The annual Port of London Draw-Off is now under way on the Thames, exposing much of the river bottom from Richmond through to Teddington Lock.

Here Richmond's 'man of the river', Keith Arthur, a presenter of Tight Lines on Sky Sports and an expert on all things fishing, offers a fascinating insight into the unusual creatures, including aquatic invaders from the Far East, that lurk beneath the murky waters.

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Keith Arthur

In 1890 a decision was made in Parliament to construct Richmond Weir and Lock, thus allowing a navigable depth to be maintained upstream to Teddington.

Once a year, usually the first ebb tide following the clocks changing, the gates are left open to their fullest extent, allowing the full range of tide right up to Teddington Weir. This is to enable rubbish to be cleared from the gates and any maintenance work to be completed.

When this happens large areas of foreshore are exposed, revealing many treasures as well as some things best left unseen.

Because the amount of fresh water coming down the Thames has decreased, and continues to do so, there is more saltwater incursion, making the water brackish.

As a result, it is by no means unusual to come across clumps of bladderwrack, which is the sort of seaweed normally found amid the crashing waves on the beach. In fact, it is far from the tourist from the seaside.

Anything hollow could contain brown shrimps — yes, the same as those caught at the seaside using a push-net.

And, as the tide starts to flood from the very lowest point, postage-stamp sized flounder — flatfish similar to plaice — can be seen swimming in the shallowest water, foraging for food.

As for freshwater species, It's possible to find both duck and painters mussels. The former grows, up to 10-12cm while the smaller painters have paler, concentric rings around the shell.

There are also invasive species. Five or six years ago I found a huge area covered with what I first thought were cockle shells. I did some research and found them to be Asian clams!

These arrivals almost certainly came as larva via the ballast in container ships to the Pool of London. These were carried upriver and, as a freshwater species, settled here.

Quagga mussels are another invasive that were first reported in Wraysbury Reservoir. The possibility is they came via yachts used in sailing competitions and somehow made it to the Thames. They seem to have ousted the equally invasive — from Russia — zebra mussels from the river. Or perhaps the latter simply died out.

You may find a Chinese mitten crab, so called because their claws are covered in what looks like velvet, lurking in a pool close to or under the bank. These too probably came in ballast and have been here for at least 40 years.

Under large stones you may find baby eels, elvers. If you turn stones, PLEASE replace them to their original position.

There's no end to the inert things that appear. One year I collected a host of ancient Hornby and Clarke milk bottles. Looking more like 'specimen' bottles or cheap vases, I thought they might make a decent display in the garden. My wife didn't so I returned them to their watery grave the next day.

A similar fate to the bottles befell the sign from the old ferry by Eel Pie island. In 'southern region' green — like the trains — the enamel sign, in near-perfect condition and about 6ft x 2ft, suddenly appeared at least 25 years after it ceased to be of use. It advertised the fact one could use the ferry to make a connection to 'Southern Trains' to London for a train fare of 5/0d return! (That's 25p for younger readers!)

So, what's the weirdest thing I've found? I saw a discarded folding chair on a gravel bar, mid-river, directly in front of Ham Street car park. I waded out (in my wellies) and found some VERY thick braided fishing line attached to it.

Obviously it needed removing from the river so I kept pulling and eventually retrieved a large weight, far larger than anything normally used on the river, and a massive hook, a similar size to those I've used shark fishing.

That in itself is unusual enough but tied to the hook — and I swear this is true — was a packet that once contained haggis!

There are large catfish in the tideway, yet another unwanted invasive species, and I'm sure haggis would be a decent bait for them, but in the packet?

Perhaps the angler who used it was hoping for the Thames equivalent of Nessie…who knows?

The Draw-Off this year ends with the incoming tide on the morning of November 26th so if you are of a mind, have some wellies with decent grip, have a wander along. You never know what treasure awaits.

As the famous song said, in a different context: "There are watches you may find, wrapped up in bacon rind…"