Tillingbourne Riverfly Monitoring

Sampling Method

We aim to monitor once a month throughout the fishing season and approximately every 6-8 weeks during the winter. The procedure we follow is based on the method recommended by Cyril Bennett and taught at the 'River Invertebrate Monitoring for Anglers' one-day workshop at Leckford.

There are three stages to the monitoring process:

• Collecting the sample.

• Washing and filtering.

• Sorting and counting.

Collecting the Sample:

The sample is obtained using a standard professional 1mm mesh pond hand net with the net held downstream resting on the river-bed. The river bed upstream is vigorously 'kicked' to allow the invertebrates to be carried into the net by the current. For weedy areas we carefully sweep the net back and forth through the weed-bed whilst gently 'stirring' with the other hand. The aim is to dislodge as many invertebrates as possible whilst not uprooting and losing too much of our ranunculus weed-bed.

Sampling in Middle Beat

We start at the bottom end of our monitoring sites 'shuffling' and 'kicking' our way progressively upstream covering approximately a ten metre stretch of river.

The total sampling time of three minutes is proportionally split to take into account the different habitats. For example, in Middle Beat the weed-bed occupies about 1/4 of the site. So we sample this area for approximately 45 seconds. The remainder of the site is split equally taking into account bank-side vegetation and the differences in the river-bed i.e. stony, gravel and silt patches.

Washing and Filtering the Sample:

The next step is to remove as much debris as possible from the sample. This can be quite difficult and our procedure may change in the future as we gain more experience. We have used various methods but so far the most successful technique has been using the net as a filter. We have found the effort entailed worthwhile as it makes sorting and counting the invertebrates very much easier.

First we fill a large bucket with river water and empty the complete sample into it - we now use a much larger bucket than shown in the photo. We have found a large round bucket normally used for 'home brewing' works best.

We give the contents a really good stir and then carefully pour back through the net leaving any large stones and most of the gravel in the bucket.

We then refill the bucket with water and repeat the process trying to dislodge as many of the invertebrates as possible before straining them back through the net.

This is repeated several times until the majority of the invertebrates are in the net and separated from the debris. The invertebrates are then tipped back into a smaller clean bucket partially filled with river water ready for 'sorting and counting'.

The larger stones and pieces of weed left in the large bucket are carefully picked over to remove any 'bugs' before discarding. Any remaining debris is put to one side until later. We have found this part of the sample usually contains the larger and heavier bugs, especially cased caddis.

Sorting and Counting the Sample:

Following the collection of the sample we separate the invertebrates into eight target groups and then count/estimate the numbers in each group. This is the most time consuming part of the process but can be very rewarding and great fun especially with a team of enthusiastic helpers!

At Albury we are able to take the sample back to our fishing hut and the sorting and counting can be carried out in comfort either under cover or, when the weather allows, sitting down outside.

Sorting and Counting in the Sunshine

We find it best to divide the sample into several parts. The sample is poured into large white photographic trays - we use two. Then with 'oven basters' we separate the various invertebrate categories and place them into either a segmented tray or into smaller photographic trays prior to counting.

The aim is to count the numbers in each of the eight target groups, grading them as follows:

A: 1-9

B: 10-99

C: 100-999

D: 1000+

Normally we start by concentrating on the  larger invertebrates such as mayfly nymphs and caddis, and these can be easily counted to a reasonable accuracy. However, with freshwater shrimp (gammarus) and often olives and yellow-may-duns there can be considerable numbers. In the time available it is impossible to count them all. For these we estimate the numbers. However, with the grading system used counting is not normally too much of a problem. After a bit of experience it is relatively easy to estimate if the numbers are in the tens or hundreds or even in the thousands. Occasionally, as a quality check, an attempt has been made to rigorously count the numbers - this can be a day long job!