Diets

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Diets
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Why are diets important?What information do they
tell us?What questions do they answer?

• Fishs view
• Source of energy (bioen)
• Managers view
• Stocking
• Aquaculture
• Nutrients
• As an indicator of environmental change
• Nutrient loading, change in pop density

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How do we get diets?

• Fish collection
• what should we be cautious of? Sources of error/
  bias due to methods selection?
• Regurgitation
• Post capture digestion
• Alteration in behavior due to capture
• Traps
• Did you use bait?

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Sampling strategies
Things to consider/keep in mind

• Diel effects
• Seasonal effects
• Both of the above affect predator and prey
  behavior
• Fish size
• Ontogenetic shifts? What age/size class is your
  diet representing?

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Sampling strategies cont
Do stomach contents accurately depict the fishs
diet?

• Digestion rates
• Too fast - protozoans
• Under representation in diets
• Could be determined by watching behavior in a
  tank
• Too slow
• Over representation in diets
• Could be solved by determining gut passage times

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Removal of gut contents

• Puking
• Dissection

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Identification of Diet Components

• Crushed/digested organism
• Find characteristic structure for each organism
• Level of identification depends on the research
  question
• Higher resolutiongreater time investment

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Quantitative diet description

• Frequency of Occurrence
• Percent composition by number
• Percent composition by weight

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Frequency occurrence

• What proportion of the diets contained one or
  more of a given food type
• Describes presence absence
• Example 18/22 bluegill contain chironomid,
  frequency of occurrence .82 or 82
• Selective or opportunistic feeding behavior is
  reflected in lower frequency occurrence

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Frequency of occurrenceDrawbacks

• High frequency of occurrence may not mean this
  diet item is of nutritional importance, only that
  it is consumed with some regularity
• Example benthic fish and algae
• Describes the uniformity with which fish feed,
  not importance of diet items

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Percent Composition by number

• Number of food items in a group relative to total
  number of diet items consumed ()
• Potential for fragmentation, count a
  characteristic part of prey item
• Dragonflies have four wings, so 4 wings 1
  dragonfly
• Percent composition by number can be used
  together with estimates of feeding rates to
  assess the effects of predators on prey
  population dynamics

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Percent Composition by Weight

• Weight of each type as a percentage of the total
  weight of the diet
• Wet and dry weights can be used
• Dry are more precise and offer more information
  about nutritional value
• Remember some component of the diets have already
  been digested!!!!
• This is the only method (described here) that
  begins to identify food importance in fish
  nutrition

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Further Analysis and Interpretation

• Selectivity indices Do fish feed at random or
  preferentially?
• Relative abundance of prey type in diet to the
  relative abundance of prey type in the
  environment
• Requires data on relative prey abundance in the
  environment
• Diet overlap indices
• When comparing different species in a community
  may want to know how much two species overlap
• Extent to which species use the same food source

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Laboratory

• Groups of two/three
• work up 1-2 diets
• Fill out data sheet including weight of each diet
  item
• Combine data with other groups
• Analyze the diets
• Freq occ, comp by number, comp by weight