Monday, 12 December 2016

Off the shelves

Following my recent pelagic post, just how deep is the deep water in the Medway?

The deepest waters are at the mouth; to the east of the Isle of Grain, the aptly named Sheerness Harbour Reach (starting in the north-east corner of the picture below) is 21 metres deep in places. That's 21 metres below the chart datum low water mark, which equates to the lowest astronomical tide. So, 21 metres is the minimum depth you'll encounter; on the highest tides you'd have a water depth of a little under 28 metres, give or take a choppy wave/trough. That would sort out most diving birds save the auks. To the south of Grain, Saltpan Reach runs for most of its length at around 13 metres deep.




At the eastern end of Saltpan Reach the deep water navigation channel, marked by the familiar red and green buoys, shallows out at seven metres, with just an incredibly narrow eleven metre channel (marked in blue on the map). To the north of this channel lays the Stoke Shelf, with a depth of just some three and a half metres below the chart datum low water mark, the similarly shallow Stoke Shoal just to the south and west (yellow circles).

South and east of the eleven metre channel is the extensive Sharpness Shelf, just two to four metres in places.

Small wonder the local tugs are kept busy piloting the large Liquefied Natural Gas tankers. The main water body alongside port facilities is dredged routinely to a depth of around 15 metres for these tankers and container ships, but for much of the southern edge the deep water lays some distance out from the shore.

No wonder the birding is hard. How important is a deep water channel for the birds? Feeding? This is a large open area of water where prey can be widespread. There is a more important navigation theory. Most seabirds can easily detect such channels, and stick over them. Theoretically, any seabird silly enough to take the dog-leg into the Medway from the Greater Thames will stay closer to the northern shore- and to date my own observations have shown this to often be the case, especially with Skuas. (This is a well known phenomenon on the Thames- the Channel runs close to the Essex shore and they get close views- but lousy lighting. Kent birders, with the sun behind them, get distant but clear views.)

The next interesting question is whether that narrow, dog-leg eleven metre deep channel at the end of Saltpan Reach is so small as to deter some birds from venturing further up estuary.

The shallower channel opens again to become Kethole Reach, running down to just south of Oakham island. The maximum depth has now dropped to seven metres. Often storm-blown seabirds just circle Kethole, especially when poor weather obscures exit routes. They either
 - (i) head back out past Grain,
 - (ii) gain height and go overland (seabirds often to the Thames, gulls and terns often south over the Medway towns), or
 - (iii) risk following the main channel further west (especially during high water).

Seabirds really don't seem to like the inner estuary. The outer estuary may be poor for seawatching, but the inner is much the poorer in comparison.

This analogy works for transiting sea duck too- by comparison, the mouth of Stangate Creek, a favoured hang-out for sea duck, is some 13 metres deep. If birds settle and take up residence, this rule goes out the window- they settle down around the best feeding areas.

I've also had idle thoughts about whether this puts some waders off routine crossings. One of the things I have noticed over last four years is that waders do not cross the main water body with any regularity; narfsiders roost on the north, sarfsiders the south.

I haven't often seen birds switching between the northern Stoke Saltings and the island complex. If they do, they more often than not switch indirectly moving about covering mud flats in the shallower parts of the estuary and using the central  mudflats such as Bishop Ooze as their stop-off crossing point. Obviously this is probably not a direct choice, the birds just make the most of available feeding time. Perusing W.G.Hale's 'New Naturalists' volume 'Waders', there are maps of wader flightlines around Morecambe/Ribble/Dee that, for the main, do similar.

But rules get broken, and yesterday was one such day when you could see waders crossing the deep water channel direct- over that narrowest point.

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First light, Queenborough, tide on the rise, already covered. Knot have been building in numbers the past couple of weeks, bang on schedule. On Deadman's Island a small group were trying to sit out the rise at Swale Ness. With the Oystercatchers occupying the drier heights, they were eventually pushed off, first south into the Swale, to West Point for a short while before turning round and out to head for Deadman's northern beaches.

In the distance, more Knot wheeled up off Stoke Saltings. With plenty of roost sites on offer there, I expected them to circle and land, but they set out south-east over Stoke Ooze. Over the next half hour another three flocks, all made for the south side. All went shortest route, shallowest water, to congregate on a dry part of the old seawall on Burntwick (blue flightlines).



These Knot may well have overflown the shelves by happy coincidence to reach a safe roost, but the Stoke Avocets went further. Narfside Avocets normally keep themselves to themselves, usually only commuting to and from Oakham, especially on higher tides. I had never seen them cross the deep water channel in four years (and I had been looking), but today they came south. They kept on a straight line over the Sharpness Shelf to turn hard south down Stangate Creek, lost to sight (but perhaps going to join the main southern roost in Funton?) These Avocets also did not cross as one flock, but three, spread out over a 40 minute spell, all on that same flightline (yellow), It was interesting to note how they stayed close to the Burntwick shoreline. They could have saved themselves some flight time by going high overland, or have entered mid-Creek to avoid potential dangers (they are not to know the wildfowlers wouldn't be aiming at them). Essentially they took shallow water for much of the route.

I was uncertain as to why any had crossed in the first place. It was not a particularly high tide, nor too strong a wind. The movement did not happen until after their original high tide roosts had settled, so it might have simply been down to disturbance; wildfowlers perhaps, or just the slow maneuvering of that oil tanker and attendant tugs close by Elphinstone Point?

Whatever the reason, all I know is I really want to watch it happen again.

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