recruitment Flashcards
what is recruitment
the number of fish surviving to enter the
fishery
- can be defined in different ways
what are the 4 different ways authors have defined recruitment
- 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
what 3 possible factors does adult recruitment depend on
- Number of eggs produced
- Survival in plankton
- Survival of young fish
is the Number of eggs produced an important factor in adult recruitment
- 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
what thesis explains the factor of Survival in the plankton in adult recruitment. what does it show?
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)
what increases the survival rate of plankton
getting out of the plankton stage as quick as possible - longer in plankton, longer to be eaten
what 2 main factors influence growth rate of larvae (and get them out of the plankton stage as quick as possible)
- Temperature - increase metabolic rates
- Food - need more food to pay for the increase in metabolic rates
- so these both factors are related
what’s the problem with estimating growth rates in the lab
Blaxter (1988) suggested that fish larvae may grow faster in the lab than in the field
what do researches need to estimate in order to look at the growth rates of fish in the wild
The age of the fish larvae
The mortality rate of the fish larvae
how can researchers estimate the age of the fish larvae to estimate the growth rates of fish in the wild
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
2 problems using otoliths to estimate the age of the fish larvae to estimate the growth rates of fish in the wild
- Preparation time - catching larvae, sectioning, cutting, microscopy etc. - expensive + requires expertise
- Resolving the different growth rings - need expertise
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?
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)
what do fish larvae eat
1.dinoflagellates (small - eat when first hatch)
2.nauplii (bigger - eat as they grow)
3.copepodite (bigger again - eat as they grow)
what factors affects prey selections for fish
- gape limitation
- wind/mixing of water column - Lasker’s stability hypothesis
- time/season - match-miss match hypothesis
explain the gape limitation factor that affects prey availability for fish
if it fits in their mouth, they can eat
explain the wind mixing factor that affects prey availability for fish
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
explain the time/season factor that affects prey availability for fish
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
what case study looks at the Match-mismatch hypothesis and how does it do that
Beaugrand et al. (2003) - looks at plankton effect on cod recruitment in the North Sea
what’s the background of the Beaugrand et al. (2003) case study
- 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?
what’s the methods used for the Beaugrand et al. (2003) case study
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
what’s the results of the Beaugrand et al. (2003) case study
- 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
what other factor may play a role in the survival of fish larvae other than food availability
predation
what is Density-dependent mortality
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
why does adult recruitment possibly depend on Survival of young fish
The number of post-larval fish (juveniles) is directly linked to number of 0-group fish e.g. atlantic-norweigon cod
