Lecture 6 - Stock Assessment Flashcards
Stock (ideal definition)
A reproductively isolated population of a species
The population dynamics of each stock are ___ of each other
independent
Stocks are identified using 1)___ and 2)___
1) distribution
2) genetics
Mark/recapture (tagging) has shown 1)___ and that there is not much 2)___ or 3)___ to/from a stock
1) fidelity
2) immigration
3) emmigration
Stock (practical definition)
A basic management unit
Maximum sustainable yield (definition)
The maximum catch that can be routinely taken from the stock year after year without resulting in stock collapse
Schaefer model
uses only catch and effort data to estimate MSY
Beverton-Holt model
uses population dynamics to estimate MSY
Why are models such as Schaefer and Beverton-Holt necessary?
Stock biomass is difficult to measure, but data on catch and effort are readily available and stock biomass is related to catch
How are stock biomass (P) related to catch-per-unit-effort (CPUE)?
If stock biomass is high, it’s easy to catch fish. If stock biomass is low, it’s hard to catch fish
Calculation for CPUE
CPUE = qP
where q = catchability coefficient (proportion of stock removed by a unit of effort)
and P = stock biomass
How is catch data collected?
1) mandatory catch reporting
2) monitoring of taxes and required dealer sale reports
3) monitoring of landings at major ports
According to a fishery scientist’s view of population growth, what adds and removes individuals from a stock biomass?
Add: growth and recruitment
Remove: natural and fishing mortality
Recruitment (definition)
The number of individuals entering the population; the new young fish
Growth (definition)
Growth of individuals, which increases the total stock biomass
Unlimited population growth is based on what kind of equation?
Exponential
Limited population growth is based on what kind of equation?
Logistic
rate of change in population calculation
rate of change = rN (1 - N/K)
where N = population size
r = reproductive capacity of the species
K = carrying capacity of the ecosystem
When is the growth rate of a population highest?
At intermediate stock biomass
Why does population growth rate vary with population size?
As a stock size is reduced, growth increases, recruitment increase, and natural mortality decreases
Components of the Beverton-Holt Model
Gross production of population (G) = per capita growth x population size + recruitment
Gross mortality of the population (M) = per capita natural mortality x population size
Fishing mortality
Using the Beverton-Holt model, how would you calculate the amount of fish that can be removed without changing the stock size?
G - M
Why does mortality decline with fishing (reduction ins stock)?
1) Caught fish don’t die a natural death
2) A fished stock is a younger stock, with a lower death rate
3) Individuals in a fished stock have access to more resources, so they are healthier and have a lower death rate
Why does gross production decline with fishing less rapidly than mortality declines?
1) Individuals in a fished stock have access to more resources, so they grow faster and have higher fecundity
2) In some cases recruitment is higher in fished stock due to a lack of competition with adults
True or false. It is easy to estimate the parameters for the Beverton-Holt model.
False
How do you estimate stock size?
1) visual surveys (coastal, aerial, fish ladders)
2) acoustic surveys
3) trawl or fishing surveys
4) tag (mark then recapture)
The age structure of a stock can be used to estimate 1)___, 2)___, and 3)___
1) growth rates
2) recruitment
3) mortality
What are some structures used to estimate the ages of fish?
1) otolith (rings of fast and slow growth bands)
2) scales
3) fin spines
4) vertebrae
What are some problems with MSY?
1) Assumes catchability (q) remains constant (increased by technological improvements)
2) Assumes stock is in equilibrium and experiences no major variations in recruitment, growth, and mortality
3) For Schaefer model, it is difficult to determine MSY BEFORE stock is overexploited
4) stocks are unstable when fished at MSY
True or false. MSY is not a target in modern fisheries management.
True
Otolith
Fish ear bone
