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Increasing scientific knowledge

Increasing scientific knowledge about eels in the River Glaven

As the title of this project suggests, eels are indeed both enigmatic and mysterious - good scientific data about their lives and their lifecycles is lacking, with much still to discover. This project aimed to gather data to improve scientific knowledge about eels via an extensive programme of monitoring, surveys, trapping, tagging and tracking of mature eels and young elvers in the River Glaven catchment. Both the University College London (UCL) and the Zoolological Society of London (ZSL) were academic partners in this work.

Surveys

We aimed to carry out at least six surveys per year at a number of locations in the river catchment, not only in the main river but also across its tributaries, lakes, ponds, ditches and marshes.  We used the first year to set a baseline and the subsequent ones to monitor change:

  • Thirteen surveys in 2015.
  • Eight surveys in 2016.
  • Ten surveys in 2017.

The majority of the surveys were by electro-fishing but we also utilised observation, sweep netting, lamping, kick sampling, fyke netting and filming, with fixed location and hand-held cameras.

Electrofishing

Electrofishing

Measuring eels caught

Measuring eels caught

The Environment Agency contributed their monitoring records, stretching back over the last 25 years, and results were collated by project officers and by a student as part of his Master of Science (MSc) thesis.

We recognise that a three year period is too brief to reveal statistically valid data on population change across the river catchment and that change will have to be assessed over many years.  However, the surveys did provide interesting local results, allowing conclusions to be drawn about the success of habitat change in attracting eels to specific areas of the river.

Trapping

Checking the Glandford Mill eel trap

Checking the Glandford Mill eel trap

Year-round trapping is an additional way to monitor the number, vigour and physical characteristics of eels in the river but, more importantly, it provides the only way of monitoring the number of elvers arriving in the river in spring and the number of adult female eels leaving the river and returning to sea to begin their return journey to the Sargasso Sea to spawn.

We installed a simple eel trap, consisting of a bucket at the top of the main bristle eel pass at Glandford Mill over the whole of spring 2015, 2016 and 2017 elver runs.  The mill owner, a project partner and volunteer, checked the trap daily, recorded results and released any trapped elvers upstream of the mill race. This allowed us to monitor elver success in reaching this mid-point along the river length, whilst recognising that some of those caught were more likely to be elvers from previous years, which had taken some time to travel that far upstream.

Netted elver

Netted elver

We also conducted trapping at 3 tidal sluices for two one-week periods in the 2017 elver run, monitoring the latest elver arrival and their success at passing the sluices and entering the river.

In addition, a study conducted by UCL MSc student Kesella Scott-Somme in 2017 employed novel passive samplers to monitor juvenile eel recruitment into the lower River Glaven and assess the impact of tide gates on migration success.

Passive elver sampler

Passive elver sampler

The passive samplers were made from recycled netting, each attached to a weight and a buoy, and deployed up and downstream of the main tidal sluices and in Cley New Cut, and also up- and downstream of one of the tidal sluices in Blakeney Freshes. Traps were checked by retrieving the sampler with a long-handled net and manually searching for eels within the netting. The study comprised two intensive sampling periods, 23-29 June and 11-16 July.

Tagging

Main river tagging locations, 2016

Main river tagging locations, 2016

Working closely with ZSL, we captured and tagged or marked (depending on eel size) 58 river-based adult eels in 2016 and 78 in 2017, in order to monitor their pattern of movement in the river. In 2017, we also tagged 17 adult eels in off-line ponds (i.e. without connection to the main river).

Captured eels were measured (Total Length, TL, mm), weighed (wet mass, g), anesthetised (Benzocaine 0.2 g L-1) and either tagged with a Passive Integrated Transponder (PIT) tag (12 x 2.12 mm, 0.1 g; 23 × 4 mm, 0.6 g, or 32 x 3.65 mm, 0.8 g, dependent on fish size, HDX, Texas Instruments) surgically implanted into the peritoneal cavity, or if too small to PIT tag (< 120 mm length) were marked on the ventral surface with visible implant elastomer (Northwest Marine Technology). Tagged eels were placed in an in-river keep net for a minimum of 0.75 hours to facilitate post-operative recovery prior to release. Fish were subsequently released in the same reach in which they were captured.

Passive Integrated Transponder PIT tag 23mm

Passive Integrated Transponder PIT tag 23mm

Eel marked with visible implant elastomer

Eel marked with visible implant elastomer

Eel post tagging

Eel post tagging

Eel movements were monitored using recapture of tagged/marked fish during electrofishing surveys and detection of tagged fish by two stationary swim-through PIT antennas installed at Glandford Mill. The antennas were positioned immediately upstream and downstream of the existing eel passes. Tag detection tests and data download were performed at 4 to 6 week intervals.

PIT antenna downstream of Glandford Mill

PIT antenna downstream of Glandford Mill