Lecture 6 - Stock Assessment

Question 1

Stock (ideal definition)

Answer 1

A reproductively isolated population of a species

Question 2

The population dynamics of each stock are ___ of each other

Answer 2

independent

Question 3

Stocks are identified using 1)___ and 2)___

Answer 3

1) distribution

2) genetics

Question 4

Mark/recapture (tagging) has shown 1)___ and that there is not much 2)___ or 3)___ to/from a stock

Answer 4

1) fidelity
2) immigration
3) emmigration

Question 5

Stock (practical definition)

Answer 5

A basic management unit

Question 6

Maximum sustainable yield (definition)

Answer 6

The maximum catch that can be routinely taken from the stock year after year without resulting in stock collapse

Question 7

Schaefer model

Answer 7

uses only catch and effort data to estimate MSY

Question 8

Beverton-Holt model

Answer 8

uses population dynamics to estimate MSY

Question 9

Why are models such as Schaefer and Beverton-Holt necessary?

Answer 9

Stock biomass is difficult to measure, but data on catch and effort are readily available and stock biomass is related to catch

Question 10

How are stock biomass (P) related to catch-per-unit-effort (CPUE)?

Answer 10

If stock biomass is high, it’s easy to catch fish. If stock biomass is low, it’s hard to catch fish

Question 11

Calculation for CPUE

Answer 11

CPUE = qP
where q = catchability coefficient (proportion of stock removed by a unit of effort)
and P = stock biomass

Question 12

How is catch data collected?

Answer 12

1) mandatory catch reporting
2) monitoring of taxes and required dealer sale reports
3) monitoring of landings at major ports

Question 13

According to a fishery scientist’s view of population growth, what adds and removes individuals from a stock biomass?

Answer 13

Add: growth and recruitment

Remove: natural and fishing mortality

Question 14

Recruitment (definition)

Answer 14

The number of individuals entering the population; the new young fish

Question 15

Growth (definition)

Answer 15

Growth of individuals, which increases the total stock biomass

Question 16

Unlimited population growth is based on what kind of equation?

Answer 16

Exponential

Question 17

Limited population growth is based on what kind of equation?

Answer 17

Logistic

Question 18

rate of change in population calculation

Answer 18

rate of change = rN (1 - N/K)

where N = population size
r = reproductive capacity of the species
K = carrying capacity of the ecosystem

Question 19

When is the growth rate of a population highest?

Answer 19

At intermediate stock biomass

Question 20

Why does population growth rate vary with population size?

Answer 20

As a stock size is reduced, growth increases, recruitment increase, and natural mortality decreases

Question 21

Components of the Beverton-Holt Model

Answer 21

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

Question 22

Using the Beverton-Holt model, how would you calculate the amount of fish that can be removed without changing the stock size?

Answer 22

G - M

Question 23

Why does mortality decline with fishing (reduction ins stock)?

Answer 23

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

Question 24

Why does gross production decline with fishing less rapidly than mortality declines?

Answer 24

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

Question 25

True or false. It is easy to estimate the parameters for the Beverton-Holt model.

Answer 25

False

Question 26

How do you estimate stock size?

Answer 26

1) visual surveys (coastal, aerial, fish ladders)
2) acoustic surveys
3) trawl or fishing surveys
4) tag (mark then recapture)

Question 27

The age structure of a stock can be used to estimate 1)___, 2)___, and 3)___

Answer 27

1) growth rates
2) recruitment
3) mortality

Question 28

What are some structures used to estimate the ages of fish?

Answer 28

1) otolith (rings of fast and slow growth bands)
2) scales
3) fin spines
4) vertebrae

Question 29

What are some problems with MSY?

Answer 29

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

Question 30

True or false. MSY is not a target in modern fisheries management.

Answer 30

True

Question 31

Otolith

Answer 31

Fish ear bone