Presentation on theme: "Exams. Preview – second half Today: Foraging and Diets Next Week: Exotics and 2 nd half of fish ids plus quiz on Moyle and Light 1996 plus first draft."— Presentation transcript:
Preview – second half Today: Foraging and Diets Next Week: Exotics and 2 nd half of fish ids plus quiz on Moyle and Light 1996 plus first draft of papers are DUE for P.Rev. Spring Breek 1 st week back: Bioenergetics Lab Peer review due Quiz on bioenergetics paper (Kitchell 1977) 2 nd week back: Kitchell’s Cabin field trip
Why Collect Diets? 1.Food webs are a big part of ecology 2.Fish’s energy: growth and reproduction 3.Aquaculture: assess stock foraging 4.Resource managers: stocking, habitat assessment 5.Environment: indicate change in habitat, population densities
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? Group discussion questions
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:
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:
Discussion What biotic factors influence fish feeding Lists: Prey abundance Size of prey 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 Lists: Community Composition
Distance till fish reacts to prey Example: General factor: Turbidity Assumptions?: For all visual feeders Water Clarity or Turbidity Distance till fish sees prey...huh? 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! 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!
Holling’s Disc Equation 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. Rate of Energy Gained = (λe – s)/(1 +λh)
Sometimes called the disk equation cause this is how he originally developed the model
Holling’s Disc Equation 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. 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 Search Encounter Pursuit Capture Handling
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 Holling’s Observations
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")
Hollings equation relates diet information to energy and time spent foraging More specific physiological energetic needs can be described using Bioenergetics Numerical Response
Organisms are not chemicals! Ecological interactions are highly organized Big effects from small changes in space/time scale Reaction vat modelForaging arena model Prey eaten Prey density Prey eaten Prey density Prey behavior limits rate Predator handling limits rate Slide from Villy Christensen, IncoFish Workshop 2006
Foraging Arena Theory “There is a horrific linkage between getting food and being food, and this creates a severe trade- off 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.
Collecting Fish Active sampling techniques (seine, short term gill nets, angling, shocking) Beware of biases -postcapture digestion -regurgitation (stressed fish) -atypical foraging behavior in traps Long term gill net, fyke net, minnow trap
Collecting Diets Collect diets by: 1. Gastric Lavage 2. Stomach Removal -Remember fish size, population density
Experimental Strategies 1.Diel patterns (predators and prey) 2.Seasonal patterns (predators and prey) 3.Fish size/gender 4.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 Brown trout #3 -Example 2: How to calculate WRONG: 3/11 midges = 0.272 RIGHT: (0.25+0.25+0.33)/3 = 0.276
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 % Composition by Weight
Diet Indicies Index of Relative Importance (IRI) IRI = (% number + % weight)(FO) Consistency Overlap Selectivity *all of these are arbitrary units!
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 How does a fish decide what to eat?