How to do a Shellfish Census
out the Grid
Entry and Analysis
your research count
Ministry of Fisheries to see if your project fits
into national study objectives. Then, after finding
out what others are doing, selecting
a beach to survey, and mapping
the beach, the next step is to set up a sampling
plan. The three parts of the sampling plan are:
- What key species
and what physical conditions will you measure?
- Where will
you take your samples on the beach?
- How often
will you resample the site?
What to measure?
Imagine that you are a detective, investigating the Cockle
Community to find out what, if anything, is killing them
off. Part of your investigation will be doing a census of
the cockles and, while doing this, keeping an eye out for
other characters in the neighbourhood.
There are many different kinds of plants and animals associated
with beaches and their tide flats. You probably don't want
to try to identify and measure everything you find in your
samples, but will select the most common and important species.
Cockles, pipis and tuatua are important to count and measure.
You might want to count their predators as well (oystercatchers,
predatory snails, and some species of starfish). You will
want to indicate the presence or absence of sea grass and
abundant algae on the transects.
You will want to be observant. Shellfish are vulnerable
at many different places in their life cycle. It might be
a good idea to find out when the shellfish actually spawn
so you can note what conditions are like when they do.
to get an idea of how healthy they are, you might mark some
shellfish in one research plot with a pencil and remeasure
them two times a year to plot their growth rate. Most shellfish
grow fastest when very young and then more slowly as they
mature (like we do). Cockles, for example grow at about
10-mm a year for the first two years and then more slowly.
As they get older they stop growing and may even shrink
in size. Their rate of growth is an important index of their
health and well-being.
Biologists believe cockles have a major recruitment every
5 years but for some reason this has not happened on schedule.
Something might be killing the very small ones. So use a
magnifying lens to look for very small specimens, less than
10-mm in shell length. The best way to find them is to sieve
the sand with a kitchen-style strainer which has a wire
mesh size of about 1 millimetre. If you find large numbers
of baby shellfish, report this to your Fisheries contact
Some physical conditions are also important. You should
note the relative exposure of the beach to waves and provide
a general description of the substrate - is it a sand beach,
a muddy sand beach, a mud flat, a coarse sand beach, a pebble
You should note any major shifts in the sediment resulting
from storms. If there seems to be a problem with sediment
from erosion, you might want to take core samples of the
beach and analyse changes in particle size over time. If
there seems to be a problem with beach erosion and loss
of sand, you might want to study the influx of storm water
onto the beach through drainage systems or rivers and the
currents that move along the shoreline.
Where to measure?
are counted at sampling stations set up along a series of
transects on the beach. The initial lay-out of the guide-lines
is important and it is best to work this out on the map
you have made, the one showing the rough outlines of the
shellfish beds. Then the lines can be set out according
to the sampling plan on the day of the survey.
The physical shape of the beach and the location of the
shellfish will determine how you design the sample grid.
Be sure to keep the following points in mind:
- Make sure
the grid covers the whole beach including the area where
the shellfish are located.
- You will want
to be able to set up the same grid again in the future
so it is important to start the ends of the base-line
- extending down the length of the tidal flat - at an
easy to find landmark on one end of the beach. It should
extend toward another landmark at the far end of the beach.
If there are no available landmarks, you will have to
determine the start and end points by taking BEARINGS.
((Taking Bearings - At each point, line up two permanent
landmarks (write them down and locate them on the map)
and then turn 60 to 90 degrees and fins two other land
marks to line up. Write these down and show them on your
map. When you need to relocate the same place all you
need to do is walk out on the beach with one set of landmarks
in line until the second set is also lined up.))
- Each beach
will have it's own sampling plan needs, but to make the
information comparable between different areas, use at
least five transects spaced at 50 meter intervals along
the base-line for small beaches and 100 meter intervals
along the base line for bigger beaches.
- Paper plans
have a way of getting complicated by the real world, so
before the survey, visit the beach and check out the proposed
lay-out. Keep in mind that the grid must be laid out and
the samples taken all in the time the shellfish beds are
exposed by the low tide.
- You might
have a biologist look over your sampling grid plan and
give advice before you start.
How often to
Your team should
redo the census at least once a year, but twice a year might
be required if you believe stresses on the beach are seasonal.
This may involve sampling before (spring) and after (autumn)
the period of peak harvesting pressure.
Your team should
redo the census if there is a major die-off, pollution spill,
or other event that might impact the shellfish. If someone
reports a sudden change in the numbers of shellfish, the
team should meet and discuss the situation and perhaps take
some quick test samples before organising a major census.
Start your base-line
from an easily recognised landmark. Record this on your
map as well as any bearings needed to find the same start
point in the future. Begin the lay-out in the morning, about
three hours after a very high tide. That way your teams
can follow the tide out as they work.
Using a measured
50-metre line, set out five base-line poles or coloured
cones at 50-metre (or 100-metre) intervals along the base
line. Be sure they are all lined up with each other between
the start and end points.
team begins its sampling at the site of the base-line pole.
Next, the team will start a series of measurements perpendicular
to the base-line, at intervals of 20-metres (40-metres for
very wide beaches) out towards the low tide line. These
transects should be perpendicular to the base line. This
can be done by eye, with a participant taking one pole out
onto the beach until the line is tight. Other members of
the team can stand back and direct the outer marker left
or right until the line appears to be at a right angle with
the base line.
A more exact
method of making sure the line is perpendicular is to have
a knot tied in the middle of the 20-metre line. One student
holds the knot underfoot at the reference pole while two
others take the end poles to either side so they line up
with the base-line. When the line is tight and the three
poles are in line with the next poles down the beach, the
knot is released and the pole (A) at one end is used to
scribe a circle on the sand using the far end as a centre.
The circle must pass the supposed perpendicular point. The
(A) pole is returned to the base line, the knot checked
to be sure it is still at the reference pole and then the
second pole (B) is used to scribe an arc out to the perpendicular.
The point where the two arcs cross will be perpendicular
to the reference pole. This point is marked with a team-member's
foot or any convenient marker. The (A) pole is then taken
to the reference pole and the (B) pole backed out until
the 20-metre line is straight. It should pass directly over
the perpendicular marker. The (B) pole is pushed into the
sand and that is the place for the second sample.
After the second
sample is taken, the (A) pole is brought forward until the
20-metre line is tight and it lines up with the reference
pole and the (B) pole that is still in the sand. The third
sample is taken. This process of "walking" the
poles out from the original reference pole is repeated after
each survey site so the team moves in a straight line.
team (and the results of your project) will benefit with
the involvement of all interested parties right from the
start. Conservation groups, local iwi, people who harvest
the shellfish, people who live near the beach, a representative
from the local or regional council (a biologist or resource
manager is ideal), your local fisheries officer, businesses
or business associations that might be involved in some
way with the beach. The Ministry for the Environment can
help provide information about the Resource Management Act
and other laws protecting the environment. The Department
of Conservation can help provide biological advice and information
about related conservation issues.
By inviting everyone to participate, your findings will
be more credible and of greater interest to everyone in
the community. Association with a particular adult group
- such as Rotary or the Royal Forest and Bird or a photographic
society - will add support to the actual process of sampling
and then analysing the results.
Divide the group
into teams. People who have done the census in the previous
year might become mentors for the newcomers. Have at least
four people on each sampling team plus one supervisor for
each two transects.
with a video or still camera, might roam between teams and
take photos of interesting specimens and of the activities
(useful for later media coverage, bulletin boards, etc.).
At least two
adults should supervise the whole operation.
- Hat, sunglasses
and sun-protection cream to protect against UV.
- Shoes that
can get wet are a good idea. Going barefoot is often a
bad idea due to sharp shells and broken glass.
- Drinks and
- A towel
1X 50 metre long line (100-metre for very long beaches)
to set up base line intervals.
1 marker pole or cone for each sampling lane down the beach.
team will require:
- 1 X 20 metre
long line (40-metre for very wide beaches) attached to
a vertical pole at either end.
- 2 buckets
or scoops to gather sea water
- 1 plastic
sieve (large) with 5-mm mesh (available from the Plastic
- 1 shovel
- 1 quadrat
sampling frame (see below)
- 2 two litre
ice cream containers to put sieved animals in.
- 1 plastic
tray with scribed size lanes from 5 to 55-mm.
- 1 clipboard
with coded datasheets.
- 1 waterproof
identification sheet with pictures and names of animals
to be recorded.
- (enough coded
plastic bags to carry specimens in for each quadrat)
is a square 315-mm on a side that limits the size of the
area sampled to one tenth of a square metre. The quadrat
can be made out of almost any rigid frame. You could make
quadrat frames out of heavy gauge wire bent to shape, lawn-edging
metal bent and pop-riveted together, aluminium edging, or
any other suitable material. A professional version would
be made of heavy gauge sheet aluminium 350-mm on a side,
150-mm deep, with a 25-mm flange bent outwards at the top
to step on when pushing it into the sand.
Measuring Tools. The basic idea is to sort the catch
into five millimetre size groups ranging from 5 to 100-mm
as measured along the longest possible axis.
You can measure
the shellfish in any way that provides accurate and reliable
measurements. One system is to put a ruler on a board with
a wood stop at the zero line. The shell is placed on the
ruler with one end against the stop. The person measuring
then reads the longest possible measurement, being careful
to look straight down to avoid parallax.
is to reproduce the measuring triangle
from the provided template and follow the instructions on
the sheet. Copies can be provided to each sampling team.
There are two sampling options. One involves each team collecting,
measuring and recording the shellfish as they work down
the beach. The second option is to have the teams simply
collect the shellfish and put them into coded ziplock bags.
The bags are then taken to a central counting and measuring
station or back to the classroom where they are counted
and measured. As long as the shellfish are returned to the
beach before the tide is in, they will be able to survive.
Counting and measuring on the beach is less stressful to
best carried out after an early morning high tide (8-9 AM)
by following the tide down the beach. The larger the tide,
the more time that is available for sampling.
team will work down one transect. Two or three participants
are responsible for locating the sample site, digging up
the sample, sieving it, and finally, after the specimens
have been measured and identified, returning the sample
back to its sample hole. If the specimens are measured on
the beach, two participants will count, measure and record
them. Otherwise they will bag the specimens in plastic bags
with the transect and quadrat number written on the bags
with a felt-tipped pen.
location is determined by the 20-metre (or 40-metre) line
between the two survey poles. Place the quadrat sampling
frame on the sand/mud. If you have made sheet metal frames,
push it into the mud to the flange. Dig out the contents
of the frame to the 150-mm depth with the shovel and put
the sand in the 5-mm sieve. Sieve the sand using sea water
to wash away the fine sand. The remaining material in the
sieve is sorted and all live cockles are counted and measured.
Other live animals are identified according to the interests
and abilities of the group.
which option is elected, two people on each sampling team
measure and record or the specimens are collected in coded
plastic bags and returned to a central measuring and counting
station. The option selected often depends on the number
of people available, weather conditions, and the beach conditions
(it being difficult to write data in high winds and sort
specimens and measure accurately in ankle deep water).
on the skills of the participants, a preselected range of
key plants and animals should be identified and noted as
present on each transect. (If the scientific name and exact
identification is not possible, general descriptions are
often sufficient - worm like, orange animal about 18-mm
long, green, lettuce like algae). Important key shellfish,
such as cockles and pipi should be measured and identified.
Unusual discoveries should be noted and perhaps photographed.
measured to the nearest 5-mm in shell length. Each specimen
is placed on the measuring board or the measuring triangle.
Measure the largest diameter (the length - of the shellfish).
Sort the measured shellfish into 5-mm groups; <5-mm,
5 to 10-mm, 10 to 15-mm, 15 to 20-mm and so on.
of shellfish in each group are then counted and this number
is entered onto the datasheet in the appropriate place.
A special datasheet
is used to record all data. It can be photocopied and used
by all data recorders. Be sure to fill out all the spaces
on the form. One form is used for each transect and the
quadrats are coded to match the sampling map. For example,
the first transect is called A and the first quadrat on
that transect is called 1. So the team sampling the A transect
will label their plastic bags A1, A2, A3, A4 etc.
one person should do the counting or measuring and another
should record the information. Be sure the counter/measurer
and the recorder communicate well with each other. The recorder
should have neat hand writing. It is difficult to write
clearly in field conditions and the recorders must make
a special effort to write each entry well.
writes the quadrat number for each station (or from the
Ziplock bag when the contents of the coded bag are emptied
onto a white plastic tote tray). The selected shellfish
are identified, counted, and measured.
the numbers of shellfish of each size group onto the datasheet,
put the shellfish that are between 5 and 10 mm in length
in the box labeled <10. Those between 10 and 15-mm long
in the box labeled <15. The quadrat number is listed
along the top of the form, so the numbers of shellfish in
each size group are in vertical rows.
Once the process
of counting and measuring is completed, the creatures should
be returned to the beach roughly where they came from. This
is called "non-destructive" sampling and is a
good conservation practice.
If the shellfish
are taken elsewhere for measuring, keep them cool and covered
with a small amount of water. They can be returned on the
following day at low tide.
Enter the shellfish
census data onto a spreadsheet. A Microsoft Excel (for IBM
computers) template is
available to get you started. The use of spreadsheets can
be an important part of the student learning experience.
You will need to add to the groups on the template to cover
the numbers of transects and quadrats and size ranges. The
use of spreadsheets can be an important part of the learning
experience. If the participating group does not have a computer,
perhaps a company can help by contributing a computer to
the project or by collecting and treating the data.
Since the data will be used in a national shellfish database
it is critical that the spreadsheet template is used. Once
the format of the spreadsheet is set up, enter the data
into the appropriate cells. It is useful for two people
to enter the data together, with one reading the tally sheets
and ticking off each entered figure while the other enters
it into the appropriate cell in the spreadsheet. Two other
people should check the data with the same procedure. Be
sure to save your spreadsheet frequently while entering
Fisheries managers require the raw data, so supplying this
in correct format is the minimum analysis required. However,
the results of the shellfish census will also be extremely
useful for the local community in conserving their shellfish
resources. For schools it can meet the numerical skills
requirements of their curriculum. Using data people have
collected themselves for a real scientific project makes
data entry and analysis of maximum interest.
Groups can select
from a variety of statistical tests and procedures as these
fit with their needs. We suggest the following as a basic
What are you
trying to discover?
- The numbers
and distribution of shellfish. This will help determine
if the shellfish beds are increasing or decreasing in
size or moving from one part of the beach to another.
It might also reveal patterns associated with beach conditions
such as storm water drain pipes or heavy recreational
- The size
of the shellfish at each location. This will show
if shellfish are moving into deeper water as they grow
older or perhaps not growing as fast in one area as another.
- The presence
of young shellfish entering the population and their
growth from year to year. This is indicated by finding
very small shellfish.
So you will
want to make a graph showing the distribution
of the shellfish at the stations, a graph showing the
average size of the shellfish at the stations, and
a graph showing the numbers of shellfish of each size class,
or a size-frequency graph.
The size frequency graph will show the growth of a very
young year-class over successive years.
When you repeat
the survey you can make graphs showing change in the population
size over time.
spreadsheet programmes, like Excel, include ways to analyse
and graph the data. Good graphs help people visualise the
numbers and sizes of shellfish present on the beach during
the time of your census. Later, when you have a series of
counts over successive seasons or years, the data can be
graphed to show changes in shellfish populations through
The first step
is to print out some of your data as tables. You can, for
example, print out a table showing shellfish distribution,
with the sample lane letters down the left side and the
distance from the beach along the top of the table. Each
cell in the table would contain the numbers of shellfish
found at the appropriate sampling station. Have students
write the numbers onto the map of your beach in the appropriate
place on the transect grid. If you look at a topographic
map showing heights, you'll see the map makers have drawn
lines connecting stations of equal height. Try to do the
same with your shellfish data to create a map with peaks
and valleys of shellfish populations.
Print out a
2D and a 3D graph showing the same data. Where were the
most shellfish found? Discuss why some stations have more
shellfish than others. Were there any special features of
those stations that made them better habitats for shellfish?
showing sizes of shellfish per station. Graph the average
sizes at each station and compare the data with the numbers
present. Is there a relationship between the sizes of shellfish
and the numbers found per station? Is there a relationship
between sizes and their distribution on the beach?
Make a graph
of the total number of shellfish of each size class. This
is called a size-frequency
graph and can show important information about recruitment
and mortality in the population. If, for example, you have
a good settlement of young, there will be a peak number
of small shellfish. Future measurements of the same population
will show this size class growing larger. If this peak vanishes
quickly, it would indicate the young shellfish died off
for some reason.
the total number of a particular shellfish over your beach
will give an idea of the total size of that resource. This
can be done by multiplying the AREA of the beach in square
metres times the average DENSITY of the shellfish per square
density of shellfish = S/Q*N Where S is the total number
of shellfish of a particular species. Q is the area of the
quadrat ( 0.1m2) and N is the total number of samples (quadrats)
taken in the survey.
the National Database
You can provide
invaluable information to the national shellfish database
in several ways,
- Enter your
information on the web form.
- Send the information
by e-mail according to the required format.
- Mail a disk
with the data in an Excel spreadsheet (only useful for
- Send a written
- The National
co-ordinator for the project is Bob Drey , Ministry of
Fisheries, PO Box 3437, Ponsonby, Auckland Fax: 09 377
4245. Email DREYB@fish_ak.fish.govt.nz
data to make a difference
Data by itself
is useless. Your group will have to make the data meaningful
to people in your community and to the other members of
your Beach Care group. The data will become more meaningful
as you repeat the survey and begin to show trends and changes.
Here are some ideas of what to do with your results:
- Send copies
of your report to the Local and Regional Council.
- Produce a
newsletter to let people know about your studies.
- Contact the
local newspaper and conduct an interview.
- Give an interview
on the local radio station.
- Talk at a
local conservation group meeting.
- Write an article
for a conservation magazine or local newspaper.
- Organise a
community group to discuss methods of protecting the resource.
- Make a public
display showing the results of the surveys and put this
at the entranceway to the beach along with a brief explanation.
If your shellfish
populations are having a problem, the community will need
to decide what to do about it and the scientific results
of your survey will be the deciding factor as to what is
done and how effective it is.
Always remember that in future, people will wonder about
the changes they see on the beaches. Will they find more
shellfish or none at all? Whatever they find they will not
have to guess about what conditions were really like in
the past because your data will be kept to show exactly
what the conditions were. If the programme goes well, your
community will, in concert with the Ministry of Fisheries,
be able to meet the needs of future generations for these