How to do a Shellfish Census

Laying out the Grid
The Sampling Team
Checklist of Equipment
Sampling Procedure
Data Entry and Analysis
Making your research count

The Sampling Plan

Contact the 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.

Another option, 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 right away.

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 beach?

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?

Shellfish populations 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 measure?

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.

How to lay out the grid

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.

Each survey 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.

Sampling Team

Your science 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.

A Photographer, 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.

Basic Equipment Checklist

Personal gear:

  • 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 nibbles
  • A towel

Scientific gear:

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.

Each sampling 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 Centre).
  • 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)

The 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.

Another method 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.

Sampling Procedure

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 the shellfish.

Sampling is 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.

Each 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.

The quadrat 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.

Measuring and Counting

Depending on 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).

Ideally, depending 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.

Shellfish are 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.

The numbers of shellfish in each group are then counted and this number is entered onto the datasheet in the appropriate place.

Recording Information

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.

When recording, 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.

The recorder 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.

When entering 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.

Data entry and analysis

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 the data.

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 theme:

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 use.
  • 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.

Most modern 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 time.

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?

Print tables 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.

Calculating 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 metre.

The average 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.

Reporting to 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 IBM format).
  • Send a written report.
  • The National co-ordinator for the project is Bob Drey , Ministry of Fisheries, PO Box 3437, Ponsonby, Auckland Fax: 09 377 4245. Email

Using your 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 shellfish resources.