Life History and Demography

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Life History and Demography
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Life in the Slow Lane

• Large, long-lived, at risk
• Stellars sea cows
• Great auks
• Pelagic sharks (lamnid and carcharhinid)
• Swordfishes (Xiphias )
• Groupers (Serranidae)
• Rock fishes (Scorpaenidae)
• Sturgeons (Acipenseridae)

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Age at Maturity or First Reproduction

• Age at which an organisms first reproduces is a
  critical factor for population growth (Cole 1954)
• Usually defined as the age where 50 of the
  females reproduce
• Reproducing early is important for population
  growth
• An annual semelparous species can produce as many
  offspring as an iteroparous species by adding
  just one additional offspring to its clutch

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Delayed Age at Maturity or First Reproduction

• Many marine organisms have delayed reproduction
• Bluefin tuna may first spawn when 8-9 years old
• Loggerhead turtle may not reproduce until 25-30
  years old
• Deep sea fish (Orange Roughy) may not mature
  until 20-40 years old

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Fecundity

• For mammals and birds, fecundity may not increase
  with size/age
• Often determinant growth
• Fixed number of offspring per year
• For many fishes, reptiles and invertebrates,
  fecundity is function of size/age
• Often indeterminant growth
• Number of eggs function of size of organism
• Volume increases exponentially with size (volume
  length3)

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Low Fecundity

• Many larger marine organisms have low fecundity
• Many sea birds typically produce only one
  offspring and only every other year at most
• Many large whales also produce one offspring in
  some case every 2-5 years

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Reproductive Value

• Reproductive value to population is a function of
  the age of organism
• RV current reproductive output residual
  (future) reproductive output
• Current reproductive output is birth rate at
  current stage
• Residual is sum of expected output at all future
  stages
• Dependent on whether population is increasing or
  decreasing (if decreasing, future reproduction
  worth more)

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Reproductive Value
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Life History Strategies r vs. K vs. ?

• Classic r vs. K selection (Pianka 1970) doesnt
  apply well in marine environments
• r selected species typically have high fecundity,
  rapid development to maturity, small size with
  short lifespan
• K selected species are typically low fecundity,
  slow to mature, large size with long lifespan
• Species like abalone are long-lived, relatively
  large, slow to mature BUT very fecund
• Marine species like these dont fit r vs. K
  dichotomy

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Fecundity vs. Age at Maturity
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Population Response to Fishing

• Long lived species relative to short-lived
  species
• Long-lived species fluctuate less
• New recruits constitute a small part of the
  population
• Fishing strongly truncates size distribution

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Fishing Effects on Long-Lived Species

• Management strategies based on fishing mortality
  may not apply well to long-lived species
• Accurately estimating the fishing mortality (F)
  may be very difficult
• The impact may more a function of the age
  distribution of the population
• Assessments based simply on biomass may be
  erroneous (e.g. large population with lots of old
  individuals)

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Population Response to Fishing

• Species with skewed sex ratios are more
  vulnerable to exploitation
• Sequential hermaphrodites are species that change
  sex during their lifetime
• Species like groupers are first female and then
  switch to males as the get larger/older
• Fishing mortality can skew sex ratio dramatically
  increasing femalesmales

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Fishing Mortality and Life History
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Population Growth and Fishing Mortality

• Different life history parameters (survival of
  adults, survival of eggs, reproductive output)
  affect population growth differently
• In long-lived organisms, generally survival of
  subadults and adults more strongly affects
  population growth than survival of larvae or
  reproductive output
• Increases in per capita egg production or larval
  survival that might accompany low population
  levels is unlikely to offset adult mortality
• Compensation in growth and survival is lower in
  longer-lived species

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Loggerhead Turtle (Caretta caretta)
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Loggerhead Turtle Populations

• Loggerhead turtle conservation prior to the 1980s
  focused mostly on improving survival of eggs and
  hatchlings
• Studies by Crouse et al. (1987) demonstrated a
  much greater effect of saving the mature
  reproductive females than saving individual
  hatchlings
• The use of TEDs (turtle excluder devices) in
  trawl fisheries would result in greater increases
  in population growth by increasing survival of
  large female turtles

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Allee Effects in the Sea

• Allee Principal (Odum 1959)
• Refers to situation where an increase in
  population density results in increased per
  capita reproduction
• Inverse density dependence
• Positive density dependence
• Depensation
• The reverse is that as population density
  decreases, per capita reproduction decreases

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Allee Effects in Reproduction

• Allee effects are not equally likely in all life
  history strategies
• Broadcast spawners (eggs and sperm broadcast) are
  particularly vulnerable
• Reduced fertilization may occur with organisms
  only meters away
• Mobile organisms (fish) can aggregrate increasing
  fertilization success

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White Abalone
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Abalone Population Failure

• White abalone (Haliotis sorenseni) used to be
  abundant in southern California/Baha below 25
  meters
• Not fished until 1965, then fished intensively in
  early 1970s ending in 1983, species is now listed
  as endangered
• Abalone must be within 1 m for fertilization
• Recruitment failed as the result of reduced adult
  density below the threshold for fertilization

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Allee Effects in Reproduction

• Free spawning (broadcast sperm, retain eggs)
• Little evidence of reduced fertilization success
• Direct sperm transfer
• Male and sperm limitation possible
• Male size can limit fertilization (spiny lobster)
• Reproduction may fail entirely below threshold
  density

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Allee Effects in Settlement and Recruitment

• Conspecifics as chemical cues for settlement
• Low adult numbers may reduce settlement of result
  in extremely high densities
• Conspecific adults as refuge for juveniles
• Urchins and sand dollars survive better near
  adults (Tegner and Dayton 1977, Highsmith 1982)
• Groups of adults may survive better
• Better cope with physical stress
• Better defense against predators

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Gregarious Recruitment in Red Sea Urchins
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Examples of Allee Effects

• Dieoff of the black sea urchin Diadema antillarum
  in the Caribbean occurred in 1983-84
• Three consequences of dieoff
• Reduction in egg production (density independent)
• Reduction in eggs fertilized (positive den.
  depend.)
• Increase in body size-fewer adults (neg. den.
  depend.)
• Positive and negative density dependence
  cancelled each other
• Reduction of density independent egg production
  created small stable populations

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Black Sea Urchin (Diadema antillarum)
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Demography and the Deep Sea

• Life history of many organisms is very slow in
  cold, dark depths
• Organisms may grow slowly and mature at older
  ages
• Its estimated that the abyssal clam Tindaria
  callistiformis takes 100 yrs to reach 8 mm
• Deep sea fish may take 10-20 years or more to
  mature

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Deep Sea Clam Tindaria
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Demography and Deep Sea Fisheries Orange Roughy

• Orange Roughy (Hoplostethus atlanticus) is
  distributed worldwide deep waters 500-1500 m
• In the most developed fishery in New Zealand,
  harvest peaked in 1989 at 57,000 t but now down
  to 15,000 t
• It was harvested based on life history
  assumptions without any data
• Data have shown that the fishing mortality
  targets were way off

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Orange Roughy (Hoplostethus atlanticus)
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Demography and Deep Sea Fisheries Orange Roughy

• Newer age-based demography based on otoliths
• Otoliths are calcareous structures with
  observable growth rings (like tree rings)
• Since Boehlert (1985) first suggested measuring
  age from otoliths, this has been a major means of
  determing life history parameters
• Orange roughy can live up to 150 years old (among
  oldest known marine species)
• Mature between 20 and 40 years
• Produce comparatively small numbers of eggs
• They also aggregate around sea mounts in austral
  winter (June-Aug)

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Migratory Species

• Many species migrate over significant distances
  during their life cycle
• For species where the move long distances
  relative to reserve size
• Susceptible to displaced fishing outside of
  reserve
• Polacheck (1990) showed for sessile species, only
  20 of population in reserve will preserve 20 of
  unexploited spawning stock
• Highly migratory species may require nearly 60
  of population in a reserve to protect same 20 of
  spawning stock

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Northern Cod

• Northern cod (Gadus morhua) move offshore in the
  fall and onshore during spring and summer
• Simulations of the population collapse during the
  1990s showed that reserves that contained lt40 of
  population would not prevent collapse
• Reserves would need to contain nearly 80 of
  population to avoid collapse
• Again, issue is displaced (increased) fishing
  outside of the reserve

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Disjunct Life History Stages

• Many species have life histories such that one
  phase occupies a habitat very different than
  another
• Many invertebrates (e.g. blue crabs) and fishes
  (e.g. Nassau grouper) have specific spawing
  grounds
• May need to consider dispersal corridors than
  link nursery grounds with spawning grounds

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Grouper Spawning Aggregation
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Blue Crab Fishery

• The blue crab in Chesapeake Bay has a complex
  life cycle
• Mating occurs in upper tributaries, females
  migrate to lower bay (higher salinity) to spawn
  eggs and hatch larvae
• Larvae migrate out of bay and postlarvae migrate
  in from shelf
• Reserves targeted lower bay, but huge declines of
  spawning stock (85) resulted (Seitz et al. 2001)

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Blue Crab (Callinectes sapidus)
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Blue Crab

• Females were still heavily exploited before they
  reached the spawning grounds (no protected
  corridor)
• Recently, large increase in protection of 75 of
  spawning grounds and migratory routes did not
  restore stocks
• Displaced fishing outside reserves continued to
  reduce spawning stock

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Disjunct Life History

• Many vertebrate species also have disjunct and
  vulnerable life histories
• Marbeled murrelets (Brachyramphus marmoratus)
  distributed in narrow band from Aleutians to
  California
• Although nearshore seabirds most of year, nest in
  old growth Pacific coast conifers
• So severely threatened by logging of old growth

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Disjunct Life History

• Salmon species (five species in western N.A.) all
  at risk because of inland life history
• Sockeye (Red)
• Coho (Silver)
• Chinook (King)
• Pink
• Chum
• Although climate change has impacted ocean going
  adults, land use has been the biggest impact
• Dams, overfishing, introduced predators and loss
  of habitat have reduced many (including winter
  run chinook, southern Coho runs) to very low
  abundances
• Migrating salmon require healthy watersheds for
  spawning, intact watersheds for rearing and
  adequate estuary habitat for outmigration

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Chinook Salmon (Oncorhynchus tshawytscha)
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Chinook Salmon Life History
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Coho Salmon Management Units
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Sedentary Adults and Dispersal

• The dispersal life history may be very important
  for species with sessile adults (many
  invertebrates, urchins, abalones)
• The effectiveness of reserves (size and spacing)
  depends strongly on dispersal distance (strongly
  correlated with development time)
• Simulation models (Quinn et al. 1993, Morgan and
  Botsford 2001) showed that reserve effectiveness
  (time to extinction) was greatly increased by
  retention/return to reserve

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Sedentary Adults and Dispersal

• Reserve effectiveness was also related to size
  and spacing of reserves relative to larval
  dispersal distance
• Shorter dispersal resulted in higher population
  abundances
• Reserve size and spacing most important for
  species with limited dispersal
• Less important for species long distance larval
  dispersal

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Life History and Management

• Life history is a critical factor putting species
  at risk
• Age at maturity
• Fecundity
• Frequency of reproduction
• Life history may also determines what management
  strategies may be feasible
• Strategies for more rapidly reproducing species
  may not be as effective

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Life History and Management

• Size limits or slot limits (leave small and
  large) may work for some species
• Size limits may not be effective with long-lived
  species since truncation will still occur with
  increased pressure on large sizes
• With deep water species, limits may not work
  because trauma of capture (all die)
• More comprehensive management options (closures,
  quotas, moratoria) are likely needed for species
  with vulnerable life histories