Wednesday, 12 October 2016

Hogsmill Electrofishing

Hogsmill Lane
Over the last two days members of the Environment Agency have been undertaking fish surveys at four locations along the Hogsmill. This is from Hogsmill Lane (just off Villiers Road) to Oakland Way-Chamber Mead KT19 upstream, just  across the borough boundary.

This type of survey is  usually undertaken by boat - although backpacks were used in the shallows upstream at Ewell. Circular electrofishing 'anodes' attract the fish, which swim towards the the charged equipment, where they are stunned but not harmed. They are then decanted to large buckets for further examination or 'processing'.

During the initial survey  at Elmbridge Meadows, a surprising catch of ~100 fish was made in the first of three runs. This is virtually one fish every metre of the ~90m netted area and included: chub, dace, roach, stone loach, gudgeon, stickleback, minnow, bullhead and an eel.

At  Worcester Park Road-Tolworth  A240  a similar number of fish were caught. By far the majority of the minor species were minnows, which were not located in the Ewell site. A rudd caught at this location was a surprise, as it would be more at home in still water, so had probably been introduced or had somehow introduced itself.
Measuring a rudd


The coarse fish are measured and in some cases a scale sample is taken - presumably for DNA analysis. The minor species are not  measured, although it can  be difficult for an untrained eye to tell them from the many young chub - which are measured - and  demonstrate natural recruitment. No perch were located, although there is a shoal below the gauging station at Watersplash Close in Kingston, so maybe they cannot negotiate the weir.

Silver eel about to return to spawn
eel on eel board Hogsmill lane
Fun and games are averted, as the eels are measured by using a special 'eel board'. This prevents some of the larger animals from thrashing around and aids examination of the condition of the animal. Some were transforming into silver eels; laying down fat for the journey to the Sargasso in the Caribbean where they breed. The eel board facilitates gentle return to the water of the animal without too much handling.





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