Deben Fish Survey August 2022

By Richard Verrill

It is well recognised that estuaries provide essential breeding grounds and nurseries for many fish species. They also provide corridors for migratory species. Estuaries provide a very dynamic environment with constant changes in tide, temperature and salinity. Intertidal areas provide particularly important refuge and feeding grounds for small fish.The variety of the shoreline in the Deben provides an abundance of different nursery environments with sandy beaches, shingle beaches, mudflats and salt marsh.

Up until 2014 Eastern Inshore Fisheries and Conservation Authority (EIFCA) had done regular fish surveys of Suffolk estuaries including the Deben but changes in priorities, finance and covid had made this more difficult. Stephen Thompson from EIFCA approached the River Deben Association and suggested some Citizen Science to help fill these gaps. In 2021 five volunteers from the RDA spent a training day with EIFCA and Suffolk Wildlife Trust where we had instruction in small fish sampling science and techniques. Stephen Colclough from the Institute of Fisheries science lead this training, he has particular experience in this field. Subsequent to this training EIFCA decided that they could not continue with this project but having seen the enthusiasm of the RDA volunteers I approached the RDA committee asking if they would pay for us to use Steve Colclough to direct a RDA Citizen Science fish study. The RDA agreed and so in April this year Steve and I looked at various potential sampling sites. The last fish samples done by EIFCA used 3 sites; one at Bawdsey. one at Ramsholt and a third at the beach opposite Waldringfield golf course. We felt it would be good to use one of these sites as we might get some comparable data. Bawdsey was chosen because it is relatively firm under foot and has excellent access from the road.

On the 16th August 4 RDA volunteers, Steve Colclough from IFM and 2 fisheries scientists from EIFCA met to do the first RDA fish sample. Previous studies had used 45 metre seine nets deployed from a boat. Steve Colclough had developed a technique for citizen science using a 15 metre seine net deployed from the shore and also potentially using winged fyke nets.  The seine net had a 3mm knotless mesh and was 2.7 metres deep with floats on the top and a weighted rope on the bottom. Using Steve’s advice and experience we were able to wade into the river to a depth of 4-5 feet and let the net flow with the current parallel to the beach, subsequently both end of the net were hauled onto the beach in a horseshoe shape. Many small fish were caught in our first trawl they were rapidly transferred into containers refilled with sea water. All fish were then identified measured  in perspex water containing measures and returned to the river.

We completed 3 trawls as the tide rose and managed to process over 300 fish in the day. We had such a good sample we decided we did not need to use fyke nets as well.

I have attached a record of our data below:

As the nets and exact location of the EIFCA survey were different it is difficult to draw any accurate conclusions but within these limits it is  interesting to make some comparisons.

In 2013 EIFCAs “mouth sample” used the edge of the salt marsh upstream from Bawdsey beach and in 2014 EIFCA  used the steep shingle bank downstream from the quay. The 2013 sample had over 1500 Goby, no Sea Bass and 30 Sand Smelt. In 2014 the EIFCA  sample showed no Goby or Sea Bass, 45 Sand smelt and 28 Sprat.

As can be seen from the RDA data over the 3 samples we caught 151 Goby, 15 Sea Bass, 39 Sand Smelt and no Sprat. These differences are likely to be explained as we sampled the beach area which has mud at low tide and a mixture of sand and shingle further up the gently inclined beach. Goby are usually found in abundance in salt marsh and much less likely to be found on a steep shingle beach as surveyed in 2014. Our sample probably illustrates a sample taken between the sites used in 2013 and 2014. Our Sand Smelt catch was very similar to EIFCA’s findings. In Their previous studies sand smelt appeared to be fairly evenly distributed in the whole river. Our Bass catch was significant as the 2013 and 2014 samples showed no Bass at the mouth but some further up the river.  With the increase in sea temperatures Bass are becoming more abundant on our coast and there is some evidence that they are now spawning in the Southern parts of the North Sea. We caught no Sprat but they are recognised to swim in large shoals and are nomadic so it would be expected that their abundance would vary year to year.

It was very reassuring that our findings were similar to those of EIFCA despite the different net size and techniques. Steve Colclough has produced some scientific papers which suggest that small Citizen Science studies can produce accurate and very valid data. The study done by the RDA provides further validation to the techniques used.

On discussion with the EIFCA fish scientists they are keen that further studies should take place. The Gold Standard for fish sampling would be a sample in May followed by a second sample in September to track the changes taking place over the summer spawning season. The RDA volunteers have shown both skill and enthusiasm and so we are now exploring further funding to pay for expert assistance and equipment to do more studies and add more data to our charts.

Richard Verrill

Richard was a GP at Framfield House Surgery Woodbridge for 31 years and retired in 2016.

He has sailed and fished on the Deben for over 30 years and was previously on the committee of the RDA.