Mortality

Question 1

How is mortality expressed?

Answer 1

as an instantaneous loss rate → rate of change in stock size at a particular moment (not over one year, one day, etc.)

Question 2

How would you describe this equation?

Answer 2

Question 3

What is Z represent?

Answer 3

typically used to denote the total instantaneous loss rate

Question 4

Instantaneous loss rate →

Answer 4

current population numbers can be described as the exponential decay of previous population numbers

Question 5

N1/N0 is an expression of what?

Answer 5

Survival (S)

Question 6

Survival

Answer 6

annual proportion of population that survives

Question 7

What does this expression interpret?

Answer 7

Relationship between instantaneous mortality and annual survival rates

Question 8

What are the two components of mortality?

Answer 8

F & M

Question 9

F

Answer 9

instantaneous fishing mortality rate

Question 10

M

Answer 10

instantaneous natural mortality rate

Question 11

Z =

Answer 11

F+M

Question 12

F Includes

Answer 12

all deaths from fishing activities
Direct removals: fish caught & kept
Indirect removals: fish that are not kept but are still killed from fishing activities (discards, cryptic mortality)

Question 13

M includes

Answer 13

mortality from many sources (starvation, disease, senescence, spawning stress, predation, abiotic factors, etc.)
Often assumed constant over life of fish

Question 14

Explain Baranov’s catch equation:

Answer 14

Question 15

WHy is Baranov’s catch equation so useful?

Answer 15

Links things that we observe (catch) with things we want to know about but don’t have numbers. Does this base on the exponential decay model.

Question 16

M is a critical parameter in fisheries stock assessments. Why?

Answer 16

M relates directly to productivity of stock → higher M, higher optimal fishing mortality (generally)

Question 17

WHy is M difficult to measure?

Answer 17

do not observe directly); easier for unfished populations (age/size composition of catch and exponential decay)

Question 18

What are the different methods used to estimate M?

Answer 18

Information-intensive methods
Information-limited methods

Question 19

Information-intensive methods

Answer 19

tagging, catch-curve analysis, integrated stock assessment models

Question 20

Information-limited methods

Answer 20

estimation using meta-analyses and life history theory/parameters

Question 21

What is a common Mark-recapture method?

Answer 21

Lincoln-Peterson method

Question 22

Lincoln-Peterson method

Answer 22

This estimates population abundance

Question 23

What are the steps in the Lincln-Peterson method?

Answer 23

Collect and tag nA animals
Sample again (may include fishery) and collect RB animals → RA,B marked (from 1)
Fraction of marked animals from 2 (RA,B/RB) should equal the fraction of population marked in 1 (i.e., nA/N where N is population size at 1)

Question 24

How does the Lincoln-Petersen method estimate population?

Answer 24

We can estimate the population as the number that are tagged times the number of times in the second sampling period divided by the number that were marked.

Question 25

How do we used the Lincoln_peterson method to estimate mortality?

Answer 25

The estimation of mortality cannot be accurately determined on its own, but rather over multiple years. As steps two and three are completed over time, the number of tagged individuals will gradually decrease due to mortality. By observing this change, valuable information regarding mortality can be obtained.

Question 26

What is assumed in the Lincoln-Peterson method?

Answer 26

tagged & untagged fish mix randomly before capture and have equal catchability by fishery; no tag loss

tagging process does not affect survival

Question 27

If catch age or length is known, what can this inform?

Answer 27

can use simple regression methods to recover estimate of Z

Z = F + M → can determine M if we know F

Question 28

What does the catch curve analysis assume?

Answer 28

constant mortality once recruited;
catch proportional to abundance;
constant fishing vulnerability across ages/lengths;
constant recruitment

Question 29

How would recruitment affect natural mortality?

Answer 29

High recruitment in the older age class would make this line start to flatten out.

Question 30

What are three life history parameters that have been correlated with natural mortality?

Answer 30

Maximum age
Growth
Reproductive effort

Question 31

Maximum age methods - often used

Answer 31

The inverse relationship between maximum age (Tmax) and M

Most informative for non- or lightly-exploited stocks

Question 32

Methods based on growth

Answer 32

Smaller and faster-growing species should have higher rates of natural mortality

can use the von Bertalanffy growth equation.