1 Fish Diet, Condition, and Growth FW 479 Feb. 23 and 25, 2004 • Condition : What does it mean? How do we measure if a fish is “healthy”? • Growth : What are typical patterns?, How is growth measured? • Diet : How is it analyzed? What can it tell us?
FW 479 Feb. 23 and 25, 2004. Fish Diet, Condition, and Growth. Condition : What does it mean? How do we measure if a fish is “healthy”? Growth : What are typical patterns?, How is growth measured? Diet : How is it analyzed? What can it tell us?. Mgt: Angler Surveys. - PowerPoint PPT Presentation
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Fish Diet, Condition, and Growth
FW 479Feb. 23 and 25, 2004
• Condition: What does it mean? How do we
measure if a fish is “healthy”?
• Growth: What are typical patterns?, How is
growth measured?
• Diet: How is it analyzed? What can it tell us?
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Foodweb Effects of Fish & Non-harvest Values•Effects on competitors, prey, and predators•Transfer of nutrients and contaminants
Mgt: Angler Surveys
Mgt: Stocking, Aquaculture, Habitat
Mgt: Regulations
Fish•Abundance•Size comp.•Age comp.•Behavior•Production•Habitat use
Weight typically increases as approx. the cube of length
Weight = aLengthb
Log(weight) = log(a) + b*log(length)
Caveats: e.g., water versus lipids as weight
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What are typical patterns of fish growth?
Age AgeGrowth, though indeterminate, often slows after the onset of reproduction.
One way to assess growth is to compare mean size at age.
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Measuring Growth: von Bertalanffy
Age
Based on….. tlLKdt
dl
If we integrate, this becomes…. 01 ttK
t eLl
von Bertalanffy growth analysis can be conducted with size at age, or mean size at age, data.
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Measuring Growth: Back Calculations
Fraser Lee Method
Scale radius
Fis
h T
L
TL when scales start
growing
TL at capture
Scale radius at capture
annulus
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Fraser Lee Back-Calculations
Scale radius
Fis
h T
L
TL when scales start growing
TL at capture
Scale diameter at capture
1st annulus
2nd
Back-calculated TL at 1st annulus
Back-calculated TL at 2nd annulus
annulus
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“Typical” Pattern of Growth Rates
An
nu
al G
row
t h
Incr
emen
t ( m
m)
Back-calculated TL (mm)
Note, this relates to the von B:
tlLKdt
dl
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Growth Rates: Gizzard Shad example
Back-Calculated TL (mm)50 100 150 200A
nn
ual
Gro
wth
In
crem
ent
(mm
)
0
20
40
60
80
100
120
140
25
TL when scales start growing
growth increment during 1st year of life
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Comparing Gizzard Shad Growth Rates
Gro
wth
In
crem
ent
(mm
)
Back-calculated TL (mm)50 100 150 200
0
20
40
60
80
100
120
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http://biology.usgs.gov/wfrc/cook.web/estuary.htm
Why do fisheries managers look at so many fish guts?
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Diet-related Questions
•What are fish eating?•What’s the best food for fish?•How much are fish eating? Is there enough food?
•How are fish affecting other food web components?
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Reason: •general characterization of diet •mainly qualitative in terms of amount eaten
Data required: •gut contents (number and taxa)
Limitations:•relative comparisons of amount of food in gut
can be made, but conclusions regarding absolute consumption are tenuous
•typically a first step
Fish Diet Composition
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Analysis:•% contribution of prey taxa (or size classes)
•may be expressed as % by number, by weight, or by calories
Fish Diet Composition
•IRI = index of relative importance(% by # + % by volume) * (% freq. of occurrence)
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Analysis:•Stable isotopes
Fish Diet Composition
From
Van
der
Zan
den
et
al. 1
99
7 C
JFA
S 5
4:1
14
2-1
15
8.
Trophic position = ((fish δ15N – mussel δ15N)/3.4) + 2
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Stable isotopes
Fish Diet Composition
From Post 2002 TREE 17(6):269-277
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Feeding Selectivity Indices
Reason: •determine what prey types (taxa or size
classes) are most important•assumption is that preferred prey are important or beneficial to consumer
Data required:•gut contents AND availability in envt
Analysis: One example is Chesson’s alpha:ri = proportion of prey item i in dietpi = proportion of prey item i in environment n = total number of prey types1/n = neutral selection