissue.
• Fish adding weight over time1. Relate time (age) to length2. Relate length to weight
0 5 10 15 20 25 30
0.0
0.1
0.2
0.3
0.4
Total length (cm)
Wei
ght (
kg)
Length‐weight
3.020.0141 LengthWeight
Weight for an age‐3 fish
0 1 2 3 4 5 6 7
5
10
15
20
25
30
Age
Tota
l len
gth
(cm
)
23.9 cm3.02
3.02
0.01410.014 4
821
20
WeightWeightWeig
Lengt
t
h
h
Weight for an age‐6 fish
0 1 2 3 4 5 6 7
5
10
15
20
25
30
Age
Tota
l len
gth
(cm
)
3.02
3.02
0.01410.014 9
721
36
WeightWeightWeig
Lengt
t
h
h
28.6 cm
Age‐weight
0 1 2 3 4 5 6 7
0.0
0.1
0.2
0.3
Age
Wei
ght l
engt
h (k
g)
23.9 cm
28.6 cm
Age & timeYears
Age‐7
Age‐6
Age‐4
Age‐5
Age‐1
Age‐2
Age‐3 Age‐3
Age‐7
Age‐6
Age‐5
Age‐2
Age‐4
Age‐1
Age‐6
Age‐5
Age‐4
Age‐3
Age‐1
Age‐2
Age‐7
Age‐5
Age‐4
Age‐3
Age‐1
Age‐2
Age‐6
Age‐7
Age‐4
Age‐3
Age‐1
Age‐2
Age‐5
Age‐6
Age‐7
Age‐3
Age‐1
Age‐2
Age‐4
Age‐5
Age‐6
Age‐7
Age‐1
Age‐2
Age‐3
Age‐4
Age‐5
Age‐6
Age‐7
Age‐1
Age‐2
Age‐3
Age‐4
Age‐5
Age‐6
Age‐7
Age & timeYears
Age‐7
Age‐6
Age‐4
Age‐5
Age‐1
Age‐2
Age‐3 Age‐3
Age‐7
Age‐6
Age‐5
Age‐2
Age‐4
Age‐1
Age‐6
Age‐5
Age‐4
Age‐3
Age‐1
Age‐2
Age‐7
Age‐5
Age‐4
Age‐3
Age‐1
Age‐2
Age‐6
Age‐7
Age‐4
Age‐3
Age‐1
Age‐2
Age‐5
Age‐6
Age‐7
Age‐3
Age‐1
Age‐2
Age‐4
Age‐5
Age‐6
Age‐7
Age‐1
Age‐2
Age‐3
Age‐4
Age‐5
Age‐6
Age‐7
Age‐1
Age‐2
Age‐3
Age‐4
Age‐5
Age‐6
Age‐7
Survival & growth
Harvest process in fish
0 1 2 3 4 5 6 7
020
040
060
080
010
00
Age
Abu
ndan
ce
No harvest
Harvest process in fish
0 1 2 3 4 5 6 7
020
040
060
080
010
00
Age
Abu
ndan
ce
No harvestHarvest of 4+ fish
Harvest process in fish
0 1 2 3 4 5 6 7
020
040
060
080
010
00
Age
Abu
ndan
ce
No harvestHarvest of 4+ fishHarvest of 3+ fish
Harvest process in fish
0 1 2 3 4 5 6 7
020
040
060
080
010
00
Age
Abu
ndan
ce
No harvestHarvest of 4+ fishHarvest of 3+ fishHarvest of 2+ fish
Biomass
0 1 2 3 4 5 6 7
0.0
0.1
0.2
0.3
Wei
ght l
engt
h (k
g)
0 1 2 3 4 5 6 7
020
040
060
080
010
00
Age
Abun
danc
e
Combine the 2 to get yield
Yield
0 1 2 3 4 5 6 7
010
2030
4050
60
Age
Abu
ndan
ce
Area under the curve is total yield
Trade off
1. Harvesting a lot of fish2. Harvesting fewer, but larger fish
Lets look at this
Harvest related to size
http://www.mdwfp.com/media/218652/creel_limits_pt._3_chaper_1.pdf
Flip the VBGF
Recall, the VBGF predicts length at age
Can rearrange equation to predict age given length
0( )(1 )K age tageLength Length e
Proof0
0
0
0
(
(
( )
(
)
)
)
0
)
)
1
1
lo
(1
(1
(g 1 )
log 1
age
age
age
age
age
age
K age t
K age t
K age t
K age t
Length Length
LengthLength
LengthLength
LengthLength
LengthLength
Len
e
e
e
e
K
gthLength
age t
K
0
0
( )
log 1 ageLengthLe
age t
t agK
ngthe
Example
Proposed minimumlength limit1. 8 inches (203 mm)2. 12 inches (304 mm)3. 14 inches (356 mm)4. 15 inches (381 mm)
0
4000.30.1
LengthKt
8 inch limit
0
log 1
203log 1400.1
0.3
0.10.3
0.
0
3.05
10.2
1 16.46
ageLength
t ageK
age
age
ageage
Length
12 inch limit
0
log 1
304log 1400
1.427
4.7574.
0.10.3
0
8
.10.3
0.157
age
t ag
L
eK
age
a
engthLength
ge
ageage
Yield per recruit model!
Diagnosing growth overfishing
Growth overfishing
No growth overfishing
Diagnosing growth overfishing
Growth overfishing
No growth overfishing
ASSESSING OVERFISHING?
Is there a way to evaluate the effects of fishing on a population?
Total mortality (Z)
Z = F + MWhere,
F = Fishing mortalityM = Natural mortality
0 1 2 3 4 5 6 7
020
040
060
080
010
00
Age
Abu
ndan
ce
Harvest process in fish
Fewer older fish with harvest.
![Overfishing karina nina_ivoslav_10-7[1]](https://img.pdfslide.us/doc/110x75/554c1892b4c905f1518b4fa8/overfishing-karina-ninaivoslav10-71.jpg)
