Download - Size Structure
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Size Structure
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Learning Objectives
• Construct and interpret length-frequency distributions
• Describe, calculate, and interpret Swingle’s ratios• Describe the development and interpretation of
standardized length categories• Calculate and interpret PSS indices• Describe differences associated with using
traditional and incremental size-structure indices• Identify the dynamic rate functions and relations
with size structure indices
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Size Structure
• Methods of measurement of fish population structure
• Calculation of indices• Interpretation of structural indices
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Fish Length and Weight
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Considerations
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Indices
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Homer Swingle
• Faculty member at Auburn University
• One of the first to use experimental ponds to obtain insight on management
• Interested in “balanced” fish populations in ponds
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Balance
• “The interrelationships in fish populations are satisfactory if the populations yield, year after year, crops of harvestable fish that are satisfactory in amount when the basic fertilities of the bodies of water containing those populations are considered. Such populations are considered to be ‘balanced populations’ and the species within such a population are ‘in balance.’”
• “Balance then denotes a condition within a population such that if 100 pounds of fish are harvested one year the correct numbers of replacements will be provided from the population so that a satisfactory poundage of fish of desirable size may be harvested in succeeding years. If the population provides too many replacements, these fish will not reach a satisfactory size for harvesting; conversely, if too few replacements are provided, the capacity of the body of water to produce will not be fully utilized and the harvestable poundage will seriously decline.”
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Swingle’s F/C Ratio
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Swingle’s F/C Ratio
• F = • C =
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Swingle’s F/C Ratio
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Swingle’s F/C Ratio
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Swingle’s Y/C Ratio
• Y = • C =
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Swingle’s Y/C Ratio
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Swingle’s Y/C Ratio
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Swingle’s At
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Swingle’s At
Species Minimum weightBLG, RESF, and similar sunfishes 0.1Crappies 0.26LMB 0.4Bullheads 0.3GZS 0.5CHCF 0.5Gar 1.0Buffalo 1.0Carp 1.0
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Swingle’s At
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Swingle’s At
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Swingle’s At
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Swingle’s At
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Swingle’s E
• Swingle’s E =
• Lower bound of balanced with 1 “C” and 1 “F” species is 1.4:1 (BLG:LMB). Therefore, there is 1 pound of LMB for every 2.4 lbs of fish (100 × 1 / 2.4 = 41.6%)– LMB (balanced) =– LMB (desired)=
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Other Swingle Indices
• A value =
• I value =
• S value =
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Jenkins and Morais Metric
• AP/P ratio– AP = – P =
– Plotted on a log10 vs log10 scale
– Curve should be above the 1:1 line to have sufficient prey for predators
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Jenkins and Morais Metric
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Swingle Ratios and Similar Indices
• Potential problems and practicality???
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Length-Frequency Histograms
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Guidelines
• ROT…sample 100 fish > stock-length
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Guidelines
• Y-axis
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Guidelines
• X-axis are bins• “bin bias”
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Bin Bias
Length (mm)
0 100 200 300 400
Rel
ativ
e fr
eque
ncy
(%)
0.00
0.05
0.10
0.15
0.20
0.25
0.30
Length (mm)
0 100 200 300 400
Rel
ativ
e fr
eque
ncy
(%)
0.00
0.02
0.04
0.06
0.08
Length (mm)
0 100 200 300 400
Rel
ativ
e fr
eque
ncy
(%)
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
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Length-Frequency Indices
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Length Categorization
Weithman and Anderson (1978) Gabelhouse (1984)
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Length Categorization
• Stock length =
• Quality length =
• Preferred length =
• Memorable length =
• Trophy length =
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Length Categorization
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Proportional Stock Density (PSD)
• Proportion of stock-length fish that are quality length or greater
• PSD = 100 × (# of fish > minimum quality length / # of fish > minimum stock length)
• Round to nearest whole number!
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PSD-WAE Example
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PSD-WAE Example
Length (mm)
200 300 400 500 600 700 800
Rel
ativ
e fr
eque
ncy
(%)
0
2
4
6
8
10
12
14
16
18
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PSD
200 300 400 500 600 700 800
Num
ber
0
2
4
6
8
Length (mm)
200 300 400 500 600 700 800
Num
ber
0
2
4
6
8
S Q
S = 29Q = 17
S = 29Q = 17
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RSDs
• RSD = 100 × (# of fish > specified length / # of fish > stock length)
• Round to nearest whole number!• Specified length (e.g., RSD-35)• Standard length categories
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RSD-WAE Example
Substock (< 250 mm) = 7Stock (250 mm) = 29Quality (380 mm) = 17Preferred (510 mm) = 10Memorable (630 mm) = 6Trophy (760 mm) = 0
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200 300 400 500 600 700 800
Num
ber
0
2
4
6
8
Length (mm)
200 300 400 500 600 700 800
Num
ber
0
2
4
6
8
S Q
PSD = 59RSD-P = RSD-M = RSD-370 =
PSD = 59RSD-P = RSD-M = RSD-370 =
P M T
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Traditional versus Incremental RSDs
• Traditional RSDs• Incremental
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RSD-WAE Example
Length (mm)
200 300 400 500 600 700 800
Rel
ativ
e fr
eque
ncy
(%)
0
2
4
6
8
10
12
14
16
18
S Q P M T
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RSD-WAE Example
SS =S-Q = Q-P =P-M =M-T =T =
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RSD-WAE Example
Length (mm)
200 300 400 500 600 700 800
Rel
ativ
e fr
eque
ncy
(%)
0
2
4
6
8
10
12
14
16
18
S Q P M T
SQ QP PM MT TSS
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Traditional versus Incremental RSDs
• Incremental RSDs
• Traditional RSDs
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Proportional Size Structure (PSS)
• Confusion in terminology with Proportional Stock Density and Relative Stock Density
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Proportional Size Structure
Current NewPSD PSSQ
RSD-Q PSSQ
RSD-P PSSP
RSD-M PSSM
RSD-T PSST
RSD S-Q PSSSQ
RSD Q-P PSSQP
RSD P-M PSSPM
RSD M-T PSSMT
Terminology
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Size Structure
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Size Structure
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Size Structure
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Biases
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Biases
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Biases
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Balance
• Balanced populations have predictable PSD (or PSSQ)
• Examples– BLG – Crappie – LMB
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Balance
AT, but with biomass
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Additional insight
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Size Structure Indices
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Size Structure Indices
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Size Structure Indices
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“The Classic Story of BLG and LMB PSDs”
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What about other species?
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What about other species?