Transcript Life History and Demography - UC Davis: Environmental

Life History and Demography
Life in the Slow Lane
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Large, long-lived, at risk
Stellar’s sea cows
Great auks
Pelagic sharks (lamnid and carcharhinid)
Swordfishes (Xiphias )
Groupers (Serranidae)
Rock fishes (Scorpaenidae)
Sturgeons (Acipenseridae)
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
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
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)
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
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)
Reproductive Value
Life History Strategies: r vs. K vs. ?
• Classic r vs. K selection (Pianka 1970) doesn’t
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 don’t fit r vs. K
dichotomy
Fecundity vs. Age at Maturity
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
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)
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 females:males
Fishing Mortality and Life History
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 longerlived species
Loggerhead Turtle (Caretta caretta)
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
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
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
White Abalone
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
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
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
Gregarious Recruitment in
Red Sea Urchins
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
Black Sea Urchin
(Diadema antillarum)
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
Deep Sea Clam Tindaria
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
Orange Roughy (Hoplostethus
atlanticus)
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)
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
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
<40% 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
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
Grouper Spawning Aggregation
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)
Blue Crab (Callinectes sapidus)
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
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
Disjunct Life History
• Salmon species (five species in western N.A.) all at risk
because of inland life history
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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
Chinook Salmon
(Oncorhynchus tshawytscha)
Chinook Salmon Life History
Coho Salmon Management Units
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
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
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
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