Lecture 5 - Estimating animal abundance: Catch-effort

Question 1

What is the catch?

Answer 1

a known number of animals are removed from a habitat

Question 2

What is effort?

Answer 2

Trapping method

Question 3

What is the harvest period?

Answer 3

period of time that animals are trapped/caught/removed

Run=time on graph of catch-recovery

Question 4

What is catch-effort?

Answer 4

a known number of animals are removed (=catch) from a habitat by trapping (=effort) over a period of time (=harvest period)

Question 5

What is catchability?

Answer 5

The slope of the Leslie catch-effort plot and is the rate at which the trap catches decreases. CPUE decreases as accumulated catch (ki) increases
- slope is exploitation

Question 6

How do you use the model to estimate the initial size of a population?

Answer 6

the initial size N of an exploited population at the end of the harvest by extrapolating the line to the x-axis

Question 7

When does this method work?

Answer 7

When the population has been exploited to the point where harvesting at each sample time requires greater and greater catching effort.
ie. # of fish caught in a net declines after each successive fishing trip at a constant rate dictated by the slope.

Question 8

When is the method used and why?

Answer 8

When a season of harvest is over to calculate the initial population at the beginning of the season

Question 9

Why is there the possibility of huge bias to be committed?

Answer 9

Related to the size of the net, volunteerism in reporting from the commercial sector, and is bycatch reported at all?

Question 10

What data is required for the model?

Answer 10

1) catch and effort are used to calculate CPUE

2) accumulated catch ki

Question 11

What do you do with the data that is collected?

Answer 11

Calculate CPUE and ki and plot CPUE against ki

Calculate the linear regression and extrapolate to the x-axis to determine N (hat)

Question 12

What happens when CPUE=0?

Answer 12

Represents the exhaustion of the catch

Question 13

What are the assumptions of the catch-effort model?

Answer 13

1) population is closed
2) individuals have the same probability of being caught throughout the sampling period
3) an animal can be harvested to the point of near extermination

Question 14

What is meant by a closed population?

Answer 14

N is constant and there are no births/deaths or immigration/emigration

Question 15

How good is the extrapolation?

Answer 15

It is only as good as the R2 value and if the variability is large the probability model may not be a good fit

Question 16

What should a linear plot of CPUE(Leslie plot) reveal?

Answer 16

a linear plot should reveal a straight line

Question 17

What happens if the linear plot regression line is not straight?

Answer 17

Then the basic assumptions of the model are violated

Question 18

What do you check for the fit of the model?

Answer 18

The R2 (variability of data from the mean/expected?) 
Parametric assumption/meet statistical assumptions?

Question 19

What are the parametric assumptions in order of importance?

Answer 19

(No bias)
Homogeneity of variance
Independence
Normality (less important because analysis is robust to variations in normality)
- Brought in on top of model specific assumptions

Question 20

What can the regression equation be used for?

Answer 20

To determine N (when x=0) but there may be rounding errors (using Krebs this isn’t as much of an issue)

Question 21

What does Krebs recommend to do in addition to the Leslie plot?

Answer 21

A Ricker Semi-log plot that is used to visually inspect the CPUE data for linearity

Question 22

What does the Ricker Semi-log plot make possible?

Answer 22

Makes it possible to visually inspect the data to make sure the underlying assumptions are not being violated
The plot may straighten the line and improvement in the line implies assumptions (parametric) are not violated

Question 23

What is the 10% rule?

Answer 23

A conservative approach to catch-recovery
Allow the population to recover
Only harvest 10% of k

Question 24

What is a disadvantage of the removal method?

Answer 24

catch-effort has poor precision and a large potential for bias
and it doesn’t work if you don’t meet parametric assumptions

Question 25

What is the rise in the catch-recovery?

Answer 25

the catch (n, population size)

Question 26

What do you get if the parametric assumptions are met? Advantage

Answer 26

you can get a population estimate from commercial estimates

Question 27

If you fit a linear regression model, what ‘baggage’ have you brought?

Answer 27

The fact that parametric assumptions must be met. It becomes a parametric analysis with all the ‘baggage’ that comes with it.

Question 28

What happens if a population drops below k/2?

Answer 28

approaches extinction/extirpation/ collapse

Question 29

What is happening if the population is above k/2?

Answer 29

hopefully recovery

Question 30

What is the MSY?

Answer 30

Maximum Sustainable Yield

• The inflection point at the maximum rise/run
• Stay above for best management

Question 31

Why do we test for linearity?

Answer 31

So that we can extrapolate the line down to the x-axis and get a good estimate

Question 32

What kind of slope do we want?

Answer 32

steep slope, can’t use model if slope approaches 0

Question 33

What is the x-axis where y=0?

Answer 33

where the population total at start of the fishery is and CPUE is 0 (ie no catch)

Question 34

What do we need to know to get k/2 and how do we get it?

Answer 34

Must know K

- only way to know K is to monitor a lot, volunteerism alone won’t work

Question 35

What does the Leslie plot give?

Answer 35

The initial population at the end of the catch, but only works if you can draw the population down with a constant slope (linearity)

Question 36

What happens when CPUE=0

Answer 36

The population is 0