While on the Otava I had plenty of scope  for observing my companions in their own fishing approach and also for experimenting by myself.  I wrote about one of these fascinating episodes in Part 51 of my Frontier series in Fish & Fly.  While on the same river, I had another hugely illustrative experience.  It was on one of the most downstream sections of the Otava we fished.  There was a long pool, very well defined by a run-in concentrated on the left bank, a slow middle section and a broad, shallow glide at the tail: in all about 75 metres in length, and, frankly, utterly perfect mixed trout and grayling water.  Paul and Sue Sissons had fished this pool the previous day so I was eager to see the effect that their nymph (Paul) and dry fly (Sue) approach had had on the fishing potential here.  They had both caught a lot of fish from this pool, with Paul taking mostly trout at the fast water of the run-in area, while Sue had caught mainly grayling in the lower half of the pool.  I eased in right on the broken water of the tail and fished a plume tip to the rising fish (pale wateries), slowly wading up the entire pool’s length.  On the far bank (left bank) were overhanging trees, and the most concentrated feed and foam lane,  though there were grayling also away from the overhanging foliage.  In all cases, however, I observed the need for supreme accuracy in order to have a chance of a particular target grayling rising to the plume tip.  The trout, as always, came much more readily, even to a fly placed significantly off the rise area.  In my first passage up the pool, over a period of perhaps half an hour, I caught two trout and nine grayling, all between 25cm and 38cm.  I was aware of several, repeated rejections, entirely from grayling, to the size 19 plume tip.  After a short break, just as I had done on the rapid, upstream, a couple of days earlier (described in the Frontier article), I repeated the fishing process, again entering at the tail, but this time with a size 21 plume tip.  There were, unsurprisingly, fewer fish showing, but still a scatter of pale wateries.  It all felt manifestly different now.  In spite of the rising fish, one could sense the high state of ‘awareness’ of some danger in the pool.  The rises were infrequent and very subtle, and almost exclusively in the most ‘protected’ lies, under the overhang or on tiny vagaries of the current which made presentation difficult.  This second run up the pool produced five grayling, and curiously included both my smallest (20cm) and largest fish (40cm) of the session.  It was altogether a fascinating reaffirmation of past lessons learned.  Diminishing returns, certainly; but more how the trout will always be the first to be caught, and then become uncatchable, and for the grayling to become challenging very quickly, although they will continue to feed, even with the obvious, or recent, presence of an angler, but will become highly cautious in their rise and will also require supreme accuracy, both in terms of position on the feed lane and timing – because a grayling, particularly the larger fish, will not rise until it is ready to do so, even allowing several duns to drift past.  This is quite unlike trout behaviour.  On a healthy river like the Otava, the invertebrate populations are immense, and support surprisingly high populations of both trout and grayling.  My companions mostly felt that there were more trout and grayling on this river, though I felt the opposite.  Although the trout, particularly the rainbows, were generally first to be caught in a session, working the water, especially with a representative dry fly (of prevalent pale wateries) such as the plume tip, in all cases on the lower sections of river we fished, produced a dominance of grayling numbers as well as the larger fish caught by the group.  The Otava is stunning for a fly fisher and many of us were reluctant to move on, though we had a date with Jan Siman who was going to show us his beloved Tepla Vltava.  I was not alone in thinking that we would miss Otava, however, with its myriad possibilities of clean waters, its stream of pale wateries and enhanced, varied challenges for each of us.  My own fishing over four days had given me almost 250 fish, two thirds of which had been grayling, upwards of 20cm, with a best fish, a grayling of 41cm. I had enjoyed enthralling ‘experimental’ sessions such as that described above, and generally fishing that had taken me by surprise.  I could not have expected such rewarding fishing, either in terms of numbers, quality, size, or various challenge, and I know that my companions felt similarly, although several were keen to see just how the Vltava would compare.