Recruitment Flashcards
recruitment
number of fish surviving to enter the fishery
recruitment: defined by authors in different ways:
- 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
adult recruitment depends on 3 possible factors
- number of eggs produced
- survival in plankton
- survival of young fish
- number of eggs produced
recruitment = poorly related to size of spawning stock
annual variations in recruitment = combination of factors, incl spawning stock
occasional v large year classes = irregular (e.g. N.Sea, 4-11yrs), can sustain stocks for several years, not a function of spawning stock
- Survival in the plankton
Ricker-Foerster Thesis:
longer in plankton -> longer to be eaten
what influences growth rate of larvae?
2 main factors:
1. temp
2. food
To do field research need ways to estimate:
- age of fish larvae
- mortality rate of fish larvae
ageing of fish larvae
Otoliths - daily growth rings
allows growth rate to be calculated
problems using otoliths
- preparation time - sectioning, cutting, etc.
- resolving the different growth rings
estimating mortality rate
calculated from size-frequency histograms
PROBLEM - catching larvae quantitatively (avoidance)
quantified by comparing depth-integrated day/night catches
Field studies - link between larvae + food
what do fish larvae eat?
Dinoflagellates
Nauplii
Copepodite
Prey selection
factor affecting food selection
gape limitation
Density-dependent mortality
when there are lots of fish larvae they may crop down their prey so much that survival is reduced
- recruitment
does larval survival influence actual recruitment?
may be mortality of juvenile fish that is more important
number of post-larval fish = directly linked to number of 0-group fish (Arcto-Norwegian Cod)
