It had been a while since we had met someone on the river for a chat, but Ben Norrington had been on our list since the very beginning of the project nearly a year ago. According to many, Ben was the ‘eel man’, and someone we had to chat to about this interesting animal that calls the Stour its home.
After spotting his Environment Agency van we found Ben on the footpath near Flatford Mill on a warm and sunny March morning and walked with him down to the Judas Gap, the old limit of the tidal Stour before the Cattawade Barrage was put in place. You could immediately see why the eels, and many other species have trouble migrating as the structure is formidable, which of course it had to be in order to hold back the tide. Whilst we we there a duck was the only thing that we saw traverse the barrier, and it only did so by taking off from the lower reaches to fly over the top and land on the upper part of the river a few seconds later.
Ben explained how the Environment Agency has been trying to reintroduce places where eels and fish can live, including off channel refuges, the reconnecting of decoy ponds and various scrapes at points along the river bank. They also aim to help others with this task through the Fisheries Improvement Fund which provides funding for a variety of projects connected with this. The EA have been conducting fish surveys for 12 years, and 10 years at the Judas Gap, which take place during the summer to monitor migratory fish, and during this time eel numbers have been declining in the river, which mirrors the global trend. The main contributors to this are parasites, overfishing and of course barriers that prevent fish and eels from travelling along rivers to and from the sea. In fact eels trapped in bodies of water with no access to the river, and therefore the sea, can morph themselves considerably as they feel the pull to migrate but cannot travel, and this increase stress can itself result in greater risk of disease. At the Judas Gap the EA have installed a ‘fish pass’ which helps eels to navigate up the steep incline of the structure into a tank at the top. Ben has to check this every two days and monitors any activity that he finds.
Eels undertake a remarkable journey to spawn and lay eggs in the Sargasso Sea (part of the Atlantic near Bermuda) before returning to the rivers where they spend the rest of their time. Although there are around 800 species world wide, only one lives in the River Stour, the European eel (Anguilla anguilla) which goes through several stages in its life: glass eel, elver, yellow and then silver, as it matures. Female eels grow three times larger than males.
Most eels stay in the river once they have arrived (Anadromous) with only some 15-30% migrating (Catadromous). You can determine the age of fish by looking at growth rings on their scales (as you would a tree) under a microscope and also analyse the otilith (middle ear bone) in a similar way. On the Stour though the EA measure length and weight of the eels. The oldest eel they have found is 70 years old and perhaps would have stayed in the River since it was 3 years old. There is a fish survey in Bures during July if you’d like to pop down and have a look to see what happens.
To try and understand eel migration two different types of tracking devices have been used: acoustic and pit tags. The former uses hydrophones in the water to detect audio pulses emitted by the tags to track the eels progress, and the latter use passive integrated transponders (PIT) with static RFID receivers in the river, triggered as the eels pass by. Japanese scientists have also been DNA testing water samples from the ocean to determine where and when eels are present.
In the river the eels migrate upstream using a variety of techniques. They follow the flow of water, detect differences in salinity and heat to head towards fresh or sea water depending on which way they are traveling. Eels provide a huge food source for birds, otters and other predators. Victorians had eel traps along the river, but now eel fishing is banned in the UK. Ben said that generally fish stocks aren’t too bad, with the channel near Dedham Mill Pond being the place with the most concentration of fish on the whole river.
It was great to be out on the river with Ben on such a sunny March day and gain a real insight into the life of the eel. To find out more join Ben on the Fish Migration Day event at Flatford Mill on 20th April.