Laboratory 3: Age
Structure and
Survivorship
Age Structure and Survivorship

Populations consist of multiple cohorts which
vary greatly in age

Demography – the study of the vital statistics
of a population
May assign individuals to an age class based upon
life history and stage of development
 e.g. juvenile
sub-adult adult

How do we age individuals?
otoliths
scales
90
80
70
60
50
40
30
20
10
0
1988/89 F (n=802)
Fork Length (mm)
52
0
50
0
48
0
46
0
44
0
42
0
40
0
38
0
36
0
34
0
32
0
30
0
28
0
26
0
24
0
22
0
20
0
18
0
16
0
14
0
1988/89 M (n=84)
12
0
10
0
Frequency
Proportions of individuals
belonging to age groups are
referred to as the age structure
or age distribution of a
population
Age Structure and Survivorship
Cohort – group of individuals of the same age
in a population
90
80
70
60
50
40
30
20
10
0
1988/89 F (n=802)
52
0
50
0
48
0
46
0
44
0
42
0
40
0
38
0
36
0
34
0
32
0
30
0
28
0
26
0
24
0
22
0
20
0
18
0
16
0
14
0
1988/89 M (n=84)
12
0
10
0
Frequency

Fork Length (mm)
120
80
1994/95 F (n=942)
60
1994/95 M (n=316)
40
20
Fork Length (mm)
52
0
50
0
48
0
46
0
44
0
42
0
40
0
38
0
36
0
34
0
32
0
30
0
28
0
26
0
24
0
22
0
20
0
18
0
16
0
14
0
12
0
0
10
0
Frequency
100
Age Pyramids

Can be used to compare
population structure
among many different
sites or the same
population at different
times of the year or from
year to year
Tuscaloosa County, AL 2000 Census
Life Tables



Compile stats for each age class or cohort
Must first identify the number and type of age interval
for the analysis
Assign collected individuals to the appropriate age class
x
nx
lx
dx
qx
Lx
Tx
ex
0-1
93
1000
204
0.204
898
1858
1.858
1-2
74
796
452
0.568
570
960
1.21
2-3
32
344
139
0.561
248
390
1.13
3-4
14
151
97
0.642
103
142
0.94
4-5
5
54
43
0.796
33
39
0.72
5-6
1
11
11
1
6
6
0.55
6-7
0
0
0
0
0
0
Survivorship Curves



Type 1 = High survival
rate for young, low
survivor rate after a
particular old age
Type 2 = A constant rate
of mortality occurs at all
ages
Type 3 = High juvenile
mortality rate with low
mortality thereafter
Point aux Pins




Collect young-of-the-year blue crabs from seagrass beds using
seines
Collect first 100 crabs
In lab, assign crabs to age classes based on carapace width (p.23)
Construct age pyramid, life table, and survivorship curve
Point aux Pins
Seine for first 100 Y-O-Y
blue crabs (C. sapidus) and
put in bag on ice
Point aux Pins
Back in lab, measure carapace width of each
crab. Age each crab according to table on p. 23.
Record data and make tables and figures.