Mortality Flashcards
Key concepts associated with Mortality
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.
How is mortality expressed?
as an instantaneous loss rate → rate of change in stock size at a particular moment (not over one year, one day, etc.)
How would you describe this equation?
What is Z represent?
typically used to denote the total instantaneous loss rate
What is important to note about absolute loss?
will depend on population size (decrease by larger amount when population large)
Instantaneous loss rate →
current population numbers can be described as the exponential decay of previous population numbers
N1/N0 is an expression of what?
Survival (S)
Survival
annual proportion of population that survives
What does this expression interpret?
Relationship between instantaneous mortality and annual survival rates
What does this expression interpret?
Total annual mortality rate
Additive rates
Instantaneous rates are additive
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%)
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.
Why would you not just add the two percentages together?
: Z has no limit but A cannot exceed 1
What are the two components of mortality?
F & M
F
instantaneous fishing mortality rate
M
instantaneous natural mortality rate
Z =
F+M
F Includes
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)
M includes
mortality from many sources (starvation, disease, senescence, spawning stress, predation, abiotic factors, etc.)
Often assumed constant over life of fish
Explain Baranov’s catch equation:
WHy is Baranov’s catch equation so useful?
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.
M is a critical parameter in fisheries stock assessments. Why?
M relates directly to productivity of stock → higher M, higher optimal fishing mortality (generally)
WHy is M difficult to measure?
do not observe directly); easier for unfished populations (age/size composition of catch and exponential decay)
What are the different methods used to estimate M?
Information-intensive methods
Information-limited methods
Information-intensive methods
tagging, catch-curve analysis, integrated stock assessment models
Information-limited methods
estimation using meta-analyses and life history theory/parameters
What is a common Mark-recapture method?
Lincoln-Peterson method
Lincoln-Peterson method
This estimates population abundance
What are the steps in the Lincln-Peterson method?
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)
How does the Lincoln-Petersen method estimate population?
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.
How do we used the Lincoln_peterson method to estimate mortality?
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.
What is assumed in the Lincoln-Peterson method?
tagged & untagged fish mix randomly before capture and have equal catchability by fishery; no tag loss
tagging process does not affect survival
If catch age or length is known, what can this inform?
can use simple regression methods to recover estimate of Z
Z = F + M → can determine M if we know F
What does the catch curve analysis assume?
constant mortality once recruited;
catch proportional to abundance;
constant fishing vulnerability across ages/lengths;
constant recruitment
How would recruitment affect natural mortality?
High recruitment in the older age class would make this line start to flatten out.
What are three life history parameters that have been correlated with natural mortality?
Maximum age
Growth
Reproductive effort
Maximum age methods - often used
The inverse relationship between maximum age (Tmax) and M
Most informative for non- or lightly-exploited stocks
Methods based on growth
Smaller and faster-growing species should have higher rates of natural mortality
can use the von Bertalanffy growth equation.
