Age & Growth

Why are there so many studies on age & growth?

1. Conservation & management

2. Easier to study & more doable historically

General findings on Easmobrnch age & growth

• hard part bands can be used to age elasmobranchs

• Bands in hard parts represent an annual mark

• Mature late (4-25 years) & grow slowly

• Can live very long 5-over 70 years (Greenland sharks estimated 400+)

• Important to know for management and conservation of a species or population

What does a population growth curve look like?

• y-axis: # of individuals in a pop

• X-axis: time

• One showing several different lines with R values

• Represents exponential growth- growth under most ideal conditions with unlimited resources, determined by how many offspring they can have, and generation time (avg age they reproduce at)

• Shark pops can’t grow fast- not a lot of offspring, late sexual maturity, long generation time

#1 method for determining age and growth

Growth zones in calcified structures (Vertebral centra!!!, sometimes dorsal spines/caudal thorns)

• wide opaque (not transparent bands) in summer, dark

• Narrow translucent (semi, transparent bands) in winter, light

• New growth is laid down on the outer edge of the centrum

How to actually look at vertebral centrum

• section and stain bands to make more visible (silver nitrate, crystal violet)

• Use 2 people to red the same centra and see if counts are consistent

• Make an age bias plot with each reader # of bands on each axis and see how good line of fit is

What is plotted based on age estimates determined?

Growth curve

• typically pot fish length as a function of age (or # of rings/bands)

• Use one of three shark length measurements: PCL=precaudal length, FL= fork length, TL = total length

• Typical growth curve strats off fast and then levels off when they reach sexual maturity because theyre not putting energy into growth anymore, rather reproduction

Back calculation

Radius a = 1.5 cm

Radius b = 2 cm

Shark toady = 4 m

For an individual shark you can determine what its length was at each previous age (each vertebral band) based on proportionate growth relationship of shark length and vertebral radius

• the vertebral radius grows at the same rate as the body length

• Ring radius/centrium radius = proportion, then %/ length of fish at that age= fish length

• If radius to growth band 3 is 75% of the total vertebral radius then at age 3 the shark was 75% of its present length

• 1.5 cm/2cm =0.75 (75%), 0.75 × 4m = 3 m

Back calculation method 1

TLn= VRn/VRc x TLc

Total length Typical growth= Vertebra radius (at age n (ring))/Vertebra radius (at captured) x Total length (captured)

Back Calculation method 2

• plot a liner regression for shark eight against total centrum radius for sharks of a wide range of lengths, then for each individual shark product length at prior ages based on the radius to each band using regression equation

• TL = 63.2 + (16.7)R → Y= b +mx

Back Calculation tables *what do you notice, what’s this called, give example

Shows back calculated length of each shark at each age of its life

• The older the shark the smaller they are when they were 1 year old than the sharks captured at younger ages

• Causes: smaller sharks are more successful when they are younger

Rosa Lee Phenomenon: for large (old) sharks, average back calculated sizes at young ages are smaller than average back calculated sizes to the same young ages for smaller sharks

• lemon sharks that grew faster were more outgoing “bolder”→ ecologically they spend more time out in the open where theres more food but also more predators so don’t make it as much as the smaller “shyer” sharks that stay in the mangroves

2nd method for determine age & growth

Size frequency analysis:

1. examine size modes of cohorts by analyzing numerous cohorts at a point in time

2. following size distribution of n individual cohort through time

• Cohort= group of individuals born t the same time

What do you notice about this figure?

This is an example of examining size modes of cohorts by analyzing numerous cohorts at a point in time

• trimodal distribution of juvenile sharks→ indicates that they saw 3 different cohorts

• Juvenile doesn’t mean all the same age, the study looked at individuals with 1, 2 or 3 vertebral rings

• Peaks are distinct in first several years of life but then become too similar

Explain what’s going on in this figure- what size frequencies analysis are they doing?

Following size distribution of an individual cohort through time- measure the growth by following a cohort every month

• all the curves show faster growth at first and then level, even though there are 3 different ages and this is only over a 1 year period

• Have fast growth in summer period and then slows down at winter time due to colder water & less prey

3rd method for determining age & growth

Mark & recapture:

• measurement at initial capture

• Recapture later and re-measure

• Provides info on growth at specific intervals of time for sharks of different size

• Have to tag a lot not ideal

4th method used to determine age & growth

Laboratory growth analysis

• used for some species with poorly calcified vertebrae or to determine periodicity of growth zone formation

Periodicity of vertebral rings

How often are the bands laid down in vertebral rings, ie what time period does the band represent

1st method used to determine periodicity of vertebral rings?

Relative Marginal Increment analysis RMI or Marginal Increment Ratio (MIR)

• measurement of distance from last band to centrum edge for each month → bc new centrum growth is on outside edge of centrum

• If distance is very small then last band was recently laid down

• If distance is large then last band was laid down months ago

• On marginal increment plot over time, the ring was layed down when the plot drops

• Tells us that this ring was laid down between June & July, and theres one band per year

1. Ring deposition in Jan-March once a year

2. Ring deposition in January once per year

Ring deposition occurs between March and June and occurs once per year

2nd method for determining periodicity of vertebral rings

Tetracycline or oxytretracylcine (OTC) marking

• injecting OTC into a live shark leaves a permanent discrete mark on the outside edge of calcified structures (centrum) as they are being formed/laid down

• Upon recapture, vertebra centrium is dissected out and the number of bands laid down since its been OTC marked is counted to determine ring periodicity

• If shark ws injected in 2015, ecaptured in 202, and there are 5 bands distal to the OTC marked then bands were laid down annually

3rd method for determining periodicity of vertebral rings

Bomb carbon: based on decay of C14 released during atmospheric nuclear bomb testing in late 1950s/early 60s (arms race) → atmosphere food w/C14 & it made its way into food webs and plant material

• leaves a permanent known marker in vertebral central like OTC if shark was alive during 50s-60s

• Useful for still alive long-lived species, and shorter lived species with archived vertebral collections with a known capture date → if know capture date see how many years before that it was the 50s/60s

Von Bertalanffy growth equation (vb curve)

Equation that predicts fish length at any given age, based on indeterminate growth to theoretical maximum length

Lt = L infinity (1-e^-k(t-t0))

Lt: predicted length at time t

L infinity: theoretical maximum length

K: growth constant; describes rate at which an organism grows

t: time (age)

t0: time when organism would have theoretically been length 0 (a negative)

• indeterminate growth by grow slower and slower as older, never actually reaching theoretical growth max (limits)

Why are the back calculated lengths lower than the predicted and observed?

When back calculating an age your back calculating it to January (or whenever the ring was laid down) but these sharks were caught in summer so they’ve grown since then,

Study of Age & Growth in Bullsharks in Southern Gulf of Mexico

• used vertebral centra to age

• Got sharks from fishermen- unidentified to sex dont know why

• Used marginal increment to verify tempora growth ring formation & reported Von Bertalanffy curve→ found difference btw sexes, females larger

• Had chart of centrum radius vs vertebra # → bc they sample the same singular vertebra from each shark for aging and want to figure out the biggest one

• Used MI= (R - r)/(rb - rb-1): R is centrum radius, rb is radius to last ring before newest, rb-1 radius to ring before that, divide by those values to adjust for the fact that young sharks have faster growth than older ones

• Know females are sexually mature by cutting them open to see if theyre pregnant and then use youngest aged one to say when they reach sexually maturity

How to age a Greenland shark

Use carbon dating bc they can’t use rings bc they don’t have enough calcified parts

• [C14] in organisms is reflective of atmosphere when alive, when die they don’t get new C14→ it starts to decay & breakdown, so you can tell how long ago the thing died based on how much C14 is left

Use Lens of eye: eye lens proteins are put into the embryo→ these are inert (dead organic tissue in eye) so starts decaying when born like when something is dead, can use to date to when it was born

What does study and graph tell us about- from Greenland shark study

• The large sharks have low concentrations because they were conceived before bomb pulse testing occurred in 1960s

• Only the smallest sharks showed marker of 1960s bomb pulse

• Life span at least 272yrs, largest could be 379 years, reach sexual maturity 156 years

• Conservation implications- pop would not be able to rebound, generation time soooo long

• longest lived vertebrate known