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Paul Gaskell, of the Wild Trout Trust, gives an insight into what to look for when Fishing for river brown trout and the water management needed to give these wonderful fish the habitat they deserve.

The most important determining factor in successful angling is the ability to locate fish, but our quarry will not always be readily visible. Glare, a ruffled surface or just a dull day can make fish spotting a ‘low percentage’ activity, even in very clear water. We will simply be unaware of the precise location of the majority of fish from our position above the water’s surface (people who have snorkelled in crystal-clear New Zealand trout streams have attested to this fact!).

Consequently, the angler who knows where to look for fish has the best chance of spotting them when they are visible – and is at a huge advantage when relying on watercraft alone to locate unseen quarry. The best fly in the world is useless if it is cast into empty water. From an angling as well as a river management point of view, it pays to understand very clearly the features that constitute a fabulous lie for adult fish

The ingredients start with a ready food supply. Riffles are fantastic river fly factories – but not easy for fish to sit in (too much hard work in fighting the main flow, too shallow to feel really safe). A riffle supplemented with a regular dose of crash landings of terrestrial bugs from surrounding tree cover is an even better buffet. The spot for a big fish, therefore, is just downstream of the riffle ‘food factory’ in a lie where it has to expend minimal effort in order to hold station and can feel secure with some overhead depth. This is the reason that the borders between fast-flowing food and oxygen-rich currents and slower flow in front of or behind a large boulder or log are such good angling prospects. The magical border of contrast between fast and slow can be formed simply by an increase in depth as a riffle slides off into a scour pool especially the deep outside edges of bends in the river. Having said that, don’t miss out on those pockets of opportunity within the riffles themselves, where water flowing around a smaller obstruction has produced a small hollow in the bed of the stream. This is a miniature example of increased depth and reduced current speed, where a fish can sit right in among the food factory. To a degree, the increase in depth is a kind of cover in itself (at least from threats above the water!), but to really seal the deal the feeding lie must be close enough to a safe bolt hole of dense cover if it is going to qualify as super-prime real estate.

Bolt holes come in many forms but think about undercut banks with plenty of gnarly tree roots, fallen trees, water crowfoot beds, overhanging drooping sedge grass foliage and even the bent, crumpled stems of annual plants like cow parsley toppling into the water. What the fish are seeking here is some overhead cover, coupled with a bit of depth, to make them feel really safe

The list of suitable features is very long, and includes the cover deliberately introduced during habitat improvement works! By making sure there is a great diversity of current speed, flow direction and varied structural elements such as boulders, tree root systems and fallen trees, the ideal home for giant trout is an automatic by-product. Of course, the category of ‘giant’ will have different meanings, depending on the venue. All of my own biggest river fish have come from really well-defined ‘structure’ and the significant advantage of knowing these opportunities by sight is certainly not lost on me!

Great anglers will read the topography of the river in the same way a musician translates notes from a musical score. Not only is it useful to be able to judge at a glance high, medium and low-probability patches of habitat in terms of holding a trout of catchable size, the underlying understanding is invaluable when undertaking fishery habitat management. At the most basic level it allows an assessment of whether opportunities for adult fish to hold in a particular section are limited. It also allows sensible prescription measures that work with the natural. The promotion of localised riverbed scour and the creation of adjacent refuge provide the key to success.

One very common mistake is the practice of building low weirs to ‘hold back’ water and generate deeper pool habitat. Over time, the reduced current speed will cause the impounded pool to accumulate sediment and become shallower (back to square one). Not only that, you also run the risk of drowning out the food-factory riffle as well as reducing fish-spawning great adult fish habitat.

Holding back water to create depth is also bad for fly life diversity. A similar mistake (on a smaller level) is to simply dig pools into the stream bed – again these will quickly fill with sediment, as you are working in opposition to the river. Far better results are obtained by using the vast array of scour-promoting secure installations of logs (we have an ever-expanding array of these structures up our sleeves at the WTT and many are detailed in our habitat manuals on Foremost among them will be the various single and paired upstream-facing log deflectors that are designed to promote stream-bed scour. However, even before we need to get to the stage of constructing flow deflectors, the commonest mistake of all can be avoided. Simply resisting the temptation to haul out fallen trees from your river will give you amazing results. The only exception to this rule is if there is a clear and present danger of causing flooding in a sensitive area. In this case, trees are better repositioned and fixed securely in place using the same techniques used to construct flow deflectors. Just as an aside (while I am on the subject of cardinal errors), the practice of cutting back those lovely feathery tree roots that you sometimes get in river margins is totally counterproductive. It is a great way, on the other hand, of massively reducing the capacity of your stream to hold fish – so I’ll leave it up to you…

Why the high value to plentiful woody debris? It just produces a fantastically diverse array of benefits that combine fantastic cover from predation with ample pool-scouring actions. The crucial difference between damming water to produce depth, versus scour pools generated by the action of the water, is that the latter are self-cleaning to a degree. They also have a much greater throughput of helplessly drifting food items in the conveyor belt running off productive riffle areas. In fact, the formation of fantastic deep pools on the outside of river bends (with the nice sloping beach/gravel bar on the inside of the bend) is often caused by the dual scouring and gravel-bar accumulating action of fallen wood. The pools also certainly benefit from submerged fallen tree cover that can accumulate in the deeper slot on the outer edge of a meander – think of the kind of structure that our North American friends will pick out for bass fishing (and their trout fishing, come to think of it!).

It is a great shame that most anglers never get the opportunity to observe fishery biologists carrying out electrofishing surveys of rivers. A few weeks of noting down where the big concentrations of huge trout come from soon cement a pretty clear understanding of where these fish lie!

In place of such experience, you will have to trust that the few photographs of fish captures are part of a much larger catalogue of similar examples. I know of countless fish caught in precisely the sort of spots described by the wish list of conditions outlined. Clearly there is no such word as ‘always’ in relation to fish behaviour, but there is such a thing as a huge majority of cases. Even the fish found taking advantage of some opportunistic feeding in slightly weird settings are likely to routinely depend on more typical lies for holding.