pMark-Recapture Sampling
for Analysis of Population Abundance & Density
We will study the abundance and density of populations of the California Horn Snail, Cerithidia
californica, which lives in the intertidal mudflats of estuaries, lagoons, and salt marshes of coastal
California.
Theory
We will use the Mark-Recapture method to estimate population size. This method was developed
to sample populations of animals that are highly mobile, and for which it would be difficult to know
how effectively you had sampled with only a single sampling time. Although snails are not “highly
mobile,” they make a good organism with which to practice the technique. Mark-recapture involves
taking an initial random sample of animals at a first collection, marking the animals (total of M
animals), and returning them to their habitat. Then a second sampling is made at a later period (several
days to many days depending upon species mobility, reproductive potential, etc.) within the same
sample area. Within this second sample one records the total number of snails sampled (n) and the
number of marked snails recaptured in the second sample (R). It is assumed that the proportion of
marked animals that are captured (R) in the second sample (n) is proportional to the total number of
marked animals (M) in the population (N):
R
n
M
= N
or, rearranging:
N =
M*n
R
This estimate of population size is called the Lincoln - Peterson Index. It has been shown that this
estimate is biased, and slightly overestimates the size of the population. An unbiased estimate was
developed by Chapman:
Nc =
(M + 1) * (n + 1)
–1
(R + 1)
Estimates of Standard Error of the Mean and Confidence Limits, as well as a method of testing for
differences between population means (t-test) are shown on page 126 of the lab manual (Eqs. 7 & 9).
SE =
t =
Procedures
(M + 1).(n + 1) * (M – R).(n – R)
(R + 1)2.(R + 2)
| N1 – N2 |
(SE1)2 + (SE2)2
Collection Period #1: Initial Collecting & Marking
Form groups of ~3 students
Determine area to be sampled
(Each group will be responsible for 2 samples = different microhabitats)
Lay out 2 quadrats of the same size in the area to be sampled (Mark the spot!)
(Try not to trample snails in habitat)
Sample snails – Methods must be repeatable at next collection date
(For example, you may subdivide the quadrat and collect for 30
seconds in each subquadrat [you should collect a total of 20 - 50 snails per sub-quadrat] )
Collect snails in a container, initially – then spread onto paper toweling
Thoroughly dry a part of the shell & put large dot of nail polish on that area
Record the number of snails collected and the color used to mark
Return snails back to habitat, scattering them around within area sampled
(Be careful not to trample)
Wash hands when you return to lab!
Collection #2: Resampling the same quadrats
2 weeks later we will return to the area and resample the same quadrats
Use exactly the same method as you used in the first sampling
Record total nos. of snails collected in the second sample (= n) and the nos. of marked snails (= R)
Return all snails back to habitat
Wash hands when you return to lab!
Calculations:
Use Chapman modification of Lincoln-Peterson Index for calculating Population Density. You should
calculate a SE and confidence interval for each sample (Eq. 7 & 8, p. 126). Finally, we will share the
data of the different groups and test (t-test) whether there is difference between the population
estimates obtained from one particular type of snail microhabitat versus the other.
Data Sheet
Mark & Recapture
Investigators:
Date & Time:
Collection #1:
Collection #2:
Location & Habitat Description (incl. markers used for quadrat):
Distance to water:
Height above/below mean low tide:
Species i.d. & observations:
(include color and location of marking)
Data:
Sample #
(M)
Initial Capture
The Estuary system
(n)
Second Capture
(R)
# Recaptured
Total #
per meter2
Mixture of salt and fresh water.
Degree of saltiness fluctuates with rainfall and tides.
Very rich in nutrient b/c of input from river system —> very high NPP (no nutrient limitations)
California horn snail grazes on algae (e.g., sea lettuce, enteromorpha) in mudflats.
Gould's bubble snail and moon snail are carnivorous; prey on horn snail
Scallops, razor clams, fiddler crabs, mudflat crabs, nudibranchs, pipefish can all be found in
mudflat community.
Aquatic part of estuary is nursery for fish; many species of sport and commercial fish.
These fry are food for shorebirds —> great diversity (seasonal shifts b/c of migrations
Terrestrial plants adapted to saline conditions (halophytes) are found from the high tide area up to the
area where water no longer reaches by capillarity through the soil (= salt marsh community)
Plant adaptations (halophytes):
Salt excreters (actively transport salt out of leaf):
Distichlis spicata (salt grass), Limnonium (sea lavender), Spartina foliosa (cordgrass)
Salt sequesterers (put salt into non-photosynthetic components, which later fall off):
Salicornia subterminalis (pickle weed), Batis maritime (salt wort)
Salt excluder/selecter (avoids uptake):
Jaumea carnosa (Jaumea), Frankenia grandiflora (Alakali heath) [not sure about Frankenia?]
Note zonation in salt marsh; different problems in low zone vs. high zone:
Low zone (boundary between sea & marsh):
Standing water, silts & clays deposited, produce sticky mud
Controls on plants:
1. Increased periods of inundation
2. waterlogged, anerobic soils.
Spartina has a hollow tube in stem to transport O2
High zone (boundary between marsh and terrestrial community):
Decreased water availability, increased salt because of evaporation
Controls on plants:
1. Drier, saltier environment
2. Shifting sands if adjacent community is coastal strand
Note: 91% of coastal wetlands of San Diego County have been destroyed by “development” !