Age, Size and Growth ZOO 511 week 3 slides

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

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

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

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

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)

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

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

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

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

HOW TO estimate age & growth with scales or otoliths

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

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

harvest section & polish analyze

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)

How do we get from age to growth?

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

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

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

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

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)

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

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