Background to Foraging

C.S. Buzz Hollingaka "the man"

Functional response was developed based on a 1959 paper

These eating These

Sometimes called the disk equation cause this is how he originally developed the model

Holling Processes of Predation

– Search

– Encounter

– Pursuit

– Capture

– Handling

Holling’s Disk Equation

Rate of Energy Gained = (λe – s)/(1 + λh)

λ = rate of encounter with diet iteme = energy gained per encounters = cost of search per unit timeh = average handling time

Energy gained per time invested per item

Functional Response

Energy gained per time invested per item

Functional Response

Type I passive predators (spiders)

Type II most typical: predator saturation (Daphnia).

Type III Learning – occurs in predators which increase their search activity with increasing prey density (bass/goby)

Numerical ResponsePredators increase in abundance as prey density

increases.

Two Potential Mechanisms

1. Increased rate of predator reproduction when prey are abundant

2. Attraction of predators to prey aggregations ("aggregational response")

Diets

Goals – Explained in lecture, applied in lab

• Why are diets important?

• How do we get diet data?

• What do we do with diet data?

Why are diets important?What information do they tell us?What questions do they answer?• Fish’s view

– Source of energy (bioen)

• Managers view– Stocking

• Aquaculture– Nutrients

• As an indicator of environmental change– Nutrient loading, change in pop density

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?

Sampling strategies

• 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?

Things to consider/keep in mind

Sampling strategies cont…

• 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

Do stomach contents accurately depict the fish’s diet?

Removal of gut contents

• Puking• Dissection

Identification of Diet Components

• Crushed/digested organism– Find characteristic structure for each

organism

• Level of identification depends on the research question– Higher resolution=greater time

investment

Quantitative diet description

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

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%

• lower frequency occurrence = selective or opportunistic feeding

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

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 + estimates of feeding rates = effects of predators on prey

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!!!!

• begins to identify food importance in fish nutrition

Further Analysis and Interpretation• Selectivity indices: Do fish feed at

random or preferentially?

• Diet overlap indices

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

Amphipods

Isopod