Exams
Preview – second half
Today: Foraging and Diets
Next Week: Exotics and 2nd half of fish ids
plus quiz on Moyle and Light 1996
plus first draft of papers are DUE for P.Rev.
Spring Breek
1st week back: Bioenergetics Lab
Peer review due
Quiz on bioenergetics paper (Kitchell 1977)
2nd week back: Kitchell’s Cabin field trip
Foraging and Diets
Why Collect Diets?
1.
2.
3.
4.
5.
Food webs are a big part of ecology
Fish’s energy: growth and reproduction
Aquaculture: assess stock foraging
Resource managers: stocking, habitat assessment
Environment: indicate change in habitat, population
densities
Group discussion questions
What can parameters about a fish can we
measure that relate to feeding?
What environmental (abiotic) factors
influence fish feeding?
What biotic factors influence fish feeding?
Discussion
What parameters about a fish can we measure that
relate to feeding?
Lists:
Discussion
What parameters about a fish can we measure that
relate to feeding?
Lists:
-At distance react to food
-measure stomach contents (counts, weight, volumetric)
-Gape
-how long it takes for a fish to swallow prey
-rates, number per time
- attempts per diet item
- angle of attack
- are there other fish around
Discussion
What environmental factors influence fish feeding?
Lists:
Lists:
Discussion
What environmental factors influence fish feeding?
Lists:
pH
temp
Time of day
season
light level
DO
Substrate
Fishing pressure
Lists:
pollution
reproductive cycle
nest guarding
Flow velocity
turbidity
Physical Habitat
Salinity
Discussion
What biotic factors influence fish feeding
Lists:
Lists:
Discussion
What biotic factors influence fish feeding
Lists: Prey abundance
Lists:
Size of prey
Community Composition
Size of predator
Competition
Gill rakers, mouth placement, and other morphology
Indirect competition (common predator)
Level of production
Specialist or generalist or omnivore
Experience
Maturity
Gender
Vegetation
Example:
General factor: Turbidity
You are both
crazy, think
about when it
gets foggy,
you can't see
crap, little
increases in
turbidity have
a way bigger
effect, but
then once it
gets cloudy
fish just use
other senses!
prey...huh?
Distance
to prey
reacts
fish sees
till fish
Distance till
Assumptions?: For all visual feeders
I think the darker the water gets the harder
it will be to find food, and the relationship
should be linear!
No way, fish
don't care if
it gets a
little muddy,
up to a point,
then they
can't see
anything!
Water Clarity or Turbidity
Holling’s Disc Equation
Rate of Energy Gained = (λe – s)/(1 +λh)
C.S. “Buzz” Holling
Holling, C. S. 1959. The components of predation as revealed by a study of small mammal predation of the European
pine sawfly. Canadian Entomologist 91:293–320.
Sometimes called the disk equation cause this is
how he originally developed the model
Holling’s Disc Equation
Rate of Energy Gained = (λe – s)/(1 +λh)
λ = rate of encounter with diet item
e = energy gained per encounter
s = cost of search per unit time
h = average handling time
C.S. “Buzz” Holling
Search
Encounter
Pursuit
Capture
Handling
Holling, C. S. 1959. The components of predation as revealed by a study of small mammal predation of the European
pine sawfly. Canadian Entomologist 91:293–320.
Holling’s Observations
Predation rates ↑ with ↑ prey densities happens
due to 2 effects:
1. Functional response by predator
-Type 1
-Type 2
-Type 3
2. Numerical response by predator
-Reproduction
-Aggregation
Functional Response
Type I passive predators
Functional Response
Type II Handling time limited
Functional Response
Type III Learned response
Functional Response
Functional response = same # of predators in area; behavioral
change
Numerical Response
↑ predation due to ↑ predators
•
Two Potential Mechanisms
1. Reproduction
↑ prey density = ↑ consumption = ↑ predator
reproduction = ↑ rate of consumption = etc.
2. Attraction of predators to prey aggregations
("aggregational response")
Numerical Response
+
=
Increased Reproduction
+
=
Numerical Response
• Aggregational Response
Numerical Response
• Hollings equation relates diet information
to energy and time spent foraging
• More specific physiological energetic
needs can be described using
Bioenergetics
Organisms are not chemicals!
Ecological interactions are highly organized
Reaction vat model
Prey
eaten
Prey
eaten
Predator handling
limits rate
Prey density
Slide from Villy
Christensen, IncoFish
Workshop 2006
Foraging arena model
Prey behavior
limits rate
Prey density
Big effects from small changes in space/time scale
Foraging Arena Theory
“There is a horrific linkage between getting food
and being food, and this creates a severe tradeoff relationship” – Walters and Martell
Chalk board!
Hiding places – under a rock, water too shallow for preds, dark profundal zone, in a
school – but low food
Arena – high food, little protection.
How we do it
Collecting Fish
Long term gill net, fyke net, minnow trap
Active sampling techniques (seine, short term gill nets,
angling, shocking)
• Beware of biases
-postcapture digestion
-regurgitation (stressed fish)
-atypical foraging behavior in traps
Collecting Diets
• Collect diets by:
1. Gastric Lavage
2. Stomach Removal
-Remember fish size, population
density
Experimental Strategies
1.
2.
3.
4.
Diel patterns (predators and prey)
Seasonal patterns (predators and prey)
Fish size/gender
Digestion rates
-slow = over represented (mouse bones)
-fast = under represented (earthworms)
-correct for these by determining gut passage
time for each diet item
Identifying Diet Items
• Categorize diet items
• What is the question you are asking?
-More specific taxonomic keying is
more information but could be wasted
time
• Broken items: count body parts (# of
heads)
• Sub-sample small diet items
Enumerating the Diet
• The “Big 3”
1. Frequency of occurrence
2. % composition by number
3. % composition by weight
• Diet Indicies
Frequency of Occurrence
• Percent of individual diets that contain one
or more of a specific diet item
• Presence/absence indicator
- Example: 12/15 walleye diets contain
crayfish, frequency of occurrence = .8 =
80%
• High frequency of occurrence ≠
energetically important, rather selectivity of
a group of individuals
% Composition by Number
• The number of an individual diet item
relative to the total number of items in the
diet/diets
-Example 1: Brown trout #1:
Amphipod
=3
Fantail darter = 1
Amphipod % composition by
number = ¾ = .75 = 75%
% Composition by Number
Brown trout
#1
Brown trout
#2
-Example 2: How to calculate
WRONG: 3/11 midges = 0.272
RIGHT: (0.25+0.25+0.33)/3 = 0.276
Brown trout
#3
% Composition by Weight
•
Weight of one type of diet item relative to the
total diet weight
1.Wet weight: quicker to obtain
2.Dry weight: more energetically informative
•
Can be calculated similarly to examples
shown for % composition by number
Diet Indicies
• Index of Relative Importance (IRI)
IRI = (% number + % weight)(FO)
• Consistency
• Overlap
• Selectivity
*all of these are arbitrary units!
How does a fish decide what to eat?
Electivity: what's the proportion of
item an in the environment compared to
the proportion in the stomach?
If an items is rare in the environment but
prevalent in diets, it is selected for.
How do we measure that? Must also
measure invertebrates
Trichoptera (Caddisfly)
Ephemeroptera (Mayfly)
Diptera (Flies)
Non-biting Midges
Black Flies
Crane Flies
Amphipods
Isopod