Recruitment Flashcards
Recruitment
number of individuals that reach a certain life stage
For exploited stocks, recruitment is typically
when individuals are first able to be selected by the fishery
Recruiptment process
depends on population abundance and distribution, spawner fecundity, and pre-recruit survival
When does 99% of the mortality occur?
between spawning and recruitment (high pre-recruit mortality)
What is the year class affected by?
density dependent and density independent processes → large variation even in the absence of a fishery
What are the four observations from Hilborn & Walters 1992
Larger spawning stocks tend to produce larger recruitments
Total recruitment tends to stop increasing beyond some spawning stock size, and may decrease at large spawning stocks
The data are highly “scattered”
Variability in an underlying spawner-recruit relationship increases for larger spawning stocks
Divers of recruitment variability
Typically thought to be a combination of many interacting factors:
(temperature, wind speeds & direction, currents/circulation, prey availability, predation, larvae growth rates, age/size composition of spawners, etc.)
What are the possible mechanisms in the variability in recruitment
Availability of food at first-feeding
Advection of larvae into favorable/unfavorable areas (predation, food availability, environmental conditions, etc.)
What was the Houde 2008 study?
Two cohorts, each beginning with 1014 eggs (100 trillion)
Cohort A has 10% daily mortality → 2.8B fish @ 100 days
Cohort B has 10% daily mortality AND experiences >90% mortality during first-feeding → 56M fish @ 100 days
High mortality during short “critical period” led to 50x difference in recruitment
What are some “Bet hedging” strategies:
Environmental conditions are variable → fitness improved by spawning in multiple locations & times (iteroparity, batch spawning)
Age/size-dependent timing / location of spawning activity
Increased quantity & quality of eggs produced by older / larger fish (“maternal effects”)
BOFFFFs
Higher relative fecundity & higher quality eggs for older females frequently observed (BOFFFFs – big old fat fecund female fish)
Striped bass recruitment
Batch spawners; 200K eggs/kg/yr
Recruitment success depends on environmental conditions (freshwater flow, temperature)
High recruitment associated with high FW flow & cold spring temperatures
Older females spawn earlier → more likely to encounter favorable environmental conditions
Age diversity ensures spawning occurs over longer time period
Density independence
The probability of survival to recruitment is unrelated to spawning stock size (i.e., no intraspecific competition or cannibalism
Generally unreasonable → eventually resources will be limiting
May be true over some portion of spawner-recruit relationship however
Density dependent recruitment
Can define spawning success as recruits/spawners (R/S)
Density dependence implies a decrease in R/S as S increases
Why cant R/S increase indefinitely?
eventually resources will be limiting
What is related to density dependence?
Compensatory mortality
Compensatory mortality
Compensation (in recruitment process) is a decrease in spawning success (R/S) as the number of spawners increases
Increase in mortality of early life stages as spawners increase (note that compensation can also act on adult growth & survival)
What is the greatest scenario for spawning success?
Low population size -> important mechanism of population regulation.
Depensation
Depensation is a decrease in spawning success (R/S) at low spawner abundances
What are the possible mechanisms for Depensation?
constant predatory pressure, reduced reproductive success at low densities (Allee effect), impaired group behavior
What may happen if stock exhibit depensation?
they may not recover after being fished to low abundance levels
General advice
consider spawner-recruit models that at least allow for depensation
Stock-Recruit (Spawner-recruit) relationship
Relationship between spawner and recruit abundance
Why is stock-recruit relationships important?
they enable recruitment prediction
Predict stock trajectories and estimate management benchmarks
Considered in age-structured stock assessment models
What does recruitment depend on ?
egg production
What is optimally used for modeling recruitment?
mean fecundity by age x age distribution (or size)
Difficult and costly data to obtain
What are the alternative measures for modeling recruitment?
Number mature females x mean fecundity
Biomass of mature individuals
Index of abundance
What is the downside to the alternative measures?
→ Not directly proportional to egg production
Ricker’s (1975) principles
Stock-recruitment curves should pass through the origin; when there is no spawning stock there is no recruitment
Stock-recruitment curves should not fall to zero at high stock levels; there should be no point at which reproduction is completely eliminated at high densities
The recruitment rate (recruits-per-spawner) should decrease continuously with increases in the spawning stock
Recruitment must exceed spawning stock over part of the range of possible spawning stock sizes
What does principal 3 in Ricjer’s 1975 analysis does not include?
depensation
What did Hilborn & Walters (1992) add to Ricker’s 1975 principles
Continuity → mean recruitment should vary smoothly with stock size
Stationarity → average stock-recruit relationship is constant over time
Beverton-Holt model (1957)
Mortality depends on size of cohort but is not directly related to spawner abundance
Density dependent by not stock-dependent mortality.
Explain the Beverton-Holt model’s equation
Ricker Model (1954)
Mortality is stock-dependent rather than density dependent
Allows for overcompensation (decreasing recruitment at high spawning stock sizes)
What can be derived from the Beverton-Holt or Ricker model?
Steepness
Steepness
expected recruitment at 20% unfished biomass relative to expected recruitment in an unfished state → usually denoted as h or z
What is the key question to asked that is related to steepness?
Is it important to keep large spawning stock to provide recruitment?
Why is steepness an important management parameter?
higher steepness indicates population more robust, resilient to harvest pressure
Why is steepness difficult to estimate?
for most stocks due to limited contrast in the data
Need spawning stock and recruit data at both high (unfished) and low (depleted) stock levels
Fishery management reference points are typically set by
using surplus production models, per-recruit analyses, and stock-recruit relationships
Age-structured models are generally used to assess
fishery with respect to reference points and management objectives
Linking models to management
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