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Life History and Demography. Life in the Slow Lane . 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).

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life in the slow lane
Life in the Slow Lane
  • 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 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
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
  • 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
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
  • 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)
life history strategies r vs k vs
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
population response to fishing
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
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 fishing13
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
population growth and fishing mortality
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
loggerhead turtle populations
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 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 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
abalone population failure
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 reproduction22
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
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
examples of allee effects
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
demography and the deep sea
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
demography and deep sea fisheries orange roughy
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
demography and deep sea fisheries orange roughy31
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
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
  • 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
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
blue crab fishery
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
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
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 history40
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
sedentary adults and dispersal
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 dispersal45
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 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 management47
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