recruitment

Question 1

what is recruitment

Answer 1

the number of fish surviving to enter the
fishery
- can be defined in different ways

Question 2

what are the 4 different ways authors have defined recruitment

Answer 2

• Number of fish metamorphosing
• Number of fish reaching maturity
• Number of fish reaching minimum size to be caught by fishery
• Number of fish reaching a certain year class

Question 3

what 3 possible factors does adult recruitment depend on

Answer 3

• Number of eggs produced
• Survival in plankton
• Survival of young fish

Question 4

is the Number of eggs produced an important factor in adult recruitment

Answer 4

• Historically, it was felt that spawning stock
  was the main factor e.g. more eggs = more recruitment
• It turns out recruitment is often poorly
  related to the size of the spawning stock aka number of eggs produced - other factors involved

Question 5

what thesis explains the factor of Survival in the plankton in adult recruitment. what does it show?

Answer 5

Ricker-Foerster Thesis
- shows very small changes in the survival rate of plankton have large effects on the final recruitment number (because numbers are so big)

Question 6

what increases the survival rate of plankton

Answer 6

getting out of the plankton stage as quick as possible - longer in plankton, longer to be eaten

Question 7

what 2 main factors influence growth rate of larvae (and get them out of the plankton stage as quick as possible)

Answer 7

1. Temperature - increase metabolic rates
2. Food - need more food to pay for the increase in metabolic rates
   - so these both factors are related

Question 8

what’s the problem with estimating growth rates in the lab

Answer 8

Blaxter (1988) suggested that fish larvae may grow faster in the lab than in the field

Question 9

what do researches need to estimate in order to look at the growth rates of fish in the wild

Answer 9

The age of the fish larvae
The mortality rate of the fish larvae

Question 10

how can researchers estimate the age of the fish larvae to estimate the growth rates of fish in the wild

Answer 10

otoliths - daily growth rings (count them)
e.g. a dense ring will be seen in the summer when there is plenty of food, more translucent ring in the winter where food is limited

Question 11

2 problems using otoliths to estimate the age of the fish larvae to estimate the growth rates of fish in the wild

Answer 11

1. Preparation time - catching larvae, sectioning, cutting, microscopy etc. - expensive + requires expertise
2. Resolving the different growth rings - need expertise

Question 12

how can researchers estimate The mortality rate of the fish larvae to estimate the growth rates of fish in the wild. what is the problem with this?

Answer 12

can be calculated from size-frequency histograms
- problem: catching larvae quantitatively (avoidance) - quantified by comparing depth-integrated day/night
catches (bigger larvae may be at different depths etc - need a fair catch)

Question 13

what do fish larvae eat

Answer 13

1.dinoflagellates (small - eat when first hatch)
2.nauplii (bigger - eat as they grow)
3.copepodite (bigger again - eat as they grow)

Question 14

what factors affects prey selections for fish

Answer 14

• gape limitation
• wind/mixing of water column - Lasker’s stability hypothesis
• time/season - match-miss match hypothesis

Question 15

explain the gape limitation factor that affects prey availability for fish

Answer 15

if it fits in their mouth, they can eat

Question 16

explain the wind mixing factor that affects prey availability for fish

Answer 16

Lasker (1981) looked at anchovy in California - Lasker’s stability hypothesis
- Larvae feed on dinoflagellates up to 40 um in size
e.g. Gymnodinium splendens
- Prey density (phytoplankton) decreased following wind induced mixing
- Best larval survival in calmest year
- reason = phytoplankton spread across grater depths when mixing of the water increases - harder for anchovy larvae to find food

Question 17

explain the time/season factor that affects prey availability for fish

Answer 17

match-miss-match hypothesis - David Cushing 1950s
- spawning is timed so the hatching of the larvae correlates with mount of food for them e.g. spring
- anything that can shift spawning timing so they hatch early/late would be a miss-match - lead to starvation of larvae

Question 18

what case study looks at the Match-mismatch hypothesis and how does it do that

Answer 18

Beaugrand et al. (2003) - looks at plankton effect on cod recruitment in the North Sea

Question 19

what’s the background of the Beaugrand et al. (2003) case study

Answer 19

• Atlantic cod (Gadus morhua) = Overexploited in North Sea since late 1960s
• Great concern over biomass and cod recruitment
• Overfishing historically blamed for decline
• Beaugrand et al. proposed that bottom-up control could also be important aka their food (plankton)
• Fluctuations in plankton → changes in cod-recruitment?

Question 20

what’s the methods used for the Beaugrand et al. (2003) case study

Answer 20

Plankton data from CPR survey (catches plankton at a set depth) - CPR database contains >245,000 samples from 1946-present
- Looked at copepod and euphausiid nauplii
- looked at cod recruitment from ICES studies
- Principal Component Analysis
- Pearson linear correlation - linking cod recruitment to plankton index

Question 21

what’s the results of the Beaugrand et al. (2003) case study

Answer 21

• Survival of larval cod depends on 3 factors:
  1. Mean size of prey
  2. Seasonal timing
  3. Abundance of prey
• Match-mismatch confirmed
• Rising temperatures since the mid-1980s (2 key effects)
  4. Increased cod metabolism and energetic cost
  5. Reduced abundance of prey
  Result = reduced growth, survival and recruitment

Question 22

what other factor may play a role in the survival of fish larvae other than food availability

Answer 22

predation

Question 23

what is Density-dependent mortality

Answer 23

When there are lots of fish larvae they may crop
down their prey so much that survival is reduced - not enough food
- Cushing and Horwood (1994) Modelled feeding of larval fish

Question 24

why does adult recruitment possibly depend on Survival of young fish

Answer 24

The number of post-larval fish (juveniles) is directly linked to number of 0-group fish e.g. atlantic-norweigon cod