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Age, Size and Growth ZOO 511 week 3 slides

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Metrics of Size and Growth Length –PROS: easy, intuitive, history in angling, length rarely shrinks, nonlethal –CONS: lots of change in biomass not related to length Wet Weight (i.e., weighing a live fish) –PROS: nonlethal, quick, useful for large calculations (ie population biomass) –CONS: can be difficult in the field if conditions are bad Dry Weight (i.e., weighing a dehydrated fish) –PROS: accurate description of individual's mass –CONS: time intensive and lethal to fish

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3 ways to estimate growth in natural populations Recaptures of marked individuals Length-Frequency Analysis Back calculation from calcified structures # Caught

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Recaptures of marked individuals METHOD: measure individuals and give them unique marks; recapture and measure again later PROS: nonlethal, accurate individual data CONS: high effort - have to catch & mark A LOT of fish

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Length-Frequency Analysis METHOD: measure population at least once; plot length vs. frequency to find age classes; compare across age classes to estimate growth # Caught Length (mm) Age class 3 Age class 4 Age class 5 Age class 2 Age class 1

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Length-Frequency Analysis METHOD: measure population at least once; plot length vs. frequency to find age classes; compare across age classes to estimate growth PROS: nonlethal; can use historic data; can do with 1 sample CONS: “snap shot” of growth; assumes constant conditions; easy to bias sample with gear, time or location; requires lots of fish # Caught Length (mm)

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Back Calculation METHOD: Examine hard structures from individuals for age and evidence of past growth rate Periods of rapid and slow growth show up as rings

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Back Calculation METHOD: Examine hard structures from individuals for evidence of past growth rate PROS: sometimes nonlethal; accurate individual data; no repeated sampling; does not assume constant conditions; can used archived structures; can estimate over small size/time changes; CONS: sometimes lethal; can be technically challenging

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Vertebrae (sharks) Fin Rays Opercula Cleithra (pikes & relatives) Hard structures to estimate age & growth

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Otoliths (lethal) Scales (non-lethal) Hard structures to estimate age & growth

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HOW TO estimate age & growth with scales or otoliths

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Otoliths work the same way Plus they are useful for many other things But you have to kill the fish to retrieve them And they are more work to process

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Otoliths What is an otolith? Where exactly is an otolith?

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harvest section & polish analyze

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Otoliths and fishery science Unique properties: –Otolith growth is continual –Lack of resorption Complete growth and environmental record –Crystalline structure Holds trace metals Scientists use otolith composition to: –Estimate what temperatures the fish experienced in the past –Determine where the fish traveled (e.g., ocean vs. freshwater)

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How do we get from age to growth?

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Frasier-Lee Equation L t = c + (L T – c)(S t /S T ) big T means now little t means some time in the past L means fish length S means scale radius

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Frasier-Lee Equation L t = c + (L T – c)(S t /S T ) c is “Carlander’s constant” -- it will have a different value for different species

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Now we have a lot of length-at-age points. How do we summarize growth patterns from this?

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How do we compare growth between 2 populations? Insert real data here?

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Von Bertalanffy Growth Model L t = L ∞ - (L ∞ - L 0 ) –kt –L t = length at time “t” (of an avg. fish in the population) –L ∞ = length at infinity –L 0 = length at time zero (birth) –K = constant (shape of growth line)

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Von Bertalanffy Growth Model L t = L ∞ - (L ∞ - L 0 ) –kt If you give the model this It will give you these

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L t = L ∞ - (L ∞ - L 0 ) -kt Linf =523.4 Lzero =57.54 k =0.081 Linf =500.6 Lzero =28.34 k =0.080 AL WS

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