Recruitment

Question 1

Recruitment

Answer 1

number of individuals that reach a certain life stage

Question 2

For exploited stocks, recruitment is typically

Answer 2

when individuals are first able to be selected by the fishery

Question 3

Recruiptment process

Answer 3

depends on population abundance and distribution, spawner fecundity, and pre-recruit survival

Question 4

When does 99% of the mortality occur?

Answer 4

between spawning and recruitment (high pre-recruit mortality)

Question 5

What is the year class affected by?

Answer 5

density dependent and density independent processes → large variation even in the absence of a fishery

Question 6

What are the four observations from Hilborn & Walters 1992

Answer 6

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

Question 7

Divers of recruitment variability

Answer 7

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.)

Question 8

What are the possible mechanisms in the variability in recruitment

Answer 8

Availability of food at first-feeding

Advection of larvae into favorable/unfavorable areas (predation, food availability, environmental conditions, etc.)

Question 9

What was the Houde 2008 study?

Answer 9

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

Question 10

What are some “Bet hedging” strategies:

Answer 10

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”)

Question 11

BOFFFFs

Answer 11

Higher relative fecundity & higher quality eggs for older females frequently observed (BOFFFFs – big old fat fecund female fish)

Question 12

Striped bass recruitment

Answer 12

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

Question 13

Density independence

Answer 13

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

Question 14

Density dependent recruitment

Answer 14

Can define spawning success as recruits/spawners (R/S)

Density dependence implies a decrease in R/S as S increases

Question 15

Why cant R/S increase indefinitely?

Answer 15

eventually resources will be limiting

Question 16

What is related to density dependence?

Answer 16

Compensatory mortality

Question 17

Compensatory mortality

Answer 17

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)

Question 18

What is the greatest scenario for spawning success?

Answer 18

Low population size -> important mechanism of population regulation.

Question 19

Depensation

Answer 19

Depensation is a decrease in spawning success (R/S) at low spawner abundances

Question 20

What are the possible mechanisms for Depensation?

Answer 20

constant predatory pressure, reduced reproductive success at low densities (Allee effect), impaired group behavior

Question 21

What may happen if stock exhibit depensation?

Answer 21

they may not recover after being fished to low abundance levels

Question 22

General advice

Answer 22

consider spawner-recruit models that at least allow for depensation

Question 23

Stock-Recruit (Spawner-recruit) relationship

Answer 23

Relationship between spawner and recruit abundance

Question 24

Why is stock-recruit relationships important?

Answer 24

they enable recruitment prediction
Predict stock trajectories and estimate management benchmarks
Considered in age-structured stock assessment models

Question 25

What does recruitment depend on ?

Answer 25

egg production

Question 26

What is optimally used for modeling recruitment?

Answer 26

mean fecundity by age x age distribution (or size)
Difficult and costly data to obtain

Question 27

What are the alternative measures for modeling recruitment?

Answer 27

Number mature females x mean fecundity
Biomass of mature individuals
Index of abundance

Question 28

What is the downside to the alternative measures?

Answer 28

→ Not directly proportional to egg production

Question 29

Ricker’s (1975) principles

Answer 29

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

Question 30

What does principal 3 in Ricjer’s 1975 analysis does not include?

Answer 30

depensation

Question 31

What did Hilborn & Walters (1992) add to Ricker’s 1975 principles

Answer 31

Continuity → mean recruitment should vary smoothly with stock size
Stationarity → average stock-recruit relationship is constant over time

Question 32

Beverton-Holt model (1957)

Answer 32

Mortality depends on size of cohort but is not directly related to spawner abundance

Density dependent by not stock-dependent mortality.

Question 33

Explain the Beverton-Holt model’s equation

Answer 33

Question 34

Ricker Model (1954)

Answer 34

Mortality is stock-dependent rather than density dependent

Allows for overcompensation (decreasing recruitment at high spawning stock sizes)

Question 35

What can be derived from the Beverton-Holt or Ricker model?

Answer 35

Steepness

Question 36

Steepness

Answer 36

expected recruitment at 20% unfished biomass relative to expected recruitment in an unfished state → usually denoted as h or z

Question 37

What is the key question to asked that is related to steepness?

Answer 37

Is it important to keep large spawning stock to provide recruitment?

Question 38

Why is steepness an important management parameter?

Answer 38

higher steepness indicates population more robust, resilient to harvest pressure

Question 39

Why is steepness difficult to estimate?

Answer 39

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

Question 40

Fishery management reference points are typically set by

Answer 40

using surplus production models, per-recruit analyses, and stock-recruit relationships

Question 41

Age-structured models are generally used to assess

Answer 41

fishery with respect to reference points and management objectives

Question 42

Linking models to management

Fill out the graph:

Answer 42