Problem Set #8a: Life Tables

Scenario

 You are conducting research on bull trout in the Cottonwood Creek and Rattlesnake Creek drainages. The Cottonwood Creek population is a resident population of bull trout characterized by slow growth and late maturity. The drainage is relatively protected and is closed to fishing. Conversely, the Rattlesnake Creek population consists of mostly hatchery-raised fish and is characterized by fast growth and early maturity. The drainage is open to fishing and some logging is occurring in the area.

Idaho Fish and Game has collected the following age-structure information on bull trout in the Cottonwood Creek and Rattlesnake Creek drainages:

Age (yrs)	Cottonwood Creek	Rattlesnake Creek
0	3000	8000
1	200	250
2	150	100
3	75	50
4	70	5
5	45	1
6	31	0
7	14	0
8	10	0
9	6	0
10	1	0

Assume these numbers were derived by dividing the age-specific population estimates by 2 (i.e., assuming a 50:50 sex ratio). In other words, the numbers in the above table represent females only.

Age- or size-specific fecundity has not been well defined for bull trout. Available information from simulations and limited sampling suggests the following fecundity schedules for the 2 populations in question:

Age (yrs)	Cottonwood Creek (slow growth, late maturity)	Rattlesnake Creek (fast growth, early maturity)
0	0	0
1	0	0
2	0	0
3	0	0
4	0	500
5	0	500
6	0	---
7	220	---
8	220	---
9	220	---
10	220	---

Note: The numbers in this table were derived by taking the average number of eggs produced by a female and then dividing by 2 (i.e., assuming a 50:50 primary sex ratio). Consequently, the numbers in the table are female offspring produced per female of a given age in the population . Of course, this is based on the assumption that all females of that age are reproductively active.

Use this information to construct an extended life table for each population. Answer the following questions.

1. Would the life table you created best be described as a cohort, time-specific, or composite life table?  What assumptions would you have to make in order for this life table to be valid?

2. Plot the survivorship (on a log scale) and mortality curves for both populations. Are the curves for each population different? If so, how are they different and what are the implications of the observed differences?

3. What are the net reproductive rate, intrinsic rate of increase, and finite rate of population change for these 2 populations.  Define each of these parameters.

4. Are these populations increasing, decreasing, or relatively stable? How do you know?

5. When life tables are used to estimate rates of population change, we assume the population has a stable age structure. Define this concept. What is the stable age structure for the Cottonwood and Rattlesnake Creek populations? Note: you can just highlight that portion of your spreadsheet(s) that contains these values.

6. What is the finite survival rate for the time periods (1) egg to age 1, and (2) from sexual maturity (i,e., when fecundity >0) to the last-known breeding age.  

Please turn in the following (due next week):

• Hard copies of your worksheets.

• Plots of survivorship and mortality curves (with proper labels and titles).

• Answers to the questions (typed and double-spaced).

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Revised: 25 August 2011