Early Life History: Recruitment Processes

Question 1

Sources of high mortality rates in early life stages

Answer 1

predation and starvation

Question 2

Hjort’s critical period hypothesis

Answer 2

Year class strength determined by survival during a critical period
Critical period: the transition from endogenous (yolk sac) to exogenous (plankton) feeding
Limited food during critical period: high mortality

Question 3

Cushing’s match/mismatch hypothesis

Answer 3

Extension of Hjort’s theories
Also focused on starvation as key source of mortality
Emphasised the importance of timing
Fish spawning timed to coincide with plankton blooms in areas where larvae will drift to
Timing of spawning fixed, timing of plankton blooms varies

Question 4

Haddock in George’s Bank: In years when the spring bloom occurred early, why was survival was high

Answer 4

High rates of recruitment associated with early autumn blooms of high magnitude
Mechanism: positive correlation between female condition and recruitment success (adult females feed better in years of strong early autumn blooms and therefore produce more eggs with a higher chance of survival)

Question 5

Lasker’s stable ocean hypothesis

Answer 5

Laboratory and field studies on northern anchovy established:
First-feeding larvae require a minimum threshold food abundance to survive
This food requirement is higher than the average food availability in the larval rearing ground
Therefore patches of local food abundance must be very important
Data from scientific surveys showed that threshold feeding conditions occurred during calm stable ocean conditions
Lasker predicted that poor recruitment would occur in stormy or windy years

Question 6

Hjort’s second hypothesis: “Aberrant Drift”

Answer 6

Inter-annual differences in circulation influences drift of eggs and larvae away from appropriate distribution areas

Question 7

Larval transport and the flatfish life cycle

Answer 7

crucial for delivery of flatfish to suitable nursery areas
Most flatfish species spawn pelagic eggs and larvae are delivered to shallow coastal nursery areas by ocean currents
If fish are not in suitable habitat when they reach settlement size, survival is unlikely

Question 8

How do flatfish spawn in areas with weak currents

Answer 8

In areas where currents are weak or nondirectional plaice spawning grounds are close to nursery in areas

Question 9

How do flatfish spawn in areas of strong currents

Answer 9

In areas where currents are strong, eggs released in an optimum direction and distance from the settlement areas (Gibson, 1999)

Question 10

How do currents affect larvae behaviour

Answer 10

Currents have an important influence on larval distributions
However, it is now clear that larvae to not behave just as passive particles
Larvae can position themselves vertically in the water column to avoid or take advantage of directional currents

Question 11

How is temperature effecting survival of fish in early stages of life

Answer 11

Impacts on growth rates metabolic rates and rate of development
Impacts on swimming abilities (warm water more viscous)
Timing and location of spawning and peaks in planktonic food supply (potential for increased mismatch)

Question 12

What does increased CO2 levels in water do o fish

Answer 12

Impair sensory abilities of larval fish
Effects on sense of smell, hearing, and vision as well as overall activity levels
This has consequences for settlement to juvenile reef habitats, predator prey interactions and competitive abilities