Before you start
The distribution and abundance of invertebrates at the upper and lower shore can be measured by randomly sampling in both areas or systematically away from the low tide point.
- A sampling tray
- D frame net
- Identification guides, such as the FSC rocky shore or saltmarsh plants fold-out chart
- Hand lens, plastic spoon
- Equipment for measuring abiotic factors (e.g. humidity, depth, water temperature, water pH, oxygen, nitrate concentration, light)
Care should be taken when working in marine environments.Tides and weather conditions can change quickly. Always check tide times when planning fieldwork at the coast. Wear waterproof trousers and wellington boots. Do not work by yourself.
Summary of procedure
- Carry out a pilot survey to identify contrasting habitats, such as an exposed rocky shore compared to a sheltered rocky shore. Control as many other variables as you can, such as substrate, water temperature, water pH, oxygen, nitrate concentration, light
- Choose a sampling strategy, such as a transect.
- Identify animals to the lowest taxonomic level as possible.
- Record the number of individuals of each species or use the to estimate abundance.
- Take sufficient samples for your data to be statistically representative. For many statistical tests, 10-30 samples are needed in each area.
- Collect abiotic data, with sufficient repeats.
Measuring height above sea level
On a rocky shore, height above low tide is more important than horizontal distance. Use tide tables to identify the time of low tide, then use this as a reference point. You can also use the tide tables to work out how much higher your reference point is from extreme low water spring tide (ELWS) level.
Height levelling with an optical level needs two people. The first person stands at the water's edge with an optical level placed so that the eye piece is level with the top of a metre rule. This first peson asks a second person to move up the rocky shore until the point that the bottom of the second person's feet become visible through the eye piece. The rise from the two people is therefore 1 metre. Repeat the procedure to continue measuring the height.
Use a tape measure to record the ground distance between the two heights. Using trigonometry, you can calculate the actual horizontal distance from these results.
Recording rocky shore invertebrates
Interrupted belt transect
The most practical sampling strategy for rocky shores is generally the interrupted belt transect. Remember to sample at regular height intervals, rather than intervals based on horizontal distance along the ground.
50cm x 50cm quadrats are appropriate for sampling rocky shore invertebrates. Use the ACFOR scale to record the abundance of each species.