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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
Fishers/Anglers•Abundance•Behavior•Attitudes
Fishery•Catch rates•Harvest•Profit/economics•Satisfaction
Reproduction
Growth Rates
Mortality Rates
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Fish Condition
Condition Factor (Fulton’s K):
weightlength3
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
i
in
i
i
i
pr
pr
1
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Feeding Selectivity Indices (cont.)
Analysis (cont.):
Envt Gut Selectivityrare abundant positive, alpha > 1/nabundant rare negative (avoidance), alpha
< 1/nrare rare neutral, alpha = 1/nabundant abundant neutral, alpha = 1/n
Limitations: •can be difficult to interpret without other
information•environment sampling must reflect encounter
rate
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Feeding Selectivity Indices: an example
Bremigan 1992
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Functional Response CurvesReason: • determine how foraging behavior and consumption
vary as a function of prey density• implications for predator and prey• an important component of predicting total
consumption of prey
Data required: number of prey in gut and density of prey in environment
Analysis:
# co
nsum
ed
Prey density in environment
Type 1 Type 3Type 2
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Function Response Curves (cont.)
Limitation: •requires a lot of data•easier to quantify in experiments - but how
well does it translate to “real world” settings?
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Questions
If two fish eat the same amount of food, will they necessarily grow the same amount?
If two fish grew the same amount, did they necessarily eat the same amount?
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Bioenergetics
Consumption = Metabolism + Wastes + Growth
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Bioenergetics
•Typically, actual consumption rate is less than Cmax.
•p = a parameter in the model = proportion of maximum consumption.
•At maximum consumption, p=1.•The p value provides a relative measure of
consumption that can be compared across different temperatures, etc.
