Mortality

Question 1

Key concepts associated with Mortality

Answer 1

There are various factors that may lead to mortality, both directly and indirectly. It is essential to comprehend these mechanisms and their cumulative impact. One of the significant drivers of population dynamics is mortality, whether it be natural or due to fishing. These parameters are crucial in age-structured population models.

Question 2

How is mortality expressed?

Answer 2

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

Question 3

How would you describe this equation?

Answer 3

Question 4

What is Z represent?

Answer 4

typically used to denote the total instantaneous loss rate

Question 5

What is important to note about absolute loss?

Answer 5

will depend on population size (decrease by larger amount when population large)

Question 6

Instantaneous loss rate →

Answer 6

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

Question 7

N1/N0 is an expression of what?

Answer 7

Survival (S)

Question 8

Survival

Answer 8

annual proportion of population that survives

Question 9

What does this expression interpret?

Answer 9

Relationship between instantaneous mortality and annual survival rates

Question 10

What does this expression interpret?

Answer 10

Total annual mortality rate

Question 11

Additive rates

Answer 11

Instantaneous rates are additive

Question 12

How would you go about solving this equation?

What is the total annual mortality rate for a fish stock that experiences 80% mortality during winter & spring and 40% mortality during summer & fall? (Hint: it’s not 120%)

Answer 12

Using the equation that looks at the relationship between instantaneous mortality and annual survival rates for both seasons. Then add those together to get the total annual mortality.

Question 13

Why would you not just add the two percentages together?

Answer 13

: Z has no limit but A cannot exceed 1

Question 14

What are the two components of mortality?

Answer 14

F & M

Question 15

F

Answer 15

instantaneous fishing mortality rate

Question 16

M

Answer 16

instantaneous natural mortality rate

Question 17

Z =

Answer 17

F+M

Question 18

F Includes

Answer 18

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 19

M includes

Answer 19

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

Question 20

Explain Baranov’s catch equation:

Answer 20

Question 21

WHy is Baranov’s catch equation so useful?

Answer 21

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 22

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

Answer 22

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

Question 23

WHy is M difficult to measure?

Answer 23

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

Question 24

What are the different methods used to estimate M?

Answer 24

Information-intensive methods
Information-limited methods

Question 25

Information-intensive methods

Answer 25

tagging, catch-curve analysis, integrated stock assessment models

Question 26

Information-limited methods

Answer 26

estimation using meta-analyses and life history theory/parameters

Question 27

What is a common Mark-recapture method?

Answer 27

Lincoln-Peterson method

Question 28

Lincoln-Peterson method

Answer 28

This estimates population abundance

Question 29

What are the steps in the Lincln-Peterson method?

Answer 29

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 30

How does the Lincoln-Petersen method estimate population?

Answer 30

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 31

How do we used the Lincoln_peterson method to estimate mortality?

Answer 31

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 32

What is assumed in the Lincoln-Peterson method?

Answer 32

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

tagging process does not affect survival

Question 33

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

Answer 33

can use simple regression methods to recover estimate of Z

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

Question 34

What does the catch curve analysis assume?

Answer 34

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

Question 35

How would recruitment affect natural mortality?

Answer 35

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

Question 36

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

Answer 36

Maximum age
Growth
Reproductive effort

Question 37

Maximum age methods - often used

Answer 37

The inverse relationship between maximum age (Tmax) and M

Most informative for non- or lightly-exploited stocks

Question 38

Methods based on growth

Answer 38

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

can use the von Bertalanffy growth equation.